is the sign of the number of element in the set. In practice, we apply the series of tests

previous methods, to derive the distribution of the inter-event distances. The results obtained for the aftershock sequence triggered by the Al Hoceima 1994 earthquake are shown on Fig. 11.

*h kh k*

\* >

1 2 2 <sup>2</sup> 1 *<sup>i</sup> I i crit i*

\_

( ) ( ( ) )

are the mean value and variance of the sample *x*

, ......... , in this order until the null hypothesis is not rejected. We used the

^(. , *<sup>h</sup>* ) is given by

^(. , *<sup>h</sup>* ) with Gaussian kernel, has more than *<sup>k</sup>* bumps if and only

(1)

e

then the *p* value of this test is given by

= (58)

(*k*); *hcrit* (2)

(*k*) ; *hcrit* (3)

\_

, and (*ε<sup>i</sup>*

( ) <sup>µ</sup> *h k Inf h f h has k bumps or less crit* = Î { ¡ / ., ( ) } (56)

*k*

, the smoothing

(*k*) reject the null

(*k*); .........; *hcrit*

(*N* ) (*k*)

(57)

; *i* =1,*n*)

**Figure 11.** Kernel density estimated for Al Hoceima 1994 aftershock sequence (a) using the rule of thumb (b)using cross validation smoothing and (c) Estimated density obtained with one mode critical bandwidth *hcrit* = 6.4

The Fig. 11(a) gives the estimated density obtained using the rule of thumb. The cross valida‐ tion optimal smoothing method gives a bandwidth parameter *h* = 1.2, the estimated density of probability obtained is shown on Fig. 11(b). The estimated density obtained with one mode critical bandwidth equal to *hcrit* = 6.4 is shown on Fig. 11(c). On the three Figures a concentra‐ tion of the inter-event distances around the value 8.9 km, which correspond to the mode of the estimated distribution of probability. The results obtained for the Al Hoceima 2004 aftershock sequence are shown on the Fig. 12. The estimated density using the rule of thumb shown on Fig 12(a) displays a concentration of the inter-event distances around the value 9.45 km, the estimated density using the cross validation optimal smoothing has been obtained using a bandwidth parameter *h* = 1.0. The density obtained displays a concentration of the inetr-event distances around the value 7.5 km. Although, the estimated density with one mode critical bandwidth obtained equal to *hcrit* = 11, shows a concentration of the inter-event distances around the value 14.14 km.

**Figure 12.** Kernel density estimated for Al Hoceima 2004 aftershock sequence (a) using the rule of thumb (b)using cross validation smoothing and (c) Estimated density obtained with one mode critical bandwidth *hcrit* = 11

For the aftershock sequence triggered by the 21 May 2003 Zemouri earthquake (Mw 6.9), the obtained results are shown on Fig. 13.

the threshold completeness *mc*

events in the Algeria-Morocco region.

, J.A. Pelàez2

tom. Control , 19, 716-723.

**Author details**

M. Hamdache1

**References**

threshold completeness *mc*

, we used the maximum likelihood to estimate above the

Scaling Properties of Aftershock Sequences in Algeria-Morocco Region

2

derived using the

http://dx.doi.org/10.5772/54888

69

. The results obtained are close to 1.0, the typical universal value

for aftershock sequence. The Omori-Utsu law for aftershock decay and Bath's law for the difference between magnitude and the largest aftershock and mainshock as modified by Shcherbakov et al., (2004a, b)., while using *AIC* as a measure to select the most appropriate model between the first stage and two stage Omori-Utsu model, the rate of decay of aftershocks was found to follow in all case, except the May 21, 2003 Zemouri aftershock sequence, the first stage Omori-Utsu law, which mean the Omori-Utsu model without secondary aftershock. The decay of aftershock trigged by the May 21, 2003 Zemouri earthquake exhibit a better fit with a two stage Omori-Utsu model, denoted model 4 in the text. The later allow us to include in the modelisation the secondary aftershock of magnitude 5.8 *mbLg*. The study of the Omori-Utsu law has been complemented of aftershock sequences. This procedure is an alternative to the Omotri-Utsu law based on the physics of the static fatigue mechanism. The study of Guten‐ berg-Richter relationship and the modified Bath's law, allowed us to perform the partitioning of the energy released during each aftershock sequence by the main shock and aftershocks.

Correlation integrale. Using non parametric estimation approach, especially the kernel density estimation, we derive the estimation of the density of the probability of the inter-event

This study is a first attemp to perform analysis of aftershock sequneces triggered by main

[1] Akaike, H. (1974). A new look at the statistical model identification, IEEE Trans. Au‐

[2] Aki, K. (1965). Maximum likelihood estimate of b in teh formula Log N = a- bM and

[3] Armorese, D. (2007). Applying a change of point detection method on frequency-

its confidence limits,. Bull. Earthq. Res. Inst. Tokyo Univ. , 43, 237-239.

magnitude distributions.Bull. Seismology Society of America, doi.

distances. The mode of this density highlight how the spatial clusters of the events.

The spatial analysis has been performed using the fractal dimension *D*

and A. Talbi1

1 Seismological Survey Department. C.R.A.A.G. Algiers, Algeria

2 Department of Physics, University of Jaén, Jaén, Spain

**Figure 13.** Kernel density estimated for Zemouri 2003 aftershock sequence (a) using the rule of thumb. (b) using cross validation smoothing and (c) Estimated density obtained with one mode critical bandwidth *hcrit* = 6.6

The estimated density using the cross validation optimal smoothing has been obtained with a bandwidth parameter *h* = 1.0. The density displays an inter-event distances concentration around the value 4.29 km as shown on Fig. 13(b). The estimated density with one mode critical bandwidth *hcrit* = 6.6; displays a concentration of the inter-event distances around 9.0 km.

**Figure 14.** Kernel density estimate for Laalam 2006 aftershock sequence ((a), (b) and (c)). The Graphs on (d) and (e) display the kernel density estimate for El Asnam 1980 aftershock sequence.

#### **7. Conclusion**

Aftershock sequences in Algeria-Morocco region have been analyzed in order to estimate and derive with accuracy the parameters of the most important scaling laws in statistical seismol‐ ogy. For each aftershock sequence the threshold complteness magnitude *mc* has been derived using different approach, we have choose the one giving the "most" stability of the fit of the cumulative number by a straight line. For the magnitude above the threshold magnitude above

the threshold completeness *mc* , we used the maximum likelihood to estimate above the threshold completeness *mc* . The results obtained are close to 1.0, the typical universal value for aftershock sequence. The Omori-Utsu law for aftershock decay and Bath's law for the difference between magnitude and the largest aftershock and mainshock as modified by Shcherbakov et al., (2004a, b)., while using *AIC* as a measure to select the most appropriate model between the first stage and two stage Omori-Utsu model, the rate of decay of aftershocks was found to follow in all case, except the May 21, 2003 Zemouri aftershock sequence, the first stage Omori-Utsu law, which mean the Omori-Utsu model without secondary aftershock. The decay of aftershock trigged by the May 21, 2003 Zemouri earthquake exhibit a better fit with a two stage Omori-Utsu model, denoted model 4 in the text. The later allow us to include in the modelisation the secondary aftershock of magnitude 5.8 *mbLg*. The study of the Omori-Utsu law has been complemented of aftershock sequences. This procedure is an alternative to the Omotri-Utsu law based on the physics of the static fatigue mechanism. The study of Guten‐ berg-Richter relationship and the modified Bath's law, allowed us to perform the partitioning of the energy released during each aftershock sequence by the main shock and aftershocks. The spatial analysis has been performed using the fractal dimension *D* 2 derived using the

Correlation integrale. Using non parametric estimation approach, especially the kernel density estimation, we derive the estimation of the density of the probability of the inter-event distances. The mode of this density highlight how the spatial clusters of the events.

This study is a first attemp to perform analysis of aftershock sequneces triggered by main events in the Algeria-Morocco region.

#### **Author details**

(a) (b) (c)

**Figure 13.** Kernel density estimated for Zemouri 2003 aftershock sequence (a) using the rule of thumb. (b) using cross

The estimated density using the cross validation optimal smoothing has been obtained with a bandwidth parameter *h* = 1.0. The density displays an inter-event distances concentration around the value 4.29 km as shown on Fig. 13(b). The estimated density with one mode critical bandwidth *hcrit* = 6.6; displays a concentration of the inter-event distances around 9.0 km.

)

(d) (e)

**Figure 14.** Kernel density estimate for Laalam 2006 aftershock sequence ((a), (b) and (c)). The Graphs on (d) and (e)

Aftershock sequences in Algeria-Morocco region have been analyzed in order to estimate and derive with accuracy the parameters of the most important scaling laws in statistical seismol‐ ogy. For each aftershock sequence the threshold complteness magnitude *mc* has been derived using different approach, we have choose the one giving the "most" stability of the fit of the cumulative number by a straight line. For the magnitude above the threshold magnitude above

(c)

validation smoothing and (c) Estimated density obtained with one mode critical bandwidth *hcrit* = 6.6

68 Earthquake Research and Analysis - New Advances in Seismology

(a) (b)

display the kernel density estimate for El Asnam 1980 aftershock sequence.

**7. Conclusion**

M. Hamdache1 , J.A. Pelàez2 and A. Talbi1

1 Seismological Survey Department. C.R.A.A.G. Algiers, Algeria

2 Department of Physics, University of Jaén, Jaén, Spain

#### **References**


[4] Bath, M. (1965). Lateral inhomogeneties in the upper mantle. Tectonophysics, , 2, 483-514.

Chabli. (2009). Are seismological and geological observations of the Al Hoceima (Mo‐

Scaling Properties of Aftershock Sequences in Algeria-Morocco Region

http://dx.doi.org/10.5772/54888

71

[17] Gardner, J. K, & Knopoff, L. (1974). Is the sequence of earthquake in southern Cali‐ fornia, with aftershock removed, Poissonian ? Bull. Seism. Soc. Am., 64 (5),

[18] Goltz, C. (1998). Fractal and chaotic properties of earthquake, in Lecture Notes in

[19] Grassberger, P, & Procaccia, I. (1983). Measuring the strangeness of strange attrac‐

[20] Gutenberg, R, & Richter, C. F. (1944). Frequency of Earthquake in California. Bull.

[21] Guo, Z, & Ogata, Y. (1997). Statistical relation between the parameters of aftershocks in time, space and magnitude. Journal of Geophys. Res. 102(B2)., 2857-2873.

[22] Guttorp, P, & Hopkins, D. (1986). On estimating varying b-values, Bull. Seismol. Soc.

[23] Hamdache, M, & Pelaéz, J. A. and K. Yelles Chaouche. (2004). The Algiers, Algeria earthquake (Mw 6.8) of the 21 May 2003: Preliminary report. Seism. Res. Letters, n3.

[24] Hamdache, M, Pelaéz, J. A, & Talbi, A. and López Casado, C. (2010). A unified cata‐ log of main earthquakes for Northern Algeria from A.D. 856 to 2008. Seism. Res.

[25] Kagan, Y. Y. (2004). Short-term proprieties of earthquake catalogs and models of

[26] Khattri, K. N. (1995). Fractal description of seismicity of India and inferences regard‐

[27] Kisslinger, C, & Jones, L. M. (1991). Proprieties of Aftershocks in Southern California.

[28] Maouche, S, Meghraoui, M, Morhange, C, Belabbes, S, Bouhadad, Y, & Haddoum, H. (2011). Active coastal thrusting and folding, and uplift rate of the Sahel Anticline and

[29] Marcellini, A. (1995). Arrhenius behavior of aftershock sequences. J. Geophys. Res. ,

[30] Marcellini, A. (1997). Physical model of aftershock temporal behavior. Tectonophy‐

[31] Marzocchi, W, & Sandri, L. and new insights on the estimation of the b-value and its

Zemouri earthquake area (Tell Atlas, Algeria). Tectonophysics, 509, , 69-80.

earthquake source. Bull. Seismol. Soc. Am. 94 (4), 1207-1228.

ing earthquake hazard. Curr. Sci., 69. , 361-366.

uncertainty. Annals of Geophysics. n6., 46

J. Geophy. Res. , 103(24), 453-24.

rocco Rif) 2004 earthquake (M=6.3) contradictory. Tectonophysics,

Earth Sciences, Springer, New York, 175 pp.

tors. Physics D, , 9, 189-208.

Am. 76 n°, 3, 889-895.

May/June 2004., 75

Lett. , 81, 732-739.

100, 6463-6468.

sics , 277, 137-146.

Seismol. Soc. Am. 34. , 158-188.

1363-1367.


Chabli. (2009). Are seismological and geological observations of the Al Hoceima (Mo‐ rocco Rif) 2004 earthquake (M=6.3) contradictory. Tectonophysics,

[17] Gardner, J. K, & Knopoff, L. (1974). Is the sequence of earthquake in southern Cali‐ fornia, with aftershock removed, Poissonian ? Bull. Seism. Soc. Am., 64 (5), 1363-1367.

[4] Bath, M. (1965). Lateral inhomogeneties in the upper mantle. Tectonophysics, , 2,

[5] Beauval, C, Hainzl, S, & Scherbaum, F. (2006). The Impact of the spatial uniform dis‐ tribution of seismicity on probabilistic seismic-hazard estimation. Bulletin Seismolo‐

[6] Beldjoudi, H, Guemache, A, Kherroubi, A, Semmane, F, Yelles-chaouche, K. A, Djel‐ lit, H, Amrani, A, & Haned, A. (2009). The Laalam (Bejaia, Nort-East Algeria) Moder‐ ate earthquake (Mw=5.2) on March 20, 2006. Pure and Applied Geoph.

[7] Bender, B. (1983). Maximum-Likelihood estimation of b-values for magnitude group‐

[8] Calvert, A, Gomez, F, Seber, D, Baranzagi, M, Jabour, N, Ibenbrahim, A, & Demnati, A. (1997). An integrated geophysical investigation of recent seismicity in the Al Ho‐

[9] Console, R, Lombardi, A. M, Murru, M, & Rhodes, R. (2003). Bath's law and the self-

[10] Daley and Vere-Jones(2003). An introduction to the theory of point process, 2nd Ed.,

[11] El Alami, S. O, Tadili, B. A, Cherkaoui, T. E, Medina, F, Ramdani, M, Ait-brahim, L, & Hanafi, M. (1998). The Al Hoceima earthquake of May 26, 1994 and its after‐

[12] Eneva, M. (1996). Effect of limited data sets in evaluating the scaling properties of spatially distributed data : an example from minning-induced seismic activity, Geo‐

[13] Enescu, B, & Ito, K. (2002). Spatial analysis of the frequency-magnitude distribution and decay rate of aftershock activity of the 2000 Western Tottori earthquake. Earth

[14] Enescu, B, Mori, J, Masatoshi, M, & Kano, Y. (2009). Omori-Utsu law c-values associ‐ ated with recent moderate earthquakes in Japan, Bull. Seismol. Soc. of Am. 99, 2A, ,

[15] Frohlich, C, & Davis, S. D. (1990). Single-link cluster analysis as a method to evaluate spatial and temporalproperties of earthquake catalogues. Geophy. J. Inter. , 100,

[16] Galindo-zaldivar, J, Chalouan, A, Azzouz, O, San, C, De Galdeano, F, Anahnah, L, Ameza, P, Ruano, A, Pedrera, A, Ruiz-constan, C, Marin-lechado, M, Benmakhlouf, A. C, Lopez-garrido, M, Ahmamou, R, Saji, F. J, & Roldan-garcia, M. Akli., and A.

shocks : a seismotectonic study. Annali di Geofisica, , 41, 519-537.

ceima region of North Morocco. Bulletin Seism. Soc. of Am. 87. , 637-651.

483-514.

gy Society of America. n6 , 96, 2465-2471.

70 Earthquake Research and Analysis - New Advances in Seismology

ed data, Bull. Seismol. Soc. Am. 73, n°, 3, 831-851.

Springer-verlag. New-York. Berlin Heidelberg., 1

similarity. J. Geophys. Res. 108, 2128.

phys., J. Int., , 124, 773-786.

Planets Space , 54, 847-859.

884-891.

19-32.

DOIs00024-009-0462-9.


[32] Mikumo, T, & Miyatake, T. (1979). Earthquake sequences on a frictional fault model with non-uniform strenghs and relaxation times, Geophys. Journal R. Astron. Soc., , 59, 497-522.

[47] Peláez, J. A, Chourak, M, Tadili, B. A, Brahim, L. A, & Hamdache, M. López Casado, C., and Martínez Solares, J.M. (2007). A catalog of main Moroccan earthquakes from

Scaling Properties of Aftershock Sequences in Algeria-Morocco Region

http://dx.doi.org/10.5772/54888

73

[48] Peláez, J.A., M. Hamdache and Sanz De Galdeano. 2012. A spatially smoothed seis‐ micity forecasting model for Mw≥5.0earthquakes in northern Algeria and Morocco.

[49] Reasenberg, P. (1985). Second-order moment of central California seismicity, 1969-82,

[50] Rydelek, P. A, & Sacks, I. S. (1989). Testing the completeness of earthquake cata‐

[51] Sandri, L, & Marzocchi, W. (2005). A technical note on the bias in the estimation of the b-value and its uncertainty through the least squares technique. Annals of Geo‐

[52] Sanz de GaldeanoC., (1990). Geologic evolution of the Betic cordilleras in the West‐

[53] Shcherbakov, R, & Turcotte, D. L. (2004a). A modified form of Bath's law. Bull. Seis‐

[54] Shcherbakov, R, Turcotte, D. L, & Rundle, J. E. (2004b). A generalized Omori´s law for earthquake aftershock decay. Geophy. Res. Lett. 31. L11613, doiGL019808.

