**8. Concluding remarks**

to what is observed at the station CITT. On the other hand, the noise rose diagrams obtained at the other recording sites point out polarization azimuths that seem to be in agreement with the slope directions of the hill flanks. Only few sites (#17, #21, #22, #25) make an exception to such trend, showing a directional variability that could be linked to the local shallow lithologic features. The investigation on the characteristics of the site response at the S. Sofia hill, therefore set into evidence that the complexity of the near-surface geology, as well as the morphology strongly influence the local amplification of the ground motion and the directional effects. Findings of the present study confirm that major amplification effects do indeed take place on the sedimentary terrains which outcrop along the flanks of the hill. On the contrary, on the lava flows, a significant amplification of the vertical component of motion, is observed as a

**Figure 25.** Examples of HVNR results at representative recording sites located on lava flows (a) and on sedimentary terrains (b); solid black lines refer to the average H/V spectra, dotted grey and black lines refers to NS/V and EW/V

The results coming out from investigations performed in the two test areas allow us to draw

**•** Ambient noise is a useful tool that can be widely adopted in site response evaluation and in studies of topographic irregularities as well, especially when a simplified topography and lithology is present. However, in the presence of lateral and vertical heterogeneities, as well as velocity inversions, a combined use of noise, earthquake data and theoretical models is advisable to correctly predict the site response behavior. In such instances topographic

**•** The wedge angle of the hill appears to play an important role since a wide angle reduces

**•** Both the directional resonance and the polarization analysis confirm the presence of a directional effect transverse to the major axis of the ridge. This behavior is particularly evident in reliefs having a homogeneous simple convex morphology. In such instance the ridge oscillation can be considered similar to what is observed in civil structures, providing there is not significant soil-structure interaction and considering the building as a single-

consequence of velocity inversion effects.

spectra, respectively.

the following general considerations about topographic effects:

136 Engineering Seismology, Geotechnical and Structural Earthquake Engineering

effects are more composite and a complex wavefield is observed.

the focusing effects at the crest in favor of laterally propagating waves.

The present study has tested the use of ambient noise recordings as a speedy technique for evaluating the local seismic response in several instances where either lithologic and/or morphologic and structural features can significantly affect the response of shallow geologic formations to a seismic input. Our findings further support the reliability of the use of ambient noise recordings for preliminary characterization of dynamic properties of terrains. Its employment has, in recent years, been widely used for site amplification studies since data acquisition time and costs are significantly reduced. Moreover, the HVNR technique can be largely adopted since it requires only one mobile seismic station with no additional measure‐ ments at rock sites for comparison. The ratio between the horizontal and vertical spectral components of motion can indeed reveal the fundamental resonance frequency of the site. It also does not require the long and simultaneous deployment of several instruments to collect a useful set of earthquake data.

[5] Baratta, M. (1910). La catastrofe sismica Calabro-Messinese (28 dicembre 1908), *Soci‐*

Speedy Techniques to Evaluate Seismic Site Effects in Particular Geomorphologic Conditions: Faults, Cavities,

Landslides and Topographic Irregularities http://dx.doi.org/10.5772/55439 139

[6] Bard, P. Y. (1994). Effects of surface geology on ground motion: recent results and re‐ maining issues. In: *G. Duma (ed), Proc. 10th European Conference on Earthquake Engineer‐*

[7] Bard, P. Y. (1999). Microtremor measurements: a tool for site effect estimation? In: *Irikura et al. (ed.), The effects of surface geology on seismic motion. Balkema, Rotterdam*, ,

[8] Bard, P. Y, & Riepl-thomas, J. (1999). Wave propagation in complex geological struc‐ tures and their effects on strong ground motion. In *Wave motion in Earthquake Engi‐*

[9] Bardet, J. P, Ichii, K, & Lin, C. H. (2000). EERA, a computer program for Equivalentlinear Earthquake site Response Analyses of layered soil deposits. *University of South‐*

[10] Ben-zion, Y, Peng, Z, Okaya, D, Seeber, L, Armbruster, J. G, Ozer, N, Michael, A. J, Baris, S, & Aktar, M. (2003). A shallow fault-zone structure illuminated by trapped waves in the Karadere-Duzce branch of the North Anatolian Fault, western Turkey.

[11] Ben-zion, Y, & Sammis, C. G. (2003). Characterization of fault zones, Pure Appl. Geo‐

[12] Ben-zion, Y, & Sammis, C. G. (2009). Mechanics, Structure and Evolution of Fault Zones, Pure appl. Geophys.,*166*doi:10.1007/s00024-009-0509-y., 1533-1536.

[13] Bindi, D, Parolai, S, & Cara, F. Di Giulio G., Ferretti G., Luzi L., Monachesi G., Pacor F., Rovelli A. ((2009). Site amplifications observed in the Gubbio Basin, Central Italy:

[14] Boore, D. M. (1973). The effect of simple topography on seismic waves: implications for the accelerations recorded at Pacoima Dam, San Fernando valley, California. *Bull.*

[15] Boore, D. M, Graizer, V. M, Tinsley, J. C, & Shaka, A. F. (2004). A study of possible ground-motion amplification at the Coyote Lake dam, California. *Bull. Seism. Soc.*

[16] Borcherdt, R. D. (1970). Effects of local geology on ground motion near San Francisco

[17] Bouchon, M. (1973). Effect of topography on surface motion. *Bull. Seism. Soc. Am.*, ,

Hints for lateral propagation effects. *Bull. Seism. Soc. Am.*, 99(2A), , 741-760.

*ing. Wien, 28 Aug.-2 Sept., Balkema, Rotterdam*, , 1, 305-323.

*neering*: edited by Kausel and Manolis, WIT Press, , 2, 37-95.

*ern California, Department of Civil Engineering, user's manual*, 38p.

*età Geografica Italiana, Rom, 458 p*.

1251-1279.

*Geophys. J. Int*., , 152, 1-19.

*Seism. Soc. Am.*, 63(5), 1603-1609.

Bay. *Bull. Seism. Soc. Am.*, , 60, 29-61.

*Am.*, doi:l., 94(4), 1327-1342.

63, 615-632.

phys., , 160, 677-715.

Our results show also the importance of performing analysis to evaluate directional effects and polarization of the horizontal components of the ground motion. It has indeed to be remembered that directional effects cannot