[55] Shcherbakov, R, Turcotte, D. L, & Rundle, J. E. (2005). Aftershocks Statistics. Pure

[56] Shcherbakov, R, & Turcotte, D. L. (2006). Scaling properties of the Park-field after‐

[57] Shi, Y, & Bolt, B. (1982). The standard error of the magnitude-frequency b value. Bull.

[58] Silverman, B. W. (1986). Density estimation for statistics and data analysis. Chapman

[59] Spada, M, Weimer, S, & Kissling, E. (2010). Quantifying a potential bias in probabilis‐ tic seismic hazard assessment: Seismotectonic zonation with fractal properties. Bull.

[60] Stich, D, Mancilla, F, Baumont, D, & Morales, J. (2005). Source analysis of the Mw 6.3 2004 Al Hoceima earthquake (Morocco) using regional apparent source time func‐

[61] Tahir, M, Grasso, J. R, & Amorèse, D. (2012). The largest aftershock: How strong, how far away, how delayed?. Geophysical Research letters, L04301, doi:GL050604,

15 World Conf. Earth. Eng. 24 - 28 September 2012, Lisboa, Portugal

logues and the hypothesis of self-similarity, Nature, , 337, 251-253.

ern Mediterranean, Miocene to Present. Tectonophysics , 172, 107-119.

1045 to 2005. Seismological Research Letters , 78, 614-621.

J. Geophys. Res., , 90, 5479-5495.

mol. Soc. Am. , 94, 1968-1975.

and Applied Geophysics , 162, 1051-1076.

Seism. Soc. Am. n°5 , 72, 1677-1687.

Seism. Soc. Am. n 6 , 101, 2694-2711.

tions. J. Geoph. Res. 110 (B06306) doiJB003366.

and Hall, London.

2012., 39

shock sequence. Bull. Seismol, Soc. Am. 94. SS384., 376.

physics.


[47] Peláez, J. A, Chourak, M, Tadili, B. A, Brahim, L. A, & Hamdache, M. López Casado, C., and Martínez Solares, J.M. (2007). A catalog of main Moroccan earthquakes from 1045 to 2005. Seismological Research Letters , 78, 614-621.

[32] Mikumo, T, & Miyatake, T. (1979). Earthquake sequences on a frictional fault model with non-uniform strenghs and relaxation times, Geophys. Journal R. Astron. Soc., ,

[33] Mogi, K. (1962). Study of elastic shocks caused by the fracture of heterogeneous ma‐ terials and its relation to the earthquake phenomena, Bull., Earthquake Res., Inst.,

[34] Molchan, G, & Kronod, T. (2009). The fractal description of seismicity, Geophs. J. Int.

[35] Nerenberg, M. A. H, & Essex, C. (1990). Correlation dimension and systematic geo‐

[36] Nocquet, J. M, & Calais, E. (2003). Crustal velocity field of Western Europe from per‐ manent GPS array solutions, 1996-2001. Geoph. J. International, , 154, 72-88.

[37] Nyffengger, P, & Frolich, C. (1998). Recommandations for determining p values for aftershock sequence and catalogs. Bull. Seismol, Soc. Am. n0 5 , 88, 1144-1154.

[38] Nyffengger, P, & Frolich, C. (2000). Aftershock occurrence rate decay properties for intermediate and deep earthquake sequences. Geoph. Res. Lett. , 27, 1215-1218.

[39] Ogata, Y. (1983). Estimation of the parameters in the modified Omori formula for af‐ tershock frequencies by the maximum likelihood procedure. J. Phys. Earth. , 31,

[40] Ogata, Y. (1992). Detection of precursory relative quiescence before great earthquake

[41] Ogata, Y, & Katsura, K. (1993). Analysis of temporal and spatial heterogeinity of magnitude frequencey distribution inferred from earthquake catalogue. Geophys. J.

[42] Ogata, Y. (1999). Seismicity Analysis through Point-Process Modelling : A Review.

[43] Ogata, Y, Jones, L. M, & Toda, S. (2003). When and where the aftershock activity was depressed : Contrasting decay patterns of the `proximate large earthquake in south‐

[44] Olssen, R. (1999). An estimation of the maximum b values in the Gutenberg-Richter

[45] Omori, F. (1894). On the aftershocks of earthquake. J. Coll. Sci. Imp. Univ. Tokyo, 7. ,

[46] Ouyed, M, Meghraoui, M, Cisternas, A, Deschamp, A, Dorel, A, Frechet, F, Gaulon,

ern California. J. of Geophysics Research, n0 B6 2318, doiJB002009., 108

R, Hatzfeld, D, & Philip, H. (1981). Nature, , 292(5818), 26-31.

through a statistical model. Geophys. Res. , 97(19), 845-19.

Pure and Applied Geophysics, 155. , 471-507.

relation. Geodynamics, , 27, 547-552.

59, 497-522.

115-124.

111-120.

Int. 113, , 727-738.

Univ., Tokyo, , 40, 125-173.

72 Earthquake Research and Analysis - New Advances in Seismology

179. n doij.1365., 3, 1787-1799.

metric effects, Phys. Rev. A. 42, , 7065-7074.


[62] Thatcher, W, & Hanks, T. C. (1973). Source parameters of southern California earth‐ quake. J. Geophys. Res. , 78, 8547-8576.

[77] Wiemer, S, & Baer, M. (2000). Mapping and removing quarry blast events from seis‐

Scaling Properties of Aftershock Sequences in Algeria-Morocco Region

http://dx.doi.org/10.5772/54888

75

[78] Wiemer, S, & Wyss, M. (2000). Minimum magnitude of complete reporting in earth‐ quake catalogs: examples from Alaska, the Western United States, and Japan, Bull.

[79] Woessner, J, & Wiemer, S. (2005). Assessing the Quality of Earthquake Catalogues: Estimating the Magnitude of Completeness and Its Uncertainty, Bull. Seismol. Soc.

micity catalogs, Bull. Seism. Soc. Am., , 90, 525-530.

Seism. Soc. Am. , 90, 859-869.

Am., doi:10.1785/0120040007,, 95, 684-698.


[77] Wiemer, S, & Baer, M. (2000). Mapping and removing quarry blast events from seis‐ micity catalogs, Bull. Seism. Soc. Am., , 90, 525-530.

[62] Thatcher, W, & Hanks, T. C. (1973). Source parameters of southern California earth‐

[63] Tinti, S, & Mulargia, F. (1987). Confidence intervals of b-values for gropuped magni‐

[64] Turcotte, D. (1997). Fractals and chaos in Geology and Geophysics, Cambridge Uni‐

[65] Utsu, T. (1961). A statistical study on the occurrence of aftershocks. Geophysics, 30. ,

[66] Utsu, T. (1965). A method for determining the value of b in a formula log n = a- bM showing the magnitude-frequency relation for earthquakles, Geophys, Bull. Hokkai‐

[67] Utsu, T. (1969). Aftershocks and earhquake statistics (I)- Some Parameters which characterize an Aftershock Sequence and their Interaction. Journal Fac., Sci., Hokai‐

[68] Utsu, T. (1971). Aftershocks and Earthquake Statistics (III). Analyses of teh distribu‐ tion of earthquakes in magnitude, Time and Space with special consideration to clus‐ tering characteristics of earthquake occurrence. Jpurnal of the Faculty of Science,

[69] Utsu, T, Ogata, Y, & Matsu, R. S. ra. (1995). The centenary of the Omori formula for a

[70] Vere-jones, D. (1969). A note on the statistical interpretation of Bath`s law. Bulletin

[71] Vere-jones, D, Murakami, J, & Christophersen, A. (2005). A further note on Bath's law, The 4th International Workshop on Statistical Seismology.Tokyo. Japan.

[72] Vidal, F. (1986). Sismotectónica de la región Béticas-Mar de Alborán. PhD Thesis.

[73] Wiemer, S, & Zuniga, R. F. (1994). Zmap. A Software package to analyse seismicity

[74] Wiemer, S, & Katsumata, K. (1999). Spatial variability of seismicity parameters in af‐

[75] Wiemer, S, & Wyss, M. (2000). Minimum magnitude of completness in earthquake catalogs : Examples from Alaska, the Western United States, and Japan. Bull. Seis‐

[76] Wiemer, S. (2001). A software package to analyze seismicity: ZMAP, Seismol. Res.

quake. J. Geophys. Res. , 78, 8547-8576.

74 Earthquake Research and Analysis - New Advances in Seismology

versity Press, New York, 416 pp

521-605.

do Univ. , 13, 99-103.

tudes. Bull. Seismol. Soc. Am. 77, n°, 6, 2125-2134.

do, Univ., Ser., VII (Geophys.), , 3, 129-195.

Seismol. Soc. of Am. n4, , 69, 1535-1541.

Universidad de Granada. 457 p.

mol. Soc. Am. , 90, 859-869.

Lett., , 72, 374-383.

Hokkaido Univ. Ser. VII, Geophysics, Vol. III, n°5.

decay law of aftershock activity. J. Phys. Earth 43. , 1-33.

(abstract), EOS Trans. AGU 75 (43), Fall Meet. Suppl., 456.

tershock zones. J. Geophys. Res. 104 (B6), 13, 135-151.


**Chapter 4**

**Seismotectonic and the Hipothetical Strike – Slip**

The studied area is comprised of the Central Volcanic Range (CVR) of Costa Rica, the northwest flank of the Talamanca Cordillera, and the space between them, known as the Central Valley of Costa Rica (Figure 1). The Central Valley separates volcanic rocks of the CVR from intrusive rocks of the Talamanca Cordillera. The zone is characterized by low seismicity in the north and high seismicity in the South (Montero, 1979; Montero & Dewey, 1982; Montero and

Astorga et al. (1989, 1991) proposed the existence of a strike-slip fault across Costa Rica extend‐ ing from the Pacific to the Caribbean and passing through the central part of the country. Fan et al. (1993) stated that a diffuse transcurrent fault zone trending northeast-southwest and composed of various subparallel strike-slip faults exists in Central Costa Rica. According to Fan et al. (1993), the fault zone extends from the Pacific coast to the Caribbean across central Costa Rica, and may represent a possible plate boundary for the proposed Panama Block. Jacob et al. (1991), Fisher et al. (1994) and Marshall (2000) asured that the strike-slip tectonic boundary traverses the Central Valley of Costa Rica. The prior proposals were mentioned in many other works [Seyfriedet al.(1991),Fisher yGardner(1991),GüendelyPacheco (1992), Fanet al.(1992), Goes et al. (1993), Lundgren et al. (1993), Marshall et al. (1993), Gardner et al. (1993), Escalante y Astorga (1994), Protti y Schwartz (1994), Montero (1994), Marshall (1994), Montero et al. (1994), Fernández et al. (1994), Barboza et al. (1995), Marshall y Anderson (1995), Marshall et al. (1995), Suárez et al. (1995), Di Marco et al. (1995), Colombo et al. (1997), Güendel y Protti (1998), López (1999), Lundgren et al. (1999), Montero (1999), Yao et al. (1999), Quintero y Güendel (2000), Montero (2001), Trenkamp et al. (2002), Husen et al. (2003), Linkimer (2003), Montero (2003), DeShonetal.(2003),Norabuenaetal.(2004),Pachecoetal.(2006),Marshall etal(2006),Camacho et al. (2010)] what spread the idea of the existence of a tectonic boundary in Central Costa Rica.

> © 2013 Arce; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

© 2013 Arce; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

distribution, and reproduction in any medium, provided the original work is properly cited.

**Tectonic Boundary of Central Costa Rica**

Additional information is available at the end of the chapter

Mario Fernandez Arce

http://dx.doi.org/10.5772/54989

**1. Introduction**

Morales, 1984).

**Chapter 4**

### **Seismotectonic and the Hipothetical Strike – Slip Tectonic Boundary of Central Costa Rica**

Mario Fernandez Arce

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/54989

#### **1. Introduction**

The studied area is comprised of the Central Volcanic Range (CVR) of Costa Rica, the northwest flank of the Talamanca Cordillera, and the space between them, known as the Central Valley of Costa Rica (Figure 1). The Central Valley separates volcanic rocks of the CVR from intrusive rocks of the Talamanca Cordillera. The zone is characterized by low seismicity in the north and high seismicity in the South (Montero, 1979; Montero & Dewey, 1982; Montero and Morales, 1984).

Astorga et al. (1989, 1991) proposed the existence of a strike-slip fault across Costa Rica extend‐ ing from the Pacific to the Caribbean and passing through the central part of the country. Fan et al. (1993) stated that a diffuse transcurrent fault zone trending northeast-southwest and composed of various subparallel strike-slip faults exists in Central Costa Rica. According to Fan et al. (1993), the fault zone extends from the Pacific coast to the Caribbean across central Costa Rica, and may represent a possible plate boundary for the proposed Panama Block. Jacob et al. (1991), Fisher et al. (1994) and Marshall (2000) asured that the strike-slip tectonic boundary traverses the Central Valley of Costa Rica. The prior proposals were mentioned in many other works [Seyfriedet al.(1991),Fisher yGardner(1991),GüendelyPacheco (1992), Fanet al.(1992), Goes et al. (1993), Lundgren et al. (1993), Marshall et al. (1993), Gardner et al. (1993), Escalante y Astorga (1994), Protti y Schwartz (1994), Montero (1994), Marshall (1994), Montero et al. (1994), Fernández et al. (1994), Barboza et al. (1995), Marshall y Anderson (1995), Marshall et al. (1995), Suárez et al. (1995), Di Marco et al. (1995), Colombo et al. (1997), Güendel y Protti (1998), López (1999), Lundgren et al. (1999), Montero (1999), Yao et al. (1999), Quintero y Güendel (2000), Montero (2001), Trenkamp et al. (2002), Husen et al. (2003), Linkimer (2003), Montero (2003), DeShonetal.(2003),Norabuenaetal.(2004),Pachecoetal.(2006),Marshall etal(2006),Camacho et al. (2010)] what spread the idea of the existence of a tectonic boundary in Central Costa Rica.

© 2013 Arce; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Arce; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

component seismometers (black triangles, Figure2) and 9 digital three-component stations (open triangles, Figure 2). The signals from analog stations are telemetered to the University of Costa Rica at San Jose where they are digitized by an A/D converter and recorded on a PC computer running the SEILOG data acquisition program. The station spacing is densest in the

Seismotectonic and the Hipothetical Strike – Slip Tectonic Boundary of Central Costa Rica

http://dx.doi.org/10.5772/54989

79

**Figure 2.** Seismic stations of the Red Sismologica Nacional (RSN: ICE\_UCR) shown with triangles. Black triangles are

Historical data on earthquakes are from Rojas (1993). The recent seismicity includes shallow earthquakes of depth equal to or smaller than 30 km and intermediate/deep earthquakes with depths larger than 30 km. Both data subsets span from 1992 through 2009 and were extracted from databases of 4845 (shallow) and 7756 (intermediate/deep) events. The range of duration

The subset of 865 high-quality shallow events includes 382 located by Fernández (1995) and 82 more by Fernández (2009). They were located with 5 or more stations (7 average) and 2 read‐ ings of S wave. Their average rms residuals and horizontal and vertical errors in location are 0.3 sec, 1.8 and 2.0 km respectively. The average azimuthal gap between stations used in the hypocenter determinations is 149.2° and the average distance to the closest station is 15.3 km.

The subset of intermediate/deep earthquakes includes only those locations showing vertical error smaller than 10 km.The average latitudinal and longitudinal component of the location

analog stations. The digital stations are indicated by open triangles.

magnitudes is 1.8-6.2 and the average is 2.8.

study area and in westhern Costa Rica.

**Figure 1.** The area of interest is indicated by the rectangle and covers part of the Central Volcanic Range (numbers mark key volcanoes), the Central Valley (CV) of Costa Rica and Talamanca Cordillera. The Central Valley is a depression located between the ranges and contains the largest population centers of Costa Rica.

Olderreferenceshave beenusedto supportthehypotheticaltectonic boundary ofCentralCosta Rica [Van Andel et al. (1971), Stoiber y Carr (1973), Burbach et al. (1984), Adamek et al. (1988), Carr y Stoiber (1990) and Mann et al. (1990)] but they are not appropriate to justify the boun‐ dary because they refer to a segmentation in the Cocos Plate not in the Caribbean Plate.

This paper analyses and discusses the seismicity and faulting of Central Costa Rica in search for evidence of the strike-slip fault proposed by Astorga et al (1989, 1991), the subparallel strikeslip fault system reported by Fan et al. (1993) and the plate boundary trace in the Central Valley of Costa Rica suggested by Jacob et al. (1991), Fisher et al. (1994) and Marshall et al. (2000).

#### **2. Data and method**

Available data on faulting, historic earthquakes, instrumentally recorded shocks and source mechanisms are provided in this work. Information on faulting is compiled from Fernández & Montero (2002); and Denyer et al., (2003). The seismic data has come from the data file compiled by the RED SISMOLOGICA NACIONAL (RSN: ICE-UCR) operated by the Univer‐ sity of Costa Rica (UCR) and the Instituto Costarricense de Electricidad (ICE). This seismic network monitors the seismic activity of Costa Rica with 20 analog, short-period verticalcomponent seismometers (black triangles, Figure2) and 9 digital three-component stations (open triangles, Figure 2). The signals from analog stations are telemetered to the University of Costa Rica at San Jose where they are digitized by an A/D converter and recorded on a PC computer running the SEILOG data acquisition program. The station spacing is densest in the study area and in westhern Costa Rica.

**Figure 2.** Seismic stations of the Red Sismologica Nacional (RSN: ICE\_UCR) shown with triangles. Black triangles are analog stations. The digital stations are indicated by open triangles.

Olderreferenceshave beenusedto supportthehypotheticaltectonic boundary ofCentralCosta Rica [Van Andel et al. (1971), Stoiber y Carr (1973), Burbach et al. (1984), Adamek et al. (1988), Carr y Stoiber (1990) and Mann et al. (1990)] but they are not appropriate to justify the boun‐

**Figure 1.** The area of interest is indicated by the rectangle and covers part of the Central Volcanic Range (numbers mark key volcanoes), the Central Valley (CV) of Costa Rica and Talamanca Cordillera. The Central Valley is a depression

This paper analyses and discusses the seismicity and faulting of Central Costa Rica in search for evidence of the strike-slip fault proposed by Astorga et al (1989, 1991), the subparallel strikeslip fault system reported by Fan et al. (1993) and the plate boundary trace in the Central Valley of Costa Rica suggested by Jacob et al. (1991), Fisher et al. (1994) and Marshall et al. (2000).

Available data on faulting, historic earthquakes, instrumentally recorded shocks and source mechanisms are provided in this work. Information on faulting is compiled from Fernández & Montero (2002); and Denyer et al., (2003). The seismic data has come from the data file compiled by the RED SISMOLOGICA NACIONAL (RSN: ICE-UCR) operated by the Univer‐ sity of Costa Rica (UCR) and the Instituto Costarricense de Electricidad (ICE). This seismic network monitors the seismic activity of Costa Rica with 20 analog, short-period vertical-

dary because they refer to a segmentation in the Cocos Plate not in the Caribbean Plate.

located between the ranges and contains the largest population centers of Costa Rica.

78 Earthquake Research and Analysis - New Advances in Seismology

**2. Data and method**

Historical data on earthquakes are from Rojas (1993). The recent seismicity includes shallow earthquakes of depth equal to or smaller than 30 km and intermediate/deep earthquakes with depths larger than 30 km. Both data subsets span from 1992 through 2009 and were extracted from databases of 4845 (shallow) and 7756 (intermediate/deep) events. The range of duration magnitudes is 1.8-6.2 and the average is 2.8.

The subset of 865 high-quality shallow events includes 382 located by Fernández (1995) and 82 more by Fernández (2009). They were located with 5 or more stations (7 average) and 2 read‐ ings of S wave. Their average rms residuals and horizontal and vertical errors in location are 0.3 sec, 1.8 and 2.0 km respectively. The average azimuthal gap between stations used in the hypocenter determinations is 149.2° and the average distance to the closest station is 15.3 km.

The subset of intermediate/deep earthquakes includes only those locations showing vertical error smaller than 10 km.The average latitudinal and longitudinal component of the location

The Talamanca Cordillera is a Miocene plutonic-hypabissal volcanic complex that extends by 180 km from central Costa Rica to western Panama. Major Tertiary volcanic complexes are present in this range but large and young strato-volcanic complexes are absent, a consequence of the significant elevation of the range (de Boer et al., 1995) and the shallow, high-angle subduction in southern Costa Rica [60° according to Arroyo (2001)]. This range is the highest topographic feature of Central America and, therefore, of the Caribbean plate. This elevation

Seismotectonic and the Hipothetical Strike – Slip Tectonic Boundary of Central Costa Rica

http://dx.doi.org/10.5772/54989

81

The Central Volcanic Range is a chain of andesitic stratovolcanoes trending northwest, parallel to the MAT. The CVR Consists of five massifs-Platanar, Poas, Barva, Irazú, Turrialba--and several pyroclastic cones associated to the main volcanoes. This cordillera covers an area of

activity at the present-day edifices commenced in the Late Cenozoic and has continued throughout the range until the present. The current activity consists of fumarolic emissions

The Central Valley is a narrow trough (15 km wide, 70 km long) between the Central Volcanic and Talamanca ranges. Late Tertiary and Quaternary volcanic rocks, believed to be part of the current volcanic edifices forming the Central Volcanic Cordillera, are present in this valley as

Previous works, field investigations and assessments of neotectonic features via airphotos indicate that deformation of central Costa Rica occurs in three geographical areas: the Central VolcanicRange,theCentralValleyandthenorthernflankoftheTalamancaCordillera(Figure4).

The Central Volcanic Range faulting is divided into three sub-zones: Irazu Volcano, Bajo de la Hondura, and Poas Volcano. Irazu is a zone of northwest-trending, short length (< 20 Km), normal faults and some northeast faults whose displacement is also normal (Figure 4). Within the Bajo de la Hondura zone, in the low between Irazu and Barba volcanoes, are the southnorth trending Hondura and Patria normal faults and the strike-slip Lara fault. At Poas, in the northwest extreme of the Central Volcanic Range, the southeast- northwest-striking Viejo,

Over decades Costa Rican geologists have considered faulting absent in the Central Valley of Costa Rica. Geologic maps show several faults in the borders of the valley but only few within it (MIEM, 1982, MINAE, 1997, Tournon & Alvarado, 1997, Denyer et al., 1993). Such faults probably exist but are difficult to recognize because of the volcanic and concrete surface cover. Among the better known faults of this area are the Alajuela and Escazu. Alajuela is a 28-km long east-west reverse fault and Escazú seems to have reverse and strike-slip component (Fernández and Montero, 2002). In the last decade additional high-quality seismic data have begun to illuminate important structures within the valley. Fernandez and Montero (2002) mapped three more faults in the valley (Cipreses and Río Azul). An interesting finding is the

and hot intra-crater lakes. Barva and Platanar are dorman volcanoes of this range.

well as some Miocene sedimentary sequences.

Carbonera and Angel faults border the volcano.

and its maximum topographic feature is Irazu volcano (elevation 3400 m). The volcanic

is possibly related to the subduction of Cocos Ridge (Kolarsky et al., 1995).

5150 m2

**4. Faulting**

**Figure 3.** Tectonic Setting. Costa Rica is located on the western extreme of the Caribbean Plate. The border between this plate and the Cocos plate is the Middle American Trench (MAT) located off the Costa Rican Pacific coast. Other tectonic boundaries are the Polochic-Motagua-Chamalecon Fault System (PMCHFS), the Panama Fracture Zone (PFZ) and the North Panama Deformed Belt (NPDB). From Fernandez et al. (2004)

errors for this kind of events are 6.35 and 6.2 km. Their average rms residual is 0.4 sec and the average distance to the closest station is 30.6 km.

Earthquakes were located using P and S wave arrival times and the SEISAN program (Havskov and Ottemøller, 1999) which includes a version of the Hypocenter. A 1-D seismic velocity structure, determined by seismic refraction in northern Costa Rica (Matumoto et al., 1977), is used by the RSN to locate earthquakes in Costa Rica. Fernández (1995) located earthquakes of Central Costa Rica with the 3-D velocity structure of Protti (1994). Fernandez (1995) and Protti et al.(1996) found no significant differencies between earthquake locations obtained with both the 1-D and the 3-D models.

Focal mechanisms for major events in the area were determined by using the first motion of P-waves. The P-wave first motion data were plotted on an equal area projection of the lower hemisphere. The search of fault planes was restricted to events with at least 9 reported first motions. These inversions were performed with the FOCMEC program (Snoke et al., 1984).

#### **3. Tectonic setting and geology**

Central America is an active island arc built up by the northeast subduction of Cocos litho‐ sphere beneath Caribbean plate. The junction of these plates forms the Middle American Trench (MAT), the western boundary of the Caribbean plate (Figure 3). The present conver‐ gence rate increases along the trench from about 7.3 cm/yr off Mexico and Guatemala to 8.5 cm/yr in western Costa Rica (DeMets 2001). Seismicity suggests that the northeast dipping slab has descended to a maximum depth of 200 km in western Costa Rica (Protti et al., 1994) and to only 70 km off southern Costa Rica. (Arroyo, 2001). The subduction became shallower at the southern terminus of MAT in response to a buoyant submarine ridge (Cocos Ridge) that arrived to the trench ~5 Ma (de Boer et al., 1995), causing a decrease in the volcanic activity. The subduction of the Cocos ridge, which rises almost 2 km above the surrounding seafloor, generates high uplift and significant deformation of the whole arc in front of the present subducting ridge. A major geologic effect produced by the subduction of Cocos plate in southern Costa Rica is the uplift of the Talamanca Cordillera.

The Talamanca Cordillera is a Miocene plutonic-hypabissal volcanic complex that extends by 180 km from central Costa Rica to western Panama. Major Tertiary volcanic complexes are present in this range but large and young strato-volcanic complexes are absent, a consequence of the significant elevation of the range (de Boer et al., 1995) and the shallow, high-angle subduction in southern Costa Rica [60° according to Arroyo (2001)]. This range is the highest topographic feature of Central America and, therefore, of the Caribbean plate. This elevation is possibly related to the subduction of Cocos Ridge (Kolarsky et al., 1995).

The Central Volcanic Range is a chain of andesitic stratovolcanoes trending northwest, parallel to the MAT. The CVR Consists of five massifs-Platanar, Poas, Barva, Irazú, Turrialba--and several pyroclastic cones associated to the main volcanoes. This cordillera covers an area of 5150 m2 and its maximum topographic feature is Irazu volcano (elevation 3400 m). The volcanic activity at the present-day edifices commenced in the Late Cenozoic and has continued throughout the range until the present. The current activity consists of fumarolic emissions and hot intra-crater lakes. Barva and Platanar are dorman volcanoes of this range.

The Central Valley is a narrow trough (15 km wide, 70 km long) between the Central Volcanic and Talamanca ranges. Late Tertiary and Quaternary volcanic rocks, believed to be part of the current volcanic edifices forming the Central Volcanic Cordillera, are present in this valley as well as some Miocene sedimentary sequences.

#### **4. Faulting**

errors for this kind of events are 6.35 and 6.2 km. Their average rms residual is 0.4 sec and the

**Figure 3.** Tectonic Setting. Costa Rica is located on the western extreme of the Caribbean Plate. The border between this plate and the Cocos plate is the Middle American Trench (MAT) located off the Costa Rican Pacific coast. Other tectonic boundaries are the Polochic-Motagua-Chamalecon Fault System (PMCHFS), the Panama Fracture Zone (PFZ)

Earthquakes were located using P and S wave arrival times and the SEISAN program (Havskov and Ottemøller, 1999) which includes a version of the Hypocenter. A 1-D seismic velocity structure, determined by seismic refraction in northern Costa Rica (Matumoto et al., 1977), is used by the RSN to locate earthquakes in Costa Rica. Fernández (1995) located earthquakes of Central Costa Rica with the 3-D velocity structure of Protti (1994). Fernandez (1995) and Protti et al.(1996) found no significant differencies between earthquake locations obtained with both

Focal mechanisms for major events in the area were determined by using the first motion of P-waves. The P-wave first motion data were plotted on an equal area projection of the lower hemisphere. The search of fault planes was restricted to events with at least 9 reported first motions. These inversions were performed with the FOCMEC program (Snoke et al., 1984).

Central America is an active island arc built up by the northeast subduction of Cocos litho‐ sphere beneath Caribbean plate. The junction of these plates forms the Middle American Trench (MAT), the western boundary of the Caribbean plate (Figure 3). The present conver‐ gence rate increases along the trench from about 7.3 cm/yr off Mexico and Guatemala to 8.5 cm/yr in western Costa Rica (DeMets 2001). Seismicity suggests that the northeast dipping slab has descended to a maximum depth of 200 km in western Costa Rica (Protti et al., 1994) and to only 70 km off southern Costa Rica. (Arroyo, 2001). The subduction became shallower at the southern terminus of MAT in response to a buoyant submarine ridge (Cocos Ridge) that arrived to the trench ~5 Ma (de Boer et al., 1995), causing a decrease in the volcanic activity. The subduction of the Cocos ridge, which rises almost 2 km above the surrounding seafloor, generates high uplift and significant deformation of the whole arc in front of the present subducting ridge. A major geologic effect produced by the subduction of Cocos plate in

average distance to the closest station is 30.6 km.

80 Earthquake Research and Analysis - New Advances in Seismology

and the North Panama Deformed Belt (NPDB). From Fernandez et al. (2004)

the 1-D and the 3-D models.

**3. Tectonic setting and geology**

southern Costa Rica is the uplift of the Talamanca Cordillera.

Previous works, field investigations and assessments of neotectonic features via airphotos indicate that deformation of central Costa Rica occurs in three geographical areas: the Central VolcanicRange,theCentralValleyandthenorthernflankoftheTalamancaCordillera(Figure4).

The Central Volcanic Range faulting is divided into three sub-zones: Irazu Volcano, Bajo de la Hondura, and Poas Volcano. Irazu is a zone of northwest-trending, short length (< 20 Km), normal faults and some northeast faults whose displacement is also normal (Figure 4). Within the Bajo de la Hondura zone, in the low between Irazu and Barba volcanoes, are the southnorth trending Hondura and Patria normal faults and the strike-slip Lara fault. At Poas, in the northwest extreme of the Central Volcanic Range, the southeast- northwest-striking Viejo, Carbonera and Angel faults border the volcano.

Over decades Costa Rican geologists have considered faulting absent in the Central Valley of Costa Rica. Geologic maps show several faults in the borders of the valley but only few within it (MIEM, 1982, MINAE, 1997, Tournon & Alvarado, 1997, Denyer et al., 1993). Such faults probably exist but are difficult to recognize because of the volcanic and concrete surface cover. Among the better known faults of this area are the Alajuela and Escazu. Alajuela is a 28-km long east-west reverse fault and Escazú seems to have reverse and strike-slip component (Fernández and Montero, 2002). In the last decade additional high-quality seismic data have begun to illuminate important structures within the valley. Fernandez and Montero (2002) mapped three more faults in the valley (Cipreses and Río Azul). An interesting finding is the

small strike-slip and reverse faults, along fault line locations mapped during field studies. Results suggest that faulting has occurred during the Holocene, but movement is likely disseminated over a broad zone (100 m) instead of being concentrated along any single fault plane. At Aguacaliente, one trench intersected a trace that offset the soil horizon by approx‐ imately 30-35 cm (Woodward-Clyde, 1993). The apparent displacement was normal and a dated carbonizad log suggested that the last movement on this fault occurred less than 3700 years ago. On a trench across the Orosi fault in Cartago, Costa Rica, the most significant finding was a set of fractures cutting all the soil units and suggesting normal dip slip, down to the east. The fractures coincide with the steepened facet of the break in slope on the

Seismotectonic and the Hipothetical Strike – Slip Tectonic Boundary of Central Costa Rica

http://dx.doi.org/10.5772/54989

83

The NW-striking Frailes-Belohorizonte-Escazu fault zone extends 30 km. The fault zone is marked by scarps, slope changes, and offsets of aligned stream channels and divides. Accord‐ ing to Fernandez and Montero (2002) this fault system combines dextral and uplift movement

The Guapiles-Siquirres fault runs along the base of the Central Volcanic Range, and therefore, marks the boundary between that range and the Caribbean plain. It is a combination of two continuous reverse faults, Guapiles in the North and Siquirres-Matina in the South (Denyer et al., 2003). Soulas (1989) proposed that the Siquirres-Matina fault is the prolongation of the North Panama Deformed Belt within the territory of Costa Rica. The Guapiles-Siquirres fault is characterized by high topographic relief with uplifted terraces and deep-narrow river valleys over much of its length (Soulas, 1989). Linkimer (2003) extends this large fault to Aguas Zarcas

Neither the strike-slip fault proposed by Astorga et al. (1989) nor the set of subparalel strikeslip faults suggested by Fan et al. (1993) were found in the studied area. The trace of the strike-slip tectonic boundary suggested by Jacob et al. (1991), Fisher et al. (1994) and Marshall et al. (2000) neither was found within the Central Valley of Costa Rica. The most impor‐ tant east-west faults, the faults required by the hypothetical strike-slip tectonic boundary, of the Central Valley are Aguacaliente and Alajuela. The first one shows a component of normal slip and the second is a tipical reverse fault that connects with the Garita fault whose slip is

Well-documented historical earthquakes data from 1700 to 2006 have been analyzed in this work to understand the seismicity of central Costa Rica. Our catalog contains 15 events (Table 1), 7 of which occurred in the Poas Volcano seismic zone, one near Irazu volcano, one west of the city of Heredia and 6 south of the Central Valley. Figure 5 shows a well-defined cluster at the western end of the Central Volcanic Range (Poás volcano area) and another at the northern

colluvial fan (Woodward-Clyde, 1993).

and consists of discontinuous fault traces.

normal.

**5. Seismicity**

**5.1. Historical seismicity**

de San Carlos (not shown) for a total distance of 150 km.

flank of the Talamanca Range (south of the Central Valley).

**Figure 4.** Faults mapped in Central Costa Rica. Triangles mark volcanoes; squares show cities or towns. The aligned volcanoes mark the longitudinal axis of the Central Volcanic Range. The cities of Cartago, San José, Heredia and Ala‐ juela are located in the Central Valley of Costa Rica. The Escazú and Aguacaliente (AF) faults define the southern boundary of the Central Valley. Faults located southeast of the Navarro fault belong to the Talamanca Cordillera. PF: Picagres Fault, BF: Belohorizonte Fault, RAF: Río Azul Fault, SIF: San Ignacio Fault, RBF: Resbalon Fault, LMF: La Mesa Fault, CIF: Cipreses Fault, NUF: Nubes Fault, CAF: Cangreja Fault, SMF: Simari Fault, ATF: Atirro Fault, PCF: Pacuare Fault, LL: La Lucha, SP: Santiago de Puriscal, U/D: normal faults howing relative motion: U, upthrown block; D, down‐ thrown block. Sawteeth along solid lines indicate thrust fault. Strike-slip arrows represent strike-slip faults.

extension of the Aguacaliente and probably Rio Azul faults under the surface of San Jose, the capital of Costa Rica, which represent a significant hazard for that city.

At the Talamanca Cordillera faults trend predominantly northwest with varying fault lengths and slip directions. The most important faults in this area are Atirro, Navarro, Aguaca‐ liente, Frailes-Escazu, and Jaris. In the east, the dextral Atirro Fault is the major structure, and it splits into two branches (the Tucurrique and Turrialba faults). At the northern rim of the range, the Aguacaliente Fault marks the boundary between the range and the Central Valley. Trench excavations across the Navarro, Aguacaliente, and Orosi faults have been conducted in order to date the most recent ruptures and to identify periods of dormancy (Woodward-Clyde, 1993). Soil development along faulted surfaces and scarp morphometry was used to determine the relative deformation rates across the segments. At the Navarro fault, the trench shows evidence of faulting within the unconsolidated sediment section, where sediment deformation features are present. These features include lineaments such as small strike-slip and reverse faults, along fault line locations mapped during field studies. Results suggest that faulting has occurred during the Holocene, but movement is likely disseminated over a broad zone (100 m) instead of being concentrated along any single fault plane. At Aguacaliente, one trench intersected a trace that offset the soil horizon by approx‐ imately 30-35 cm (Woodward-Clyde, 1993). The apparent displacement was normal and a dated carbonizad log suggested that the last movement on this fault occurred less than 3700 years ago. On a trench across the Orosi fault in Cartago, Costa Rica, the most significant finding was a set of fractures cutting all the soil units and suggesting normal dip slip, down to the east. The fractures coincide with the steepened facet of the break in slope on the colluvial fan (Woodward-Clyde, 1993).

The NW-striking Frailes-Belohorizonte-Escazu fault zone extends 30 km. The fault zone is marked by scarps, slope changes, and offsets of aligned stream channels and divides. Accord‐ ing to Fernandez and Montero (2002) this fault system combines dextral and uplift movement and consists of discontinuous fault traces.

The Guapiles-Siquirres fault runs along the base of the Central Volcanic Range, and therefore, marks the boundary between that range and the Caribbean plain. It is a combination of two continuous reverse faults, Guapiles in the North and Siquirres-Matina in the South (Denyer et al., 2003). Soulas (1989) proposed that the Siquirres-Matina fault is the prolongation of the North Panama Deformed Belt within the territory of Costa Rica. The Guapiles-Siquirres fault is characterized by high topographic relief with uplifted terraces and deep-narrow river valleys over much of its length (Soulas, 1989). Linkimer (2003) extends this large fault to Aguas Zarcas de San Carlos (not shown) for a total distance of 150 km.

Neither the strike-slip fault proposed by Astorga et al. (1989) nor the set of subparalel strikeslip faults suggested by Fan et al. (1993) were found in the studied area. The trace of the strike-slip tectonic boundary suggested by Jacob et al. (1991), Fisher et al. (1994) and Marshall et al. (2000) neither was found within the Central Valley of Costa Rica. The most impor‐ tant east-west faults, the faults required by the hypothetical strike-slip tectonic boundary, of the Central Valley are Aguacaliente and Alajuela. The first one shows a component of normal slip and the second is a tipical reverse fault that connects with the Garita fault whose slip is normal.

#### **5. Seismicity**

extension of the Aguacaliente and probably Rio Azul faults under the surface of San Jose, the

thrown block. Sawteeth along solid lines indicate thrust fault. Strike-slip arrows represent strike-slip faults.

**Figure 4.** Faults mapped in Central Costa Rica. Triangles mark volcanoes; squares show cities or towns. The aligned volcanoes mark the longitudinal axis of the Central Volcanic Range. The cities of Cartago, San José, Heredia and Ala‐ juela are located in the Central Valley of Costa Rica. The Escazú and Aguacaliente (AF) faults define the southern boundary of the Central Valley. Faults located southeast of the Navarro fault belong to the Talamanca Cordillera. PF: Picagres Fault, BF: Belohorizonte Fault, RAF: Río Azul Fault, SIF: San Ignacio Fault, RBF: Resbalon Fault, LMF: La Mesa Fault, CIF: Cipreses Fault, NUF: Nubes Fault, CAF: Cangreja Fault, SMF: Simari Fault, ATF: Atirro Fault, PCF: Pacuare Fault, LL: La Lucha, SP: Santiago de Puriscal, U/D: normal faults howing relative motion: U, upthrown block; D, down‐

At the Talamanca Cordillera faults trend predominantly northwest with varying fault lengths and slip directions. The most important faults in this area are Atirro, Navarro, Aguaca‐ liente, Frailes-Escazu, and Jaris. In the east, the dextral Atirro Fault is the major structure, and it splits into two branches (the Tucurrique and Turrialba faults). At the northern rim of the range, the Aguacaliente Fault marks the boundary between the range and the Central Valley. Trench excavations across the Navarro, Aguacaliente, and Orosi faults have been conducted in order to date the most recent ruptures and to identify periods of dormancy (Woodward-Clyde, 1993). Soil development along faulted surfaces and scarp morphometry was used to determine the relative deformation rates across the segments. At the Navarro fault, the trench shows evidence of faulting within the unconsolidated sediment section, where sediment deformation features are present. These features include lineaments such as

capital of Costa Rica, which represent a significant hazard for that city.

82 Earthquake Research and Analysis - New Advances in Seismology

#### **5.1. Historical seismicity**

Well-documented historical earthquakes data from 1700 to 2006 have been analyzed in this work to understand the seismicity of central Costa Rica. Our catalog contains 15 events (Table 1), 7 of which occurred in the Poas Volcano seismic zone, one near Irazu volcano, one west of the city of Heredia and 6 south of the Central Valley. Figure 5 shows a well-defined cluster at the western end of the Central Volcanic Range (Poás volcano area) and another at the northern flank of the Talamanca Range (south of the Central Valley).


**Table 1.** Historical earthquakes in Central Costa Rica (Rojas, 1993)

The historical seismic data correlate well with previously identified faulting. For instance, at the Poas seismic zone 5 earthquakes are located along the northwest-trending faults that border the volcano from south to west (Figure 5). It is quite probable that the Carbonera and Viejo faults were responsible for the Bajos del Toro (1911, 1955) and Sarchi (1912) earthquakes. The damage zones described for the Fraijanes earthquakes (6 and 7 on Figure 5) suggest that the source could be the Angel fault. To the southeast, the epicenters of historical earthquakes are located on the periphery of the Talamanca Cordillera, where most form an alignment along the Aguacaliente fault (the Cartago earthquakes of 1834, 1841 and 1910 and the Tres Rios earthquake of 1912). The 6.4 Ms Cartago earthquake (1910) and the 5.2 Ms Tres Rios earthquake (1912) appear to be in the same seismogenic context; the 1910 event possibly strained the northwest segment of the Aguacaliente fault and, two years later the accumulated strain was released originating the Tres Rios earthquake. A similar situation could have happened at Poas when Sarchi earthquake followed the 1911 Bajos del Toro earthquake.

Additional strong evidence for the correlation between historical earthquakes and faulting comes from isoseismal maps. Montero & Morales (1988) found elongated intensity contours that clearly surround the known source of these events. For the Cartago, Tres Rios and Fraijanes earthquakes, the contoured intensity distributions relate the earthquakes to northwesttrending faults, suggesting that the Angel and Aguacaliente faults participated in the gener‐ ation of those events. Bajos del Toro, Sarchi and Patillos events have northeast-trending damage areas that disagree with the fault orientation; in these cases the lack of reports

**Figure 5.** Map showing the historical earthquakes in Central Costa Rica. Stars mark the epicenters of historical earth‐ quakes of the last two centuries. The number near each star is the number of the event in Table 1.Earthquakes 4 and 9 and 6 and 7 share the same epicentral area, respectively. Triangles indicate volcanoes. AF: Aguacaliente Fault, PF: Pica‐ gres Fault, BF: Belohorizonte Fault, RAF: Río Azul Fault, SIF: San Ignacio Fault, RBF: Resbalon Fault, LMF: La Mesa Fault, CIF: Cipreses Fault, NUF: Nubes Fault, CAF: Cangreja Fault, SMF: Simari Fault, ATF: Atirro Fault, PCF: Pacuare Fault, LL:

Seismotectonic and the Hipothetical Strike – Slip Tectonic Boundary of Central Costa Rica

http://dx.doi.org/10.5772/54989

85

This historical seismicity is considered upper-crustal seismicity by White (1991) and White & Harlow (1993). The later authors pointed out that upper-crustal earthquakes are spatially distributed along the volcanic front of the whole of Central America; they appropriately called them volcanic-front earthquakes and stated that these earthquakes pose the greatest hazards

northward the source could affect the geometry of the isoseismal map.

for the population.

La Lucha, SP: Santiago de Puriscal.

Seismotectonic and the Hipothetical Strike – Slip Tectonic Boundary of Central Costa Rica http://dx.doi.org/10.5772/54989 85

**Figure 5.** Map showing the historical earthquakes in Central Costa Rica. Stars mark the epicenters of historical earth‐ quakes of the last two centuries. The number near each star is the number of the event in Table 1.Earthquakes 4 and 9 and 6 and 7 share the same epicentral area, respectively. Triangles indicate volcanoes. AF: Aguacaliente Fault, PF: Pica‐ gres Fault, BF: Belohorizonte Fault, RAF: Río Azul Fault, SIF: San Ignacio Fault, RBF: Resbalon Fault, LMF: La Mesa Fault, CIF: Cipreses Fault, NUF: Nubes Fault, CAF: Cangreja Fault, SMF: Simari Fault, ATF: Atirro Fault, PCF: Pacuare Fault, LL: La Lucha, SP: Santiago de Puriscal.

Additional strong evidence for the correlation between historical earthquakes and faulting comes from isoseismal maps. Montero & Morales (1988) found elongated intensity contours that clearly surround the known source of these events. For the Cartago, Tres Rios and Fraijanes earthquakes, the contoured intensity distributions relate the earthquakes to northwesttrending faults, suggesting that the Angel and Aguacaliente faults participated in the gener‐ ation of those events. Bajos del Toro, Sarchi and Patillos events have northeast-trending damage areas that disagree with the fault orientation; in these cases the lack of reports northward the source could affect the geometry of the isoseismal map.

The historical seismic data correlate well with previously identified faulting. For instance, at the Poas seismic zone 5 earthquakes are located along the northwest-trending faults that border the volcano from south to west (Figure 5). It is quite probable that the Carbonera and Viejo faults were responsible for the Bajos del Toro (1911, 1955) and Sarchi (1912) earthquakes. The damage zones described for the Fraijanes earthquakes (6 and 7 on Figure 5) suggest that the source could be the Angel fault. To the southeast, the epicenters of historical earthquakes are located on the periphery of the Talamanca Cordillera, where most form an alignment along the Aguacaliente fault (the Cartago earthquakes of 1834, 1841 and 1910 and the Tres Rios earthquake of 1912). The 6.4 Ms Cartago earthquake (1910) and the 5.2 Ms Tres Rios earthquake (1912) appear to be in the same seismogenic context; the 1910 event possibly strained the northwest segment of the Aguacaliente fault and, two years later the accumulated strain was released originating the Tres Rios earthquake. A similar situation could have happened at Poas

**No. Name Latitude Longitude Year Magnitude Seismic Zone**

South of Central

South of Central

South of Central

South of Central

South of Central

South of Central

South of Central

Valley

Valley

Valley

Valley

Valley

Valley

Valley

1 Barva earthquake 10.1000 -84.2000 1772 5.6 Poás

3 Alajuela earthquake 09.9500 -84.2670 1835 5.8 Puriscal

6 Fraijanes earthquake 10.1380 -84.1840 1851 5.5 Poás 7 Fraijanes earthquake 10.1380 -84.1830 1888 5.8 Poás

10 Toro Amarillo earthquake 10.2333 -84.3000 1911 6.1 Poás 11 Sarchí earthquake 10.1916 -84.2750 1912 6.2 Poás

14 Patillos earthquake 10.0250 -83.9083 1952 5.5 Irazú 15 Toro Amarillo earthquake 10.2333 -84.3166 1955 5.8 Poás

2 Cartago earthquake 09.8250 -83.9300 1834 5.2

84 Earthquake Research and Analysis - New Advances in Seismology

4 Cartago earthquake 09.8416 -83.9100 1841 5.8

5 Alajuelita earthquake 09.8300 -84.1000 1842 5.4

8 Tablazo earthquake 09.8166 -84.0333 1910 5.2

9 Cartago earthquake 09.8416 -83.9100 1910 6.4

12 Tres Ríos earthquake 09.8666 -84.0000 1912 5.2

13 Paraíso earthquake 09.8083 -83.8800 1951 5.2

**Table 1.** Historical earthquakes in Central Costa Rica (Rojas, 1993)

when Sarchi earthquake followed the 1911 Bajos del Toro earthquake.

This historical seismicity is considered upper-crustal seismicity by White (1991) and White & Harlow (1993). The later authors pointed out that upper-crustal earthquakes are spatially distributed along the volcanic front of the whole of Central America; they appropriately called them volcanic-front earthquakes and stated that these earthquakes pose the greatest hazards for the population.

A final remark about this seismicty deals with its connection with large Costa Rican earth‐ quakes. Upper-crustal destructive earthquakes of central Costa Rica in the last one hundred years coincided with large earthquakes that took place in the country. In 1904, a 7.2 Ms magnitude subduction earthquake happened in southern Costa Rica and also 6.8 Ms event southwest of the Central Valley, and five years later the Cartago (1910), Tablazo (1910), Bajos del Toro (1911) and Sarchi (1912) earthquakes occurred in Central Costa Rica. Similarly, in 1950 the largest earthquake reported in Costa Rica occurred, a 7.7 Ms magnitude subduction event that was followed by the Paraiso (1951), Patillos (1952) and Bajos del Toro (1955) earthquakes. These data suggest that destructive events of central Costa Rica may represent seismicity triggered by large subduction events.

All of this evidence suggests that historical earthquakes did not occur randomly, and more‐ over, they did not form any lineament in an east-west direction that supports the existence of a tectonic boundary with that orientation in central Costa Rica. Those events are clearly associated with faults that have been recently mapped.

#### **5.2. Instrumental seismicity**

The epicentral distribution of 865 shallow earthquakes (0-30 km) recorded by RSN during the period 1992-2009 is plotted in Figure 6. This shallow seismicity is not uniformly distributed over the study area, that is, there are seismic clusters separated by zones of low level seismicity. On a rough scale, the seismicity of Talamanca is higher than the seismicity of the Central Volcanic Range. In the Central Valley the seismicity has the lowest rate for the whole area.

The volcán Irazu is a zone of seismic swarms that resemble volcano/tectonic. According to Fernández et al. (1998) there have been seismic swarms at Irazu in 1982, 1991, and 2007. The pattern of these swarms is a large number of very small earthquakes with few moderate events of magnitude 4 or so, but no clear mainshock larger than the other events. They have occurred on short fault of the zone, especificaly on Elia, Ariete and Nubes.

At the Bajo de la Hondura, a trough between the Irazu and Barva volcanoes, scarce but permanent seismicity has been recognized. It is a seismicity of magnitude smaller than 5. One of the recent major events was the magnitude 4.4 earthquake that occurred there on August 21,1990, at 13 km depth. The main sources of this activity are the Hondura, Patria and Lara faults.

2009). The event was located in the eastern flank of the volcano at 4 km deep and was generated

**Figure 6.** Shallow (0-30 km) seismicity of Central Costa Rica from 1992 through 2009. Several clusters represent the most important seismic zones in the studied area. Crosses are seismic event for the 1995-2009 period. Diamonds are earthquakes located by Fernandez (1995) and black circles represent earthquakes located by Fernandez (2009). Lines A-B and C-D indicate traces of cross sections. AF: Aguacaliente Fault, PF: Picagres Fault, BF: Belohorizonte Fault, RAF: Río Azul Fault, SIF: San Ignacio Fault, RBF: Resbalon Fault, LMF: La Mesa Fault, CIF: Cipreses Fault, NUF: Nubes Fault, CAF: Cangreja Fault, SMF: Simari Fault, ATF: Atirro Fault, PCF: Pacuare Fault, LL: La Lucha, SP: Santiago de Puriscal.

Seismotectonic and the Hipothetical Strike – Slip Tectonic Boundary of Central Costa Rica

http://dx.doi.org/10.5772/54989

87

In the Talamanca Cordillera the seismicity is spread all over the area but there are also dense clusters at Pejibaye, south of Cartago and Santiago de Puriscal (Figure 6). Two of these clusters correspond with isolated seismic sequences (Pejibaye and Puriscal) and the other one with a

The Pejibaye July 10 1993 (Mc = 5.3) earthquake, together with the Mc = 4.9 July 8 foreshock two days before and the Mc = 4.8 aftershock three days later represent the most extensive and wellrecorded seismic sequence in the eastern part of central Costa Rica (Fernandez, 2009). These earthquakes and many aftershocks occurred within a small area of northwest and northeasttrendingfaults.The event'sdepths are relativelyshallowandcanbe associatedwithSimarifault

which, according to focal mechanisms, is strike/slip with a high normal component.

by the Angel fault.

zone of permanent seismicity (La Lucha).

The seismic activity at Poas is mainly composedd of swarms and sporadic strong earthquakes. The swarm activity consists, like the Irazu activity, of a hundred of small earthquakes gener‐ ated during one or two months. Fernandez et al. (in prep.) have recognized seismic swarms at Poas in 1980, 1990 and 1999 According to their location, the last swarms at this area was generated by Carbonera and Angel faults. A strong 6.2 Mw magnitude earthquake hit the zone on January 8, 2009 killing 25 people and destroying many houses, several bridges and the route to Cinchona. In adition, the earthquake triggered many landslides in the epicentral area. As a consequence of such earthquake the village of Cinchona (Figure 7) had to be reubicated. The economic losses from the destructive earthquake are estimated in \$492 million (Laurent, Seismotectonic and the Hipothetical Strike – Slip Tectonic Boundary of Central Costa Rica http://dx.doi.org/10.5772/54989 87

A final remark about this seismicty deals with its connection with large Costa Rican earth‐ quakes. Upper-crustal destructive earthquakes of central Costa Rica in the last one hundred years coincided with large earthquakes that took place in the country. In 1904, a 7.2 Ms magnitude subduction earthquake happened in southern Costa Rica and also 6.8 Ms event southwest of the Central Valley, and five years later the Cartago (1910), Tablazo (1910), Bajos del Toro (1911) and Sarchi (1912) earthquakes occurred in Central Costa Rica. Similarly, in 1950 the largest earthquake reported in Costa Rica occurred, a 7.7 Ms magnitude subduction event that was followed by the Paraiso (1951), Patillos (1952) and Bajos del Toro (1955) earthquakes. These data suggest that destructive events of central Costa Rica may represent seismicity

All of this evidence suggests that historical earthquakes did not occur randomly, and more‐ over, they did not form any lineament in an east-west direction that supports the existence of a tectonic boundary with that orientation in central Costa Rica. Those events are clearly

The epicentral distribution of 865 shallow earthquakes (0-30 km) recorded by RSN during the period 1992-2009 is plotted in Figure 6. This shallow seismicity is not uniformly distributed over the study area, that is, there are seismic clusters separated by zones of low level seismicity. On a rough scale, the seismicity of Talamanca is higher than the seismicity of the Central Volcanic Range. In the Central Valley the seismicity has the lowest rate for the whole area.

The volcán Irazu is a zone of seismic swarms that resemble volcano/tectonic. According to Fernández et al. (1998) there have been seismic swarms at Irazu in 1982, 1991, and 2007. The pattern of these swarms is a large number of very small earthquakes with few moderate events of magnitude 4 or so, but no clear mainshock larger than the other events. They have occurred

At the Bajo de la Hondura, a trough between the Irazu and Barva volcanoes, scarce but permanent seismicity has been recognized. It is a seismicity of magnitude smaller than 5. One of the recent major events was the magnitude 4.4 earthquake that occurred there on August 21,1990, at 13 km depth. The main sources of this activity are the Hondura, Patria and Lara

The seismic activity at Poas is mainly composedd of swarms and sporadic strong earthquakes. The swarm activity consists, like the Irazu activity, of a hundred of small earthquakes gener‐ ated during one or two months. Fernandez et al. (in prep.) have recognized seismic swarms at Poas in 1980, 1990 and 1999 According to their location, the last swarms at this area was generated by Carbonera and Angel faults. A strong 6.2 Mw magnitude earthquake hit the zone on January 8, 2009 killing 25 people and destroying many houses, several bridges and the route to Cinchona. In adition, the earthquake triggered many landslides in the epicentral area. As a consequence of such earthquake the village of Cinchona (Figure 7) had to be reubicated. The economic losses from the destructive earthquake are estimated in \$492 million (Laurent,

triggered by large subduction events.

86 Earthquake Research and Analysis - New Advances in Seismology

**5.2. Instrumental seismicity**

faults.

associated with faults that have been recently mapped.

on short fault of the zone, especificaly on Elia, Ariete and Nubes.

**Figure 6.** Shallow (0-30 km) seismicity of Central Costa Rica from 1992 through 2009. Several clusters represent the most important seismic zones in the studied area. Crosses are seismic event for the 1995-2009 period. Diamonds are earthquakes located by Fernandez (1995) and black circles represent earthquakes located by Fernandez (2009). Lines A-B and C-D indicate traces of cross sections. AF: Aguacaliente Fault, PF: Picagres Fault, BF: Belohorizonte Fault, RAF: Río Azul Fault, SIF: San Ignacio Fault, RBF: Resbalon Fault, LMF: La Mesa Fault, CIF: Cipreses Fault, NUF: Nubes Fault, CAF: Cangreja Fault, SMF: Simari Fault, ATF: Atirro Fault, PCF: Pacuare Fault, LL: La Lucha, SP: Santiago de Puriscal.

2009). The event was located in the eastern flank of the volcano at 4 km deep and was generated by the Angel fault.

In the Talamanca Cordillera the seismicity is spread all over the area but there are also dense clusters at Pejibaye, south of Cartago and Santiago de Puriscal (Figure 6). Two of these clusters correspond with isolated seismic sequences (Pejibaye and Puriscal) and the other one with a zone of permanent seismicity (La Lucha).

The Pejibaye July 10 1993 (Mc = 5.3) earthquake, together with the Mc = 4.9 July 8 foreshock two days before and the Mc = 4.8 aftershock three days later represent the most extensive and wellrecorded seismic sequence in the eastern part of central Costa Rica (Fernandez, 2009). These earthquakes and many aftershocks occurred within a small area of northwest and northeasttrendingfaults.The event'sdepths are relativelyshallowandcanbe associatedwithSimarifault which, according to focal mechanisms, is strike/slip with a high normal component.

has been observed in the metropolitan area of San Jose in the last 5 years; it consists of 2 < Mc < 4 earthquakes whose epicenters appear to define a NW-striking lineation that coincides with the northwest end of the Rio Azul and Aguacaliente faults. In the southern border of the central valley there are seismic sources with relatively high rates of seismicity such as the Escazu and Aserri faults, both related to the Frailes-Belohorizonte Escazu fault system. The Aguacaliente fault, responsible for the 1841 and 1910 Cartago earthquakes, has had little activity in the last

Seismotectonic and the Hipothetical Strike – Slip Tectonic Boundary of Central Costa Rica

http://dx.doi.org/10.5772/54989

89

In an effort to see if earthquakes define faults, seismicity cross-sections were carried out in the studied area. Due the low number of earthquakes in some cases and the nearness between faults in other cases only two seismic cross sections were calculated, one at the Pejibaye seismic zone and other eastward of La Lucha. In the cross section A-B (Figure 8a) the hypocenters seem to define a inclined plane that dips 75° northeast, which suggests that a high-angle fault is the responsible for this seismic activity. The cross section C-D (Figure 8b) reveals that the dense seismicity cluster along the Pejibaye seismic zone is generated by an almost vertical fault. Fernandez (2009) reported a fault dipping 76° northwest as the cause of this seismicity.

Recent earthquake epicenters from 1992 through 2009 were plotted on a map of Costa Rica in order to show the characteristic local pattern of seismicity that is possibly associated with tectonic features. The plot displays a wide zone of high subduction and crustal seismic activity in Central Costa Rica which coincides with a diffuse zone rather than with a narrow longitu‐ dinal area (Fernández et al., 2007). The seismicity forms an anomalous big cluster composed of smaller clusters (Figure 9) but despite the considerable concentration of earthquakes, epicenters of either the big or smaller clusters fail to delineate any large and single NE or EW

To know whether or not the seismicity pattern is related to a hypothetical strike-slip tectonic boundary, we examined the depths of the earthquake clusters. We would expect shallow seismogenic source locations for a strike-slip tectonic boundary but deep (greater than 30 km)

three decades.

fault plane.

**Figure 8.** Seismic cross sections A-B and C-D outlined in Figure 6.

**5.3. The seismic anomaly of Central Costa Rica**

**Figure 7.** The village of Cinchona after the 2009 Cinchona Earthquake. The earthquake changed the geography of the area. Courtesy of Joanna Mendez.

Puriscal was a quiet seismic zone before 1990 but in that year there began one of the highest concentrations of seismic activity of Costa Rica in recent decades. This activity was triggered by a large earthquake from the Pacific Coast. Thousands of micro earthquakes were generated in Puriscal in the December 1990-June 1991 period, almost 30 events of Mc > 4.0 and the main event of Mc = 5.7, the Piedras Negras earthquake.

La Lucha is the most seismically active zone in central Costa Rica, however a large percentage of its present-day seismicty is microearthquake activity (Mc < 3.0). Although the epicentral distribution is diffuse, a northwest trend can be recognized, and this trend is in good agreement with that of the Frailes Fault. The main structural features associated with La Lucha seismicity are Frailes and Navarro faults.

While the Central Volcanic and Talamanca Ranges have significant seismicity (Fernandez, 1995; Fernandez et al., 1998) the number of recorded earthquakes and their magnitudes reflect very little activity within the Central Valley. During more than 20 year of records, the back‐ ground microseismicity of this valley is represented as scattered low-level activity (Fernandez, 1995). The best known and well- defined concentration of earthquakes in the valley is in Belen and seems to be associated with the Escazu fault. A more recent manifestation of seismicity has been observed in the metropolitan area of San Jose in the last 5 years; it consists of 2 < Mc < 4 earthquakes whose epicenters appear to define a NW-striking lineation that coincides with the northwest end of the Rio Azul and Aguacaliente faults. In the southern border of the central valley there are seismic sources with relatively high rates of seismicity such as the Escazu and Aserri faults, both related to the Frailes-Belohorizonte Escazu fault system. The Aguacaliente fault, responsible for the 1841 and 1910 Cartago earthquakes, has had little activity in the last three decades.

In an effort to see if earthquakes define faults, seismicity cross-sections were carried out in the studied area. Due the low number of earthquakes in some cases and the nearness between faults in other cases only two seismic cross sections were calculated, one at the Pejibaye seismic zone and other eastward of La Lucha. In the cross section A-B (Figure 8a) the hypocenters seem to define a inclined plane that dips 75° northeast, which suggests that a high-angle fault is the responsible for this seismic activity. The cross section C-D (Figure 8b) reveals that the dense seismicity cluster along the Pejibaye seismic zone is generated by an almost vertical fault. Fernandez (2009) reported a fault dipping 76° northwest as the cause of this seismicity.

**Figure 8.** Seismic cross sections A-B and C-D outlined in Figure 6.

Puriscal was a quiet seismic zone before 1990 but in that year there began one of the highest concentrations of seismic activity of Costa Rica in recent decades. This activity was triggered by a large earthquake from the Pacific Coast. Thousands of micro earthquakes were generated in Puriscal in the December 1990-June 1991 period, almost 30 events of Mc > 4.0 and the main

**Figure 7.** The village of Cinchona after the 2009 Cinchona Earthquake. The earthquake changed the geography of the

La Lucha is the most seismically active zone in central Costa Rica, however a large percentage of its present-day seismicty is microearthquake activity (Mc < 3.0). Although the epicentral distribution is diffuse, a northwest trend can be recognized, and this trend is in good agreement with that of the Frailes Fault. The main structural features associated with La Lucha seismicity

While the Central Volcanic and Talamanca Ranges have significant seismicity (Fernandez, 1995; Fernandez et al., 1998) the number of recorded earthquakes and their magnitudes reflect very little activity within the Central Valley. During more than 20 year of records, the back‐ ground microseismicity of this valley is represented as scattered low-level activity (Fernandez, 1995). The best known and well- defined concentration of earthquakes in the valley is in Belen and seems to be associated with the Escazu fault. A more recent manifestation of seismicity

event of Mc = 5.7, the Piedras Negras earthquake.

88 Earthquake Research and Analysis - New Advances in Seismology

are Frailes and Navarro faults.

area. Courtesy of Joanna Mendez.

#### **5.3. The seismic anomaly of Central Costa Rica**

Recent earthquake epicenters from 1992 through 2009 were plotted on a map of Costa Rica in order to show the characteristic local pattern of seismicity that is possibly associated with tectonic features. The plot displays a wide zone of high subduction and crustal seismic activity in Central Costa Rica which coincides with a diffuse zone rather than with a narrow longitu‐ dinal area (Fernández et al., 2007). The seismicity forms an anomalous big cluster composed of smaller clusters (Figure 9) but despite the considerable concentration of earthquakes, epicenters of either the big or smaller clusters fail to delineate any large and single NE or EW fault plane.

To know whether or not the seismicity pattern is related to a hypothetical strike-slip tectonic boundary, we examined the depths of the earthquake clusters. We would expect shallow seismogenic source locations for a strike-slip tectonic boundary but deep (greater than 30 km)

This is in excellent agreement with our results, which support the seamount domain of Central

Seismotectonic and the Hipothetical Strike – Slip Tectonic Boundary of Central Costa Rica

http://dx.doi.org/10.5772/54989

91

**Figure 10.** Cumulative numbers of located earthquakes, separated into six depth ranges, within or near the Costa Ri‐ can territory. These graphs plot earthquakes detected and located by the Red Sismologica Nacional (RSN: ICE-UCR)

Figure 11 shows a set of seamounts on the Cocos plate between the Fisher mounts and the Quepos plateau. The seamounts form a subducting rough zone that collides with the Caribbean plate generating stress, deformation and weakening of the continental crust. Onshore, in front of this zone is the seismic anomaly of Central Costa Rica. The ocean bottom in the Cocos plate between Quepos plateau and Cocos Ridge is almost flat and the seismic level in front of this rectangular area is relatively low (Figures 11). These facts also suggest that sea mounts play an important role in generating seismicity in Costa Rica. They apparently increase intraplate and interplate earthquakes onshore and therefore, in absence of them the seismic activity in

P-wave first motion is used to determine focal mechanism solutions. However, first-motion observations will frequently be in the wrong quadrant because of incorrect first-motion

Central Costa Rica would probably be lower than the current activity.

Costa Rica as the cause of the seismic anomaly.

from 1992 to 2009. Depth ranges are in km.

**6. Focal mechanisms**

**Figure 9.** Background seismicity in Costa Rica from 1992 through 2009. Circles are earthquakes. Several clusters repre‐ sent the most important seismic zones of Costa Rica. The sum of these clusters generates a zone of concentrated seis‐ micity in Central Costa Rica. MAT: Middle American Trench.

source locations for subduction zone earthquakes. Because 80% of the present-day seismicity of Central Costa Rica is shallow, we expect earthquake concentrations to be above a subduction decollement.

To test whether the seismic origin is in the subduction zone or from a much shallower transform fault earthquakes with depths in the range of 30–90 km were plotted at intervals of 10 km (Figure 10). Costa Rican earthquakes are distributed over all depths with deeper clusters to the northeast. The cluster in figure 8a approximately coincide with the results of DeShon et al. (2003) who found that earthquakes occur above 30 km depth, 95 km from the trench offshore Central Costa Rica. Our results suggest a source for the anomaly related to the subduction process, perhaps subducted seamounts on the Cocos plate that generate larger stress fields than nearby smooth subducted areas of the same plate, causing the high intraplate and interplate seismicity in central Costa Rica. Bilek et al. (2003) stated that shallow, smallermagnitude seismicity is more common in regions of seamounts subduction than in the smoother region subducting off northern Costa Rica, suggesting that subduction of topo‐ graphic highs localizes seismicity. Von Huene et al. (2004) indicate that subducted seamounts appear to remain attached to the underthrust plate more than 100 km landward of the trench axis as indicated by clustered earthquakes beneath the shelf and local uplift along the coast. This is in excellent agreement with our results, which support the seamount domain of Central Costa Rica as the cause of the seismic anomaly.

**Figure 10.** Cumulative numbers of located earthquakes, separated into six depth ranges, within or near the Costa Ri‐ can territory. These graphs plot earthquakes detected and located by the Red Sismologica Nacional (RSN: ICE-UCR) from 1992 to 2009. Depth ranges are in km.

Figure 11 shows a set of seamounts on the Cocos plate between the Fisher mounts and the Quepos plateau. The seamounts form a subducting rough zone that collides with the Caribbean plate generating stress, deformation and weakening of the continental crust. Onshore, in front of this zone is the seismic anomaly of Central Costa Rica. The ocean bottom in the Cocos plate between Quepos plateau and Cocos Ridge is almost flat and the seismic level in front of this rectangular area is relatively low (Figures 11). These facts also suggest that sea mounts play an important role in generating seismicity in Costa Rica. They apparently increase intraplate and interplate earthquakes onshore and therefore, in absence of them the seismic activity in Central Costa Rica would probably be lower than the current activity.

#### **6. Focal mechanisms**

source locations for subduction zone earthquakes. Because 80% of the present-day seismicity of Central Costa Rica is shallow, we expect earthquake concentrations to be above a subduction

**Figure 9.** Background seismicity in Costa Rica from 1992 through 2009. Circles are earthquakes. Several clusters repre‐ sent the most important seismic zones of Costa Rica. The sum of these clusters generates a zone of concentrated seis‐

To test whether the seismic origin is in the subduction zone or from a much shallower transform fault earthquakes with depths in the range of 30–90 km were plotted at intervals of 10 km (Figure 10). Costa Rican earthquakes are distributed over all depths with deeper clusters to the northeast. The cluster in figure 8a approximately coincide with the results of DeShon et al. (2003) who found that earthquakes occur above 30 km depth, 95 km from the trench offshore Central Costa Rica. Our results suggest a source for the anomaly related to the subduction process, perhaps subducted seamounts on the Cocos plate that generate larger stress fields than nearby smooth subducted areas of the same plate, causing the high intraplate and interplate seismicity in central Costa Rica. Bilek et al. (2003) stated that shallow, smallermagnitude seismicity is more common in regions of seamounts subduction than in the smoother region subducting off northern Costa Rica, suggesting that subduction of topo‐ graphic highs localizes seismicity. Von Huene et al. (2004) indicate that subducted seamounts appear to remain attached to the underthrust plate more than 100 km landward of the trench axis as indicated by clustered earthquakes beneath the shelf and local uplift along the coast.

decollement.

micity in Central Costa Rica. MAT: Middle American Trench.

90 Earthquake Research and Analysis - New Advances in Seismology

P-wave first motion is used to determine focal mechanism solutions. However, first-motion observations will frequently be in the wrong quadrant because of incorrect first-motion

**Number Date Latitude Longitude Mag Depth RMS EH EZ AZ Dip Rake** 1 90/12/22 09.883 -84.334 5.7 14.6 0.28 0.7 1.7 252.5 63.0 30.7 2 92/11/02 09.887 -83.766 3.4 06.2 0.21 0.4 0.7 060.3 72.8 -58.4 3 92/11/03 09.921 -84.138 4.1 06.5 0.30 0.6 0.9 269.0 40.0 58.0 4 92/11/12 09.745 -84.013 3.5 16.8 0.27 0.7 2.1 097.6 51.1 145.6 5 93/01/20 09.979 -84.183 3.7 11.6 0.35 0.8 2.0 230.0 90.0 45.0 6 93/05/07 09.705 -83.767 3.7 03.8 0.28 0.6 0.8 236.9 56.4 -10.3 7 93/07/09 09.756 -83.615 4.3 12.6 0.30 3.2 4.8 239.6 68.5 -57.5 8 93/07/10 09.776 -83.686 5.3 12.8 0.31 2.2 3.2 262.37 75.9 -32.4 9 93/07/13 9.735 -83.615 4.9 12.4 0.22 3.0 2.9 240.5 43.9 -22.2 10 93/07/14 09.701 -83.809 3.9 06.7 0.59 0.8 2.0 224.9 45.9 -76.0 11 94/01/11 09.812 -84.142 3.5 16.8 0.21 0.6 1.1 110.4 65.4 79.0 12 94/10/29 09.867 -84.064 3.3 06.6 0.30 0.7 0.5 253.0 84.0 -40.0 13 96/05/23 09.850 -83.988 3.1 11.4 0.36 0.7 2.0 097.0 74.0 -53.0 14 96/05/26 10.090 -83.660 4.0 14.9 0.40 2.2 3.6 210.0 50.0 -90.0 15 99/07/18 10.206 -84.228 3.2 04.8 0.31 1.0 0.5 359.0 66.1 -26.3 16 09/01/08 10.194 -84.177 6.2 03.6 0.60 2.6 2.6 025.0 47.5 -37.0

Seismotectonic and the Hipothetical Strike – Slip Tectonic Boundary of Central Costa Rica

http://dx.doi.org/10.5772/54989

93

Note. Mag.: Magnitude, RMS: root-mean-square, EH: horizontal error, EZ: vertical error, AZ: Azimuth.

**Figure 12.** Faulting and focal mechanisms. Small lettered stereo projections are fault-plane solutions for 16 carefully

selected earthquakes. BT: Bajos del Toro, PV: Poas Volcano, IV: Irazu Volcano.

**Table 2.** Parameters of focal mechanisms.

**Figure 11.** The Cocos-Caribbean tectonic boundary in front of the Costa Rican Pacific coast is the Middle American Trench. Large seamounts (Fisher Mount, Eve volcanoes, Quepos plateau) are being subducted under the Caribbean plate just in Central Costa Rica. This process causes high stress and seismicity. From Ranero and von Huene, 2000.

direction, inappropriate earthquake velocity model, station polarity reversals and incorrect direct P-arrival picks due to low signal-to-noise ratios. The method requires enough data to ideally determine fault-plane solutions. Few data or incorrect first motion observations may generate more than one or many focal mechanism solutions and changes in the earthquake location or in the seismic velocity model can significantly affect the distribution of observations on the focal sphere, changing the best-fitting focal mechanism solution. Low magnitude earthquake and seismometers locates near the nodal planes between the compressional and dilatational quadrants of an earthquake do not produce strong first motions which made difficult to determine focal mechanisms.

Because the studied area is characterized by microseismicity and truly few intermediatemagnitude earthquakes, it is really difficult to obtain a large number of reliable focal-plane solutions in central Costa Rica. After a strict selection of seismic events of the last 18 years, we only found 16 reliable focal mechanisms (Table 2, Figures 12 and 13). They show considerable variation in the sense of motion which probably reflects movement on preexisting planes of weakness that are geometrically favorable for slip but not necessarily aligned with a plane of maximum shear stress. The events exhibit reverse, normal and strike-slip faulting.

Seismotectonic and the Hipothetical Strike – Slip Tectonic Boundary of Central Costa Rica http://dx.doi.org/10.5772/54989 93


Note. Mag.: Magnitude, RMS: root-mean-square, EH: horizontal error, EZ: vertical error, AZ: Azimuth.

**Table 2.** Parameters of focal mechanisms.

direction, inappropriate earthquake velocity model, station polarity reversals and incorrect direct P-arrival picks due to low signal-to-noise ratios. The method requires enough data to ideally determine fault-plane solutions. Few data or incorrect first motion observations may generate more than one or many focal mechanism solutions and changes in the earthquake location or in the seismic velocity model can significantly affect the distribution of observations on the focal sphere, changing the best-fitting focal mechanism solution. Low magnitude earthquake and seismometers locates near the nodal planes between the compressional and dilatational quadrants of an earthquake do not produce strong first motions which made

**Figure 11.** The Cocos-Caribbean tectonic boundary in front of the Costa Rican Pacific coast is the Middle American Trench. Large seamounts (Fisher Mount, Eve volcanoes, Quepos plateau) are being subducted under the Caribbean plate just in Central Costa Rica. This process causes high stress and seismicity. From Ranero and von Huene, 2000.

Because the studied area is characterized by microseismicity and truly few intermediatemagnitude earthquakes, it is really difficult to obtain a large number of reliable focal-plane solutions in central Costa Rica. After a strict selection of seismic events of the last 18 years, we only found 16 reliable focal mechanisms (Table 2, Figures 12 and 13). They show considerable variation in the sense of motion which probably reflects movement on preexisting planes of weakness that are geometrically favorable for slip but not necessarily aligned with a plane of

maximum shear stress. The events exhibit reverse, normal and strike-slip faulting.

difficult to determine focal mechanisms.

92 Earthquake Research and Analysis - New Advances in Seismology

**Figure 12.** Faulting and focal mechanisms. Small lettered stereo projections are fault-plane solutions for 16 carefully selected earthquakes. BT: Bajos del Toro, PV: Poas Volcano, IV: Irazu Volcano.

Another important limitation to obtain more and better focal mechanisms in Central Costa Rica is the instrumentation used to detect them. We are still using short period, one component seismic sensors to detect and locate the seismicity. Due to this, the resolution of the strike for the occurring mechanisms depends on the readings at only few stations in many cases. In the future it would be more appropriate to compute the focal mechanisms using waveform inversion (Dreger & Helmberger, 1993; Zhu & Helmberger, 1996; Herrmann et al., 2008; D

Seismotectonic and the Hipothetical Strike – Slip Tectonic Boundary of Central Costa Rica

http://dx.doi.org/10.5772/54989

95

The faulting, high seismicity and strike-slip focal mechanisms do not define a consistent eastwest shear zone in central Costa Rica. Strike-slip deformation in central Costa Rica is inter‐ preted as a result of the elastic strain accumulation in the upper plate due to the subduction of seamount domain and Cocos Ridge under the Caribbean Plate. The fault orientation may reflect the northeast movement of Cocos plate, stresses caused by the subduction of sea mounts, and the compression of the Cocos Ridge in southern Costa Rica, where the rate of convergence between Cocos and Caribbean plates is maximum (DeMets, 2001). This high rate and the southnorth sliding of the Cocos plate along the Panamá Fracture Zone could be creating a favorable

White & Harlow (1993) studied the destructive shallow earthquakes in Central America and found a concentrated seismicity in the volcanic front. According to them, this volcanic front is a zone of dextral strike-slip driven by oblique subduction. Large earthquakes as that of Managua in 1972 and Tilarán in northern Costa Rica in 1973 were strike-slip earthquakes. These data indicate that strike-slip motion within the Caribbean Plate is not concentrated in Costa

Fan et al. (1993) proposed that left-lateral strike-slip motion in central Costa Rica occurs on various sub-parallel strike-slip faults that comprise a diffuse northeast-southwest strike-slip fault zone. This is inconsistent with Astorga et al. (1989, 1991) who proposed an east-west trend for the fault system of Central Costa Rica. But the proposal of Astorga et al (1989, 1991) is not

Fischer et al. (1994) stated that the seismicity after Cóbano (1990) and Limón (1991) earthquakes are constrained in a diffuse zone of faulting oriented west-east along the Central Valley of Costa Rica and that the variety of faults may reflect an early stage of a developing shear zone. In this work all currently mapped faults and lineaments are included and we find the same faulting pattern that Arias & Denyer (1991) attribute to a north-south compression that affects Costa Rica since late Miocene-Pliocene. The distribution of earthquakes and focal mechanisms indicate that seismic activity occurs on both northeast and northwest trending faults. There‐ fore, the seismicity mentioned by Fisher et al. (1994) is not likely to be due to incipient faulting but to preexistent faulting reactivated by the collision of Cocos Ridge with the Caribbean Plate

environment to form northwest lateral tears (as Frailes, for instance).

Rica but it is present all over Central America (Quintero & Guendell, 2000)

(Denyer & Arias, 1991) and by faults reactivated after large earthquakes.

´Amico et al., 2010; D´Amico et al., 2011).

supported by the data described here.

**7. Discussion**

**Figure 13.** P-wave first motion focal mechanisms, determined using pspolar routine of GMT (Graphic Mapping Tools). In all cases more than 9 P-wave polarities were used. Open circles represent downward first motions, black circles rep‐ resent upward first motion.

Focal mechanisms near Pejibaye (6, 7, 8, 9, and 10) show nearly normal-slip along planes striking northeast, suggesting a possible association with a northeast-trending faults. At Puriscal, the fault-plane solution (1) is strike-slip with reverse component. That solution indicates right-lateral motion along the northeast striking nodal plane. Based on the destruc‐ tion near Alajuela associated to the correspondent earthquake Montero (2001) chose that plane as the fault plane and proposed the Virilla fault as the responsible for the earthquake. However, the strike of the selected nodal plane is close to the orientation of the Picagres fault.

Fault-plane solutions for events from Frailes-Escazú faults (3, 4, 5, 11, 12) show thrust and strike-slip motion with a strong reverse component (3, 4, 11). These solutions suggest north‐ west striking faulting, in good agreement with the strike of the mapped faults. Event 13 suggests a high normal component along the Aguacaliente fault. When resolvable, the focal mechanisms of small to moderate sized earthquakes (M< 4.5) in the Poas area show predom‐ inantly strike-slip motion (15, 16). The fault-plane solution for the 2009 Cinchona earthquake (16) is oblique with high normal component (Rojas et al., 2009).

Another important limitation to obtain more and better focal mechanisms in Central Costa Rica is the instrumentation used to detect them. We are still using short period, one component seismic sensors to detect and locate the seismicity. Due to this, the resolution of the strike for the occurring mechanisms depends on the readings at only few stations in many cases. In the future it would be more appropriate to compute the focal mechanisms using waveform inversion (Dreger & Helmberger, 1993; Zhu & Helmberger, 1996; Herrmann et al., 2008; D ´Amico et al., 2010; D´Amico et al., 2011).

#### **7. Discussion**

Focal mechanisms near Pejibaye (6, 7, 8, 9, and 10) show nearly normal-slip along planes striking northeast, suggesting a possible association with a northeast-trending faults. At Puriscal, the fault-plane solution (1) is strike-slip with reverse component. That solution indicates right-lateral motion along the northeast striking nodal plane. Based on the destruc‐ tion near Alajuela associated to the correspondent earthquake Montero (2001) chose that plane as the fault plane and proposed the Virilla fault as the responsible for the earthquake. However,

**Figure 13.** P-wave first motion focal mechanisms, determined using pspolar routine of GMT (Graphic Mapping Tools). In all cases more than 9 P-wave polarities were used. Open circles represent downward first motions, black circles rep‐

Fault-plane solutions for events from Frailes-Escazú faults (3, 4, 5, 11, 12) show thrust and strike-slip motion with a strong reverse component (3, 4, 11). These solutions suggest north‐ west striking faulting, in good agreement with the strike of the mapped faults. Event 13 suggests a high normal component along the Aguacaliente fault. When resolvable, the focal mechanisms of small to moderate sized earthquakes (M< 4.5) in the Poas area show predom‐ inantly strike-slip motion (15, 16). The fault-plane solution for the 2009 Cinchona earthquake

the strike of the selected nodal plane is close to the orientation of the Picagres fault.

(16) is oblique with high normal component (Rojas et al., 2009).

resent upward first motion.

94 Earthquake Research and Analysis - New Advances in Seismology

The faulting, high seismicity and strike-slip focal mechanisms do not define a consistent eastwest shear zone in central Costa Rica. Strike-slip deformation in central Costa Rica is inter‐ preted as a result of the elastic strain accumulation in the upper plate due to the subduction of seamount domain and Cocos Ridge under the Caribbean Plate. The fault orientation may reflect the northeast movement of Cocos plate, stresses caused by the subduction of sea mounts, and the compression of the Cocos Ridge in southern Costa Rica, where the rate of convergence between Cocos and Caribbean plates is maximum (DeMets, 2001). This high rate and the southnorth sliding of the Cocos plate along the Panamá Fracture Zone could be creating a favorable environment to form northwest lateral tears (as Frailes, for instance).

White & Harlow (1993) studied the destructive shallow earthquakes in Central America and found a concentrated seismicity in the volcanic front. According to them, this volcanic front is a zone of dextral strike-slip driven by oblique subduction. Large earthquakes as that of Managua in 1972 and Tilarán in northern Costa Rica in 1973 were strike-slip earthquakes. These data indicate that strike-slip motion within the Caribbean Plate is not concentrated in Costa Rica but it is present all over Central America (Quintero & Guendell, 2000)

Fan et al. (1993) proposed that left-lateral strike-slip motion in central Costa Rica occurs on various sub-parallel strike-slip faults that comprise a diffuse northeast-southwest strike-slip fault zone. This is inconsistent with Astorga et al. (1989, 1991) who proposed an east-west trend for the fault system of Central Costa Rica. But the proposal of Astorga et al (1989, 1991) is not supported by the data described here.

Fischer et al. (1994) stated that the seismicity after Cóbano (1990) and Limón (1991) earthquakes are constrained in a diffuse zone of faulting oriented west-east along the Central Valley of Costa Rica and that the variety of faults may reflect an early stage of a developing shear zone. In this work all currently mapped faults and lineaments are included and we find the same faulting pattern that Arias & Denyer (1991) attribute to a north-south compression that affects Costa Rica since late Miocene-Pliocene. The distribution of earthquakes and focal mechanisms indicate that seismic activity occurs on both northeast and northwest trending faults. There‐ fore, the seismicity mentioned by Fisher et al. (1994) is not likely to be due to incipient faulting but to preexistent faulting reactivated by the collision of Cocos Ridge with the Caribbean Plate (Denyer & Arias, 1991) and by faults reactivated after large earthquakes.

Strike-slip deformation along plate boundaries is often distributed among several parallel faults (Brink et al, 1996) and shear zones are overprinted by numerous foliation-parallel brittle faults (Cunningham, 1996). Offset strike-slip faults may be connected by intervening pull apart basins but this geometric pattern is not well defined in Central Costa Rica. There are parallel faults but they do not follow a preferential direction and not all of the parallel faults are strikeslip in type. Observing the fault distribution and orientation near the Central Valley of Costa Rica, we see parallelism between the most important: Alajuela, Aguacaliente and Frailes-Escazu faults (northwest extreme). But the Alajuela Fault is a very well-known reverse fault and the Frailes-Escazu also seems to have a strong reverse component according to Denyer et al. (1993), Fernández & Montero (2002) and our results in this work. Focal mechanisms and an excaved trench suggest that in contrast the Aguacaliente fault has a significant normal component. If this is so, the central Valley of Costa Rica would not be a pull apart basin unless it represents a developed strike-slip fault system where strike-slip faults have gradually evolved into oblique thrusts or thrusts (Fuh et al., 1997).

quakes of the zone, suggest northwest motion along the Viejo, Carbonera, Angel, Frailes and

Seismotectonic and the Hipothetical Strike – Slip Tectonic Boundary of Central Costa Rica

http://dx.doi.org/10.5772/54989

97

The strike-slip fault of Costa Rica proposed by Astorga et al (1989) and the set of subparalel strike-slip faults suggested by Fan et al. (1993) were not found in the studied area. Neither the trace of the hypothetical strike-slip tectonic boundary, which according to Jacob et al. (1991), Fisher et al. (1994) and Marshall et al. (2000) cut the Central Valley of Costa Rica, was not found

According to our data, there is no a clear and well defined east-west strike-slip fault system in Central Costa Rica that might represent a tectonic boundary. The anomalous deformation and seismicity of central Costa Rica is more related to the subduction of sea mounts than to the

**•** Earthquake data were provided by the Red Sismologica Nacional (RSN) operated by the Costa Rican Electricity Company and the University of Costa Rica. They cannot be released

**•** Some plots were made using the Generic Mapping Tools version 4.2.1 (www.soest.ha‐

Thank to personnel of both Central America Seismological Center (CASC) and the Red Sismológica Nacional (RSN: ICE-UCR) for providing data to carry out this investigation. My gratitude to Sara Kruse for comments and suggestions that greatly improved the manuscript. Also thanks to Cindy Solis and Jonnathan Reyes for their help in processing the data and preparing the figures. The author is grateful to CONICIT for financial support through

Escuela de Geología, Universidad de Costa Rica, Programa PREVENTEC, Red Sismológica

proposed hypothetical strike-slip tectonic boundary for Central Costa Rica.

Aguacaliente faults.

in that valley.

to the public.

**Acknowledgements**

FORINVES program.

**Author details**

Mario Fernandez Arce\*

**9. Data and resources section**

waii.edu/gmt; Wessel and Smith, 1998).

Address all correspondence to: mario.fernandezarce@ucr.ac.cr

Nacional (RSN: ICE-UCR). San José, Costa Rica, Central America

Marshall et al. (2000) attributed the deformation of Central Costa Rica to the subduction of Cocos Ridge and the seamount domain and proposed an E-W deformation front that propa‐ gates northward into the overriding volcanic arc, as the tectonic boundary between the Caribbean plate and the Panama block. But even this deformed belt requires a set of EW strikeslip faults along its northern edge, located in the Central Valley of Costa Rica. However, the EW strike-slip faults, and therefore the EW strike-slip motion, are absent in the studied area and most active faults of that area are northwest. DeMets (2001) and Norabuena et al. (2004) estimated trench-paralell motion of the Costa Rican forearc to northwest at a rate of 7 and 8 mm/yr respectively. They suggest interseismic and post-seismic effects from forearc faults and the subduction interface, diffuse extension at the trailing edge of the forearc sliver, partitioning of slip between multiple forearc faults, northwest striking right-lateral strike-slip faults and vertical axis rotation of smaller blocks defined by short, northeast striking, left-lateral "book‐ shelf" faults as the multiple cause of the observed motion. In the same way, northeast motion could have multiple explanations.

Von Huene et al., (2003) assure that subducted seamounts are causing deformation and weakened of the upper plate which steepness the slope above them, generating great potential for tsunamigenic landslides. The sea mounts destroy the frontal prism and uplifts the conti‐ nental crust. Since this result it is clear that subducted seamount play an important role in the deforming the upper plate in central Costa Rica.

#### **8. Conclusion**

There is a seamount domain off central Costa Rica and intense crustal deformation and high seismicity onshore, in front of this seamount domain. The deformation includes an x-pattern faulting in which both northeast and northwest faults are active and have high seismicity. Focal mechanisms of small-magnitude earthquakes show normal, reverse and strike-slip motion along some faults of the studied area. Most of the historical earthquakes, the largest earth‐ quakes of the zone, suggest northwest motion along the Viejo, Carbonera, Angel, Frailes and Aguacaliente faults.

The strike-slip fault of Costa Rica proposed by Astorga et al (1989) and the set of subparalel strike-slip faults suggested by Fan et al. (1993) were not found in the studied area. Neither the trace of the hypothetical strike-slip tectonic boundary, which according to Jacob et al. (1991), Fisher et al. (1994) and Marshall et al. (2000) cut the Central Valley of Costa Rica, was not found in that valley.

According to our data, there is no a clear and well defined east-west strike-slip fault system in Central Costa Rica that might represent a tectonic boundary. The anomalous deformation and seismicity of central Costa Rica is more related to the subduction of sea mounts than to the proposed hypothetical strike-slip tectonic boundary for Central Costa Rica.

#### **9. Data and resources section**

Strike-slip deformation along plate boundaries is often distributed among several parallel faults (Brink et al, 1996) and shear zones are overprinted by numerous foliation-parallel brittle faults (Cunningham, 1996). Offset strike-slip faults may be connected by intervening pull apart basins but this geometric pattern is not well defined in Central Costa Rica. There are parallel faults but they do not follow a preferential direction and not all of the parallel faults are strikeslip in type. Observing the fault distribution and orientation near the Central Valley of Costa Rica, we see parallelism between the most important: Alajuela, Aguacaliente and Frailes-Escazu faults (northwest extreme). But the Alajuela Fault is a very well-known reverse fault and the Frailes-Escazu also seems to have a strong reverse component according to Denyer et al. (1993), Fernández & Montero (2002) and our results in this work. Focal mechanisms and an excaved trench suggest that in contrast the Aguacaliente fault has a significant normal component. If this is so, the central Valley of Costa Rica would not be a pull apart basin unless it represents a developed strike-slip fault system where strike-slip faults have gradually

Marshall et al. (2000) attributed the deformation of Central Costa Rica to the subduction of Cocos Ridge and the seamount domain and proposed an E-W deformation front that propa‐ gates northward into the overriding volcanic arc, as the tectonic boundary between the Caribbean plate and the Panama block. But even this deformed belt requires a set of EW strikeslip faults along its northern edge, located in the Central Valley of Costa Rica. However, the EW strike-slip faults, and therefore the EW strike-slip motion, are absent in the studied area and most active faults of that area are northwest. DeMets (2001) and Norabuena et al. (2004) estimated trench-paralell motion of the Costa Rican forearc to northwest at a rate of 7 and 8 mm/yr respectively. They suggest interseismic and post-seismic effects from forearc faults and the subduction interface, diffuse extension at the trailing edge of the forearc sliver, partitioning of slip between multiple forearc faults, northwest striking right-lateral strike-slip faults and vertical axis rotation of smaller blocks defined by short, northeast striking, left-lateral "book‐ shelf" faults as the multiple cause of the observed motion. In the same way, northeast motion

Von Huene et al., (2003) assure that subducted seamounts are causing deformation and weakened of the upper plate which steepness the slope above them, generating great potential for tsunamigenic landslides. The sea mounts destroy the frontal prism and uplifts the conti‐ nental crust. Since this result it is clear that subducted seamount play an important role in the

There is a seamount domain off central Costa Rica and intense crustal deformation and high seismicity onshore, in front of this seamount domain. The deformation includes an x-pattern faulting in which both northeast and northwest faults are active and have high seismicity. Focal mechanisms of small-magnitude earthquakes show normal, reverse and strike-slip motion along some faults of the studied area. Most of the historical earthquakes, the largest earth‐

evolved into oblique thrusts or thrusts (Fuh et al., 1997).

96 Earthquake Research and Analysis - New Advances in Seismology

could have multiple explanations.

**8. Conclusion**

deforming the upper plate in central Costa Rica.


#### **Acknowledgements**

Thank to personnel of both Central America Seismological Center (CASC) and the Red Sismológica Nacional (RSN: ICE-UCR) for providing data to carry out this investigation. My gratitude to Sara Kruse for comments and suggestions that greatly improved the manuscript. Also thanks to Cindy Solis and Jonnathan Reyes for their help in processing the data and preparing the figures. The author is grateful to CONICIT for financial support through FORINVES program.

#### **Author details**

Mario Fernandez Arce\*

Address all correspondence to: mario.fernandezarce@ucr.ac.cr

Escuela de Geología, Universidad de Costa Rica, Programa PREVENTEC, Red Sismológica Nacional (RSN: ICE-UCR). San José, Costa Rica, Central America

#### **References**

[1] Adamek, S, Frohlich, C, & Pennington, D. Seismicity of the Caribbean-Nazca boun‐ dary: Constraints on microplate tectonics of the Panama region. J. Geophys. Res., (1988). , 93, 2053-2075.

[13] Cunningham, W, Windley, B, Dorjnamjaa, D, Badamgarov, J, & Saandar, M. Late Cenozoic transpression in southwestern Mongolia and the Gobi Altai-Tien Shan con‐

Seismotectonic and the Hipothetical Strike – Slip Tectonic Boundary of Central Costa Rica

http://dx.doi.org/10.5772/54989

99

[14] Amico, D, Orecchio, S, Presti, B, Gervasi, D, Guerra, A, Neri, I, Zhu, G, & Herrmann, L. R. B., Testing the stability of moment tensor solutions for small and moderate earthquakes in the Calabrian-Peloritan arc region. Boll. Geo. Teor. Appl., doi:10.4430/

[15] Amico, D, Orecchio, S, Presti, B, Zhu, D, Herrmann, L, & Neri, R. B. G., Broadband waveform inversion of moderate earthquakes in the Messina straits, Southern Italy, Physics of Earth and Planetary Interiors, doi:j.pepi.2010.01.012, (2010). , 179, 97-106.

[16] De Boer, J. Z, Drummond, M. S, Bordelon, M. J, Defant, M. J, Bellon, H, & Maury, R. C. Cenozoic magmatic phases of the Costa Rican island arc (Cordillera de Talaman‐ ca), in Mann, P., ed., Geological Society of America Special Paper, Geologic and Tec‐ tonic Development of the Caribbean Plate Boundary in Southern Central America,

[17] Demets, C. A new estimate for present-day Cocos-Caribbean plate motion: Implica‐ tions for slip along the Central American volcanic arc, Geophys. Res. Lett, (2001). , 28

[18] Denyer, P, & Arias, O. Estratigrafía de la región central de Costa Rica, Rev. Geol.

[19] Denyer, P, Arias, O, Soto, G, Obando, L, & Salazar, G. Mapa Geologico de la Gran

[20] Denyer, P, Montero, W, & Alvarado, G. Atlas Tectonico de Costa Rica, Editorial Uni‐

[21] Deshon, H, Schwart, S, Bilek, S, Dorman, L, Gonzalez, V, Protti, M, Flueh, E, & Dix‐ on, T. Seimogenic zone structure of the Middle America Trench, Costa Rica, J. Geo‐

[22] Marco, G, Baunmgartner, P., Channel, J., Late Cretaceous-early Tertiary paleomag‐ netic data and a revised tectonostratigraphic subdivision of Costa Rica and western Panama, in Mann, P., ed., Geologic and Tectonic Development of the Caribbean Plate Boundary in Southern Central America: Boulder, Colorado, Geological Society of

[23] Dreger, D. S, & Helmberger, D. V. Determination of source parameters at regional distances with single station or sparse network data. J. Geophys Res., (1993). , 98,

[24] Escalante, G, & Astorga, A. Geología del Este de Costa Rica y el Norte de Panamá.

Rev. Geol. Amér. Central, v. esp. Terremoto de Limón: (1994). , 1-14.

nection, Earth and Planetary Science Letters, (1996). , 140, 67-81.

bgta0009,(2011). , 52, 283-298.

(1995). (295), 35-55.

Amer. Central, (1991). , 12, 1-59.

versidad de Costa Rica, (2003). , 81.

phys. Res. 108 (B10), 2491, (2003).

America Special Paper 295, (1995).

1162-1179.

Area Metropolitana, (1993).


[13] Cunningham, W, Windley, B, Dorjnamjaa, D, Badamgarov, J, & Saandar, M. Late Cenozoic transpression in southwestern Mongolia and the Gobi Altai-Tien Shan con‐ nection, Earth and Planetary Science Letters, (1996). , 140, 67-81.

**References**

(1988). , 93, 2053-2075.

98 Earthquake Research and Analysis - New Advances in Seismology

Central, (1991). , 12, 61-74.

pacific Council: 23 , 1989.

ture behavior. Geology, (2003). , 31(5)

ica, N. 1, (2010). , 100, 343-348.

Int., (1997). , 131, 189-208.

Boulder, Colorado, (1990). , 375-391.

strike-slip faults, J. Geophy Res., (1996). , 101(B7)

43-63.

[1] Adamek, S, Frohlich, C, & Pennington, D. Seismicity of the Caribbean-Nazca boun‐ dary: Constraints on microplate tectonics of the Panama region. J. Geophys. Res.,

[2] Arias, O, & Denyer, P. Estructura geológica de la región comprendida en las hojas topográfica Abra, Caraigres, Candelaria y Río Grande, Costa Rica. Rev. geol. Amér.

[3] Arroyo, I, Sismicidad y neotectónica en la región de influencia del proyecto Boruca: hacia una mejor definición sismogénica del Sureste de Costa Rica. 162 pp. Tesis de

[4] Astorga, A, Fernández, J, Barboza, G, Campos, L, Obando, J, Aguilar, A, & Obando, L. Cuencas sedimentarias de Costa Rica: Evolución Cretácico Superior-Cenozoica y potencial de Hidrocarburos.-Symposium on the Energy and Mineral Potencial of the Central American- Caribbean Region, San José, Costa Rica, March 6-9, 1989, Circum‐

[5] Astorga, A, Fernández, J, Barboza, G, Campos, L, Obando, J, Aguilar, A, & Obando, L. Cuencas sedimentarias de Costa Rica: Evolución geodinámica y potencial de hi‐

[6] Barboza, G, Barrientos, J, & Astorga, A. Tectonic evolution and sequence stratigraphy of the central Pacific margin of Costa Rica. Rev. Geol. Amer. Central, 18, (1995). ,

[7] Bilek, S, Schwartz, S, & Deshon, H. Control of seafloor roughness on earthquake rup‐

[8] Brink, U, Katzman, R, & Jian, L. Three-dimensional models of deformation near

[9] Burbach, G, Frohlich, C, Pennington, W, & Matumoto, T. Seismicity and tectonics of

[10] Camacho, E, Hutton, W, & Pacheco, J. A New at Evidence for a Wadatti-Benioff Zone and Active Convergence at the North Panama Deformed Belt, Bull. Seism. Soc Amer‐

[11] Carr, M, & Stoiber, R. Volcanism, in The Caribbean region, The Geology of North America, vol., H, edited by G. Dengo, and J. Case, Geological Society of America,

[12] Colombo, D, Cimini, G, & De Franco, R. Three-dimensional velocity structure of the upper mantle beneath Costa Rica from a teleseismic tomography study. Geophys. J.

the subducted Cocos plate. J. Geophys. Res., (1984). , 89, 7719-7735.

Licenciatura, Escuela de Geología, Universidad de Costa Rica, (2001).

drocarburos. Rev. Geol. Amer. Central, (1991). , 43, 25-59.


[25] Fan, G, Beck, S, & Wallace, T. A Diffuse Transcurrent Boundary Boundary in Central Costa Rica: Evidence From a Portable Aftershock Study (Abstract), Eos. Trans., AGU, 73, 345, (1992).

August 23-28, 1993, McMaster University, Hamilton, Ontario, Canada, Programme

Seismotectonic and the Hipothetical Strike – Slip Tectonic Boundary of Central Costa Rica

http://dx.doi.org/10.5772/54989

101

[38] Goes, S. D. B, Velasco, A. A, Schwartz, S, & Lay, T. The April 22, 1991, Valle de la Estrella, Costa Rica (Mw=7.7) earthquake and its tectonic implications: a broadband

[39] Güendel, F, & Pacheco, J. The 1990-1991seismic sequence across central Costa Rica: evidence for the existence of a micro-plate boundary connecting the Panama de‐ formed belt and the Middle America trench, Eos Trans. Am. Geophys. Un. 73, 399,

[40] Güendel, F, & Protti, M. Sismicidad y Sismotectónica de América Central, en: Buforn, E., Udías, A., Física de la Tierra, N° 10, Servicio de Publicaciones, Universidad Com‐

[41] Havskov, J, & Ottemøller, L. The SEISAN earthquake analysis software for Windows, Sun and Linux. Manual and software, Instutute of Solid Earth Physics, University of

[42] Herrmann, R. B, Withers, M, & Benz, H. The April 18, 2008 Illinois earthquake: an

[43] Husen, S, Kissling, E, & Quintero, R. Tomographic evidence for a subducted sea‐ mount beneath the Gulf of Nicoya, Costa Rica: The cause of the 1990 Mw = 7.0 Gulf

[44] Jacob, K, Pacheco, J, & Santana, G. Seismology and Tectonics, in Costa Rica Earth‐ quake of April 22, 1991. Reconnaissance Report, Earthquake Spectra, Supplement B,

[45] Kolarsky, R. A, Mann, P, & Montero, W. Island arc response to shallow subduction of the Cocos Ridge, Costa Rica, in Mann, P., ed., Geological Society of America Special Paper, Geologic and Tectonic Development of the Caribbean Plate Boundary in

[46] Laurent, J. Evaluación económica de pérdidas y daños. 2009. En: Barquero (Ed.): El

[47] Linkimer, L. Neotectónica del extremo oriental del Cinturón Deformado del Centro de Costa Rica, Tesis de Licenciatura, Universidad de Costa Rica, 103 , 2003.

[48] López, A. Neo and paleostress partitioning in the SW corner of the Caribbean plate and its fault reactivation potential. Tesis doctoral, Universidad de Tûbinger, Alema‐

[49] Lundgren, P, Protti, M, Donnellan, A, Heflin, M, Hernandez, E, & Jefferson, D. Seis‐ mic cycle and plate margin deformation in Costa Rica: GPS observations from 1994

to 1997, Journal of Geophysical Research, (1999). , 104(B12), 28915-28926.

terremoto de Cinchona, 8 de enero de 2009. Inf. RSN, 101‐127, (2009).

ANSS monitoring success. Seism. Res. Lett., (2008). , 79, 830-843.

Southern Central America, (1995). (295), 235-262.

of Nicoya earthquake. Geophysical Research Letters, N 8, (2003). , 29

seismic study, J. Geophys. Res., (1993). , 98, 8127-8142.

with Abstracts, (1993). , 143.

plutense de Madrid, (1998).

Bergen, Norway, (1999).

(1991). , 7, 15-33.

nia, 293 , 1999.

(1992).


August 23-28, 1993, McMaster University, Hamilton, Ontario, Canada, Programme with Abstracts, (1993). , 143.

[38] Goes, S. D. B, Velasco, A. A, Schwartz, S, & Lay, T. The April 22, 1991, Valle de la Estrella, Costa Rica (Mw=7.7) earthquake and its tectonic implications: a broadband seismic study, J. Geophys. Res., (1993). , 98, 8127-8142.

[25] Fan, G, Beck, S, & Wallace, T. A Diffuse Transcurrent Boundary Boundary in Central Costa Rica: Evidence From a Portable Aftershock Study (Abstract), Eos. Trans., AGU,

[26] Fan, G, Beck, S, & Wallace, T. The Seismic Source Parameters of the 1991 Costa Rica Aftershock Sequence: Evidence for a Transcurrent Plate Boundary. J. Geoph Res. 98,

[27] Fernández, J, Botazzi, G, Barboza, G, & Astorga, A. Tectónica y estratigrafía de la Cuenca Limón Sur. Rev. Geol. Amér. Central, v. esp. Terremoto de Limón: (1994). ,

[28] Fernández, M. Análisis sísmico en la parte central de Costa Rica y evaluación del hi‐ potético sistema de falla transcurrente de Costa Rica, Tesis de maestría, Universidad

[29] Fernandez, M, Camacho, E, Molina, E, Marroquin, G, & Strauch, W. Seismicity and neotectonic of Central America, in: Bundschuh, J., Alvarado, G. (eds), Central Ameri‐ ca- Geology, Resource and Hazards; Taylor & Francis Customerr Services, Andover,

[30] Fernandez, M, Escobar, D, & Redondo, C. Seismograph Networks and seismic obser‐ vation in El Salvador and Central America, Geological Society of America Special Pa‐

[31] Fernandez, M, & Montero, W. Fallamiento y Sismicidad del Area entre Cartago y San José, Valle Central de Costa Rica, Rev. Geol. Amer. Central, (2002). , 26, 25-37.

[32] Fernández, M, Mora, M, & Barquero, R. Los procesos sísmicos del Volcán Iraza, Rev.

[33] Fernandez, M. Seismicity of the Pejibaye-Matina, Costa Rica, region: a strike-slip tec‐

[34] Fisher, D, Gardner, T, Marshall, J, & Montero, W. Kinematics associated with late Cenozoic deformation in central Costa Rica: Western boundary of the Panama micro‐

[35] Fisher, D. M, & Gardner, T. W. Tectonic escape of the Panama microplate: Kinemat‐ ics along the western boundary, Costa Rica: Geological Society of America, Abstracts

[36] Fuh, S, Liu, C, Lundberg, N, & Reed, D. Strike-slip faults offshore southern Taiwan: implications for the oblique arc-continent collision processes, Tectonophysics,

[37] Gardner, T. W, Fisher, D. M, & Marshall, J. S. Western boundary of the Panama mi‐ croplate, Costa Rica: Geomorphological and structural constraints: International As‐ sociation of Geomorphologists, 3rd International Geomorphology Conference,

tonic boundary?, Geofisica Internacional, 48(4), 351-364, (2009).

Nacional Autónoma de México (UNAM), 85 , 1995.

73, 345, (1992).

15-28.

B9: 15,759-15,778, (1993).

100 Earthquake Research and Analysis - New Advances in Seismology

United Kingdom, 1340 , 2007.

Geol. América Central, (1998). , 21, 47-59.

plate. Geology, 22, 3: 263-266, (1994).

with Programs, (1991). , 23, A198.

(1997). , 274, 25-39.

per (2004). , 375, 257-267.


[50] Lundgren, P, Wolf, S, Protti, M, & Hurst, K. GPS meaSurements of cristal deforma‐ tion associated with the April 22, Valle de la Estrella, Costa Rica earthquake. Geo‐ phys. Res. Letters, (1993). , 20(5), 407-410.

[62] Montero, W, Camacho, E, Espinosa, A, & Boschini, I. Sismicidad y marco neotectóni‐ co de Costa Rica y Panamá. Rev. Geol. Amér. Central, v. espec., terremoto de Limón,

Seismotectonic and the Hipothetical Strike – Slip Tectonic Boundary of Central Costa Rica

http://dx.doi.org/10.5772/54989

103

[63] Montero, W, & Dewey, J. W. Shallow-focus seismicity, composite focal mechanism, and tectonic of the Valle Central de Costa Rica. Seis. Soc. Amer. Bull, (1982). , 72 [64] Montero, W. El sistema de falla Atirro-Río Sucio y la cuenca de tracción de Turrialba-Irazú: Indentación tectónica relacionada con la colisión del levantamiento del Coco,

[65] Montero, W. El terremoto del 4 de marzo de 1924 (Ms 7,0): ¿un gran temblor interpla‐ ca relacionado al límite incipiente entre la placa Caribe y la microplaca Panama. Rev

[66] Montero, W, & Morales, L. D. Zonificación sísmica del Valle Central. Memorias del

[67] Montero, W, & Morales, L. Sismotectónica y niveles de actividad de microtemblores en el suroeste del Valle Central, Costa Rica, Revista Geofísica, 21: 21-41, (1984).

[68] Montero, W. Neotectonica de la región central de Costa Rica: frontera oeste de la mi‐

[69] Montero, W. Niveles de actividad de microtemblores en el sureste del Valle Central,

[70] Norabuena, E, Dixon, T, Schwart, S, Deshon, H, Newman, A, Protti, M, Gonzalez, V, Dorman, L, Flueh, E, Lundgren, P, Pollitz, F, & Sampson, D. Geodetic, and seismic constraints on some seismogenic zone processes in Costa Rica, J. Geophys. Res.

[71] Pacheco, J, Quintero, R, Vega, F, Segura, J, Jiménez, W, & González, V. The Mw 6.4 Damas, Costa Rica, Earthquake of 20 November 2004: Aftershock and Slip Distribu‐

[72] Protti, M, Guendel, F, & Mcnally, K. The geometry of the Wadati-Benioff zone under southern Central America and its tectonic significance: results from a high-resolution

[73] Protti, M, Schwarts, S. Mechanics of back arc deformation in Costa Rica: Evidence from an aftershock study of the April 22, 1991, Valle de la Estrella, Costa Rica, earth‐

[74] Protti, M, Schwartz, S, & Zandt, G. Simultaneous inversion for earthquake location and velocity structure beneath central Costa Rica, Seis. Soc. Amer. Bull., (1996). ,

[75] Protti, M. The Most Recent Large Earthquakes in Costa Rica (1990 Mw 7.0 and 1991 Mw 7.6) and Three-dimensional Crustal and Upper Mantle P-wave Velocity Struc‐

local seismographic network, Phys. Earth Planet. Inter., (1994). , 84, 271-287.

croplaca Panama. Rev. Geológica de Amer. Central, (2001). , 24, 29-56.

(1994). , 73-82.

Rev. Geol. Amer. Centr., (2003). , 28, 05-29.

Geológica de Amer. Central, (1999). , 22, 25-62.

Costa Rica, Revista Geofísica 10-11: 105-115, (1979).

tion, Bull. Seism. Soc. America N 4, doi:(2006). , 96

quake (Mw = 7.7). Tectonics, N. 5: 1093-1107 , 13, 1994.

B11403, 1-25, (2004). , 109

86(1A), 19-31.

4\_ Seminario de Ingeniería Estructural, San José, CR, (1988).


[62] Montero, W, Camacho, E, Espinosa, A, & Boschini, I. Sismicidad y marco neotectóni‐ co de Costa Rica y Panamá. Rev. Geol. Amér. Central, v. espec., terremoto de Limón, (1994). , 73-82.

[50] Lundgren, P, Wolf, S, Protti, M, & Hurst, K. GPS meaSurements of cristal deforma‐ tion associated with the April 22, Valle de la Estrella, Costa Rica earthquake. Geo‐

[51] Mann, P, Schubert, C, & Burke, K. Review of the Caribbean neotectonic, in The Carib‐ bean region, The Geology of North America, vol., H, edited by G. Dengo, and J. Case,

[52] Marshall, J, & Anderson, R. Quaternary uplift and seismic cycle deformation, Penín‐

[53] Marshall, J. S. Evolution of the Orotina debris fan, Pacific coast, Costa Rica: Late Cen‐ ozoic tectonism along the western boundary of the Panama microplate: Geological

[54] Marshall, J. S, Fisher, D. M, & Gardner, T. W. Central Costa Rica deformed belt: Kine‐ matics of diffuse faulting across the western Panama block, Tectonics, (2000). , 19,

[55] Marshall, J. S, Fisher, D. M, & Gardner, T. W. Western margin of the Panama micro‐ plate, Costa Rica: Kinematics of faulting along a diffuse plate boundary: Geological

[56] Marshall, J. S, Gardner, T. W, & Fisher, D. M. Active tectonics across the western Car‐ ibbean-Panama boundary and the subducting rough-smooth boundary, Pacific coast, Costa Rica: Geological Society of America, Abstracts with Programs, (1995). , 27,

[57] Marshall, J. S. LaFromboise, E.J., Utick, J.D., In the wake of flat subduction: Upperplate tectonics across a steep to flat slab transition, Pacific margin, Costa Rica, Cen‐ tral America: Backbone of the Americas, Patagonia to Alaska, 3-7 April 2006, Mendoza, Argentina, GSA Specialty Meetings Abstracts with Programs, Abs. 3-12,

[58] Matumoto, T, Othake, M, Lathan, G, & Umaña, J. Crustal structure of southern Cen‐

[59] Ministerio de Industria, Energía y Minas (MIEM). Dirección de Geología, Minas y Petróleo, Mapa geológico de Costa Rica. Escala 1:200.000. San José, Costa Rica, (1982).

[60] Ministerio del Ambiente, Energía y Minas (MINAE). Dirección Superior de Geología, Minas e Hidrocarburos, Mapa geológico de Costa Rica. Escala 1:500.000. San José,

[61] Montero, W, Neotectonics and related stress distribution in a subduction- collisional

tral America, Bull. Seismol. Soc. Am., 67: 1:121-134, (1977).

zone: Costa Rica, Profil: Stuttgart, (1994). , 125-141.

Geological Society of America, Boulder, Colorado, (1990). , 375-391.

sula de Nicoya, Costa Rica. GSA Bulletin, (1995). , 107(4), 463-473.

Society of America, Abstracts with Programs, (1994). , 26(7), A207.

Society of America, Abstracts with Programs, (1993). , 25(6), A284.

phys. Res. Letters, (1993). , 20(5), 407-410.

102 Earthquake Research and Analysis - New Advances in Seismology

468-492.

A124.

(2006). (2), 38.

Costa Rica, (1997).


ture of Central Costa Rica, Ph.D. dissertation, University of California, Santa Cruz, 116 , 1994.

[88] Von Huene, R, Ranero, C, & Watts, P. Tsunamigenic slope failure along the Middle America Trench in two tectonic settings. Marine Geology, (2004). , 203, 303-317. [89] White, R, & Harlow, D. Destructive Upper-Crustal Earthquakes of Central America

Seismotectonic and the Hipothetical Strike – Slip Tectonic Boundary of Central Costa Rica

http://dx.doi.org/10.5772/54989

105

[90] White, R. Tectonic inplications of upper-crustal seismicity in Central America, In: Slemmons, D., Engdahl, E., Zoback, M., Blackwell, eds, Neotectonics of North Ameri‐

[91] Woodward-Clyde: A preliminary evaluation of earthquake and volcanic hazards sig‐ nificant to the major populations centers of the Valle CentralCosta Rica. Final Report

[92] Yao, Z, Quintero, R, & Roberts, R. Tomographic Imaging of P- and S- wave velocity

[93] Zhu, L, & Helmberger, D. Advancement in source estimation technique using broad‐ band regional seismograms. Bull. Seism. Soc. Am., (1996). , 86, 1634-1641.

structure Veneta Costa Rica. Journal of Seismology (1999). , 3, 177-190.

ca, Boulder Colorado, Geological Society of America, Decade Map (1991). , 1

prepared for Ret Corporation, San José, Costa Rica, (1993).

Since 1900. Bull. Seims. Soc. Am., (1993). , 83


[88] Von Huene, R, Ranero, C, & Watts, P. Tsunamigenic slope failure along the Middle America Trench in two tectonic settings. Marine Geology, (2004). , 203, 303-317.

ture of Central Costa Rica, Ph.D. dissertation, University of California, Santa Cruz,

[76] Quintero, R, & Guendell, F. Stress Field in Costa Rica, Central America, Journal of

[77] Ranero, C, & Von Huene, R. Subduction erosion along the Middle America conver‐

[78] Rojas, W. Catálogo de sismicidad histórica y reciente en América Central: Desarrollo y Análisis. Tésis de Licenciatura en Geología, Universidad de Costa Rica, 91 , 1993.

[79] Rojas, W, Montero, W, Soto, G. J, Barquero, R, Boschini, I, Alvarado, G. E, & Vargas, A. Contexto geológico y tectónico local, sismicidad histórica y registro sísmico instru‐ mental, In: Barquero, R. (Ed.): El terremoto de Cinchona, 8 de enero de 2009. Inf. In‐

[80] Seyfried, H, Astorga, A, Hubert, A, Calvo, C, Wolfgang, K, Hannlore, S, & Jutta, W. Anatomy of an evolving Island Arc: tectonic and eustatic control in the south Central American forearc area, in: McDonald, D.I.M (Ed.): Sea level Changes at active plate margins: Processes and Products. Spec. Publs. Int Assoc. Sediments, (1991). , 12,

[81] Snoke, J. A, Munsey, J, Tiague, W, & Bollinger, A. C. G. A., a program for focal mech‐ anism determinations by combined use of polarity and SV-P amplitude ratio data,

[82] Soulas, J. Tectonica activa, informe de mision de consultuoria P. H. Siquirres, Institu‐

[83] Stoiber, R, & Carr, M. Quaternary volcanic and tectonic segmentation of Central

[84] Suárez, G, Pardo, M, Domínguez, J, Ponce, L, Montero, W, Boschini, I, & Rojas, W. The Limón, Costa Rica, earthquake of April 22, 1991: Back arc thrusting and collision‐

[85] Tournon, J, & Alvarado, G. Carte géologique du Costa Rica: notice explicative; Mapa geológico de Costa Rica: folleto explicativo, échelle-escala 1 500 000.-Ed. Tecnológica

[86] Trenkamp, R, Kellog, J, Freymueller, J, & Mora, H. Wide plate margin deformation, southern Central America and Northwestern South America, CASA GPS observa‐

[87] Van Andel, T. H, Heath, G. R, Malfait, B. T, Heinrichs, D. F, & Ewing, J. I. Tectonics of the Panama Basin, eastern equatorial Pacific. Geological Society of America Bulle‐

al tectonics in a subduction environment. Tectonics, (1995). , 14(2), 518-530.

to Costarricense de Electricidad (ICE), Internal report, (1989).

de Costa Rica, 80 pp. + Mapa geológico de Costa Rica, (1997).

tions, Journal of South American Earth Sciences, (2002). , 15, 157-171.

116 , 1994.

Seismology, (2000). , 4, 297-319.

104 Earthquake Research and Analysis - New Advances in Seismology

terno RSN: (2009). , 7-33.

earthquakes, 55(3): 15., (1984).

tin, (1971). , 82, 1489-1508.

America: Bull. Volc. (1973). , 37(3), 304-323.

217-240.

gent margin, Nature (2000). , 404, 748-752.


**Chapter 5**

**Modeling Dynamic-Weakening and**

**High-Velocity Slip Experiments**

Additional information is available at the end of the chapter

Earthquakes are associated with slip along fault-zones in the crust, and the intensity of dynamic-weakening is one of the central questions of earthquake physics (Dieterich, 1979; Reches and Lockner, 2010). Since it is impossible to determine fault friction with seismological methods (Kanamori and Brodsky, 2004), the study of fault friction and earthquake weakening has been usually addressed with laboratory experiments (Dieterich, 1979) and theoretical

The experimental analyses of dynamic-weakening were conducted in several experimental configurations: bi-axial direct shear (Dieterich, 1979; Samuelson et al, 2009), tri-axial confined shear (Lockner and Beeler, 2002), and rotary shear apparatus (Tsutsumi and Shimamoto, 1997; Goldsby and Tullis, 2002; Di Toro et al., 2004; Reches and Lockner, 2010). The direct shear apparatus allows high normal stress and controlled pore water pressure (up to ~200 MPa) with limited slip velocity (up to 0.01 m/s) and limited slip distance (~10 mm) (Shimamoto and Logan, 1984). These slip velocities and displacements are significantly smaller than those of typical earthquakes (0.1-10 m/s and up to 5 m, respectively). In order to study high velocity and long

While many studies indicated a systematic weakening with increasing slip-velocity (Dieterich, 1979; Di Toro et al., 2011), recent experimental observations revealed an opposite trend of dynamic-strengthening particularly under high velocity (Reches and Lockner, 2010; Kuwano and Hatano, 2011). This strengthening was attributed to dehydration of the fault gouge due to frictional heating at elevated velocities (Reches and Lockner, 2010; Sammis et al., 2011). If

> © 2013 Liao and Reches; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2013 Liao and Reches; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

slip distance, experiments have been conducted in rotary shear machines.

Zonghu Liao and Ze'ev Reches

models (Ohnaka and Yamashita, 1989).

http://dx.doi.org/10.5772/54889

**1. Introduction**

**Dynamic-Strengthening of Granite in**

**Chapter 5**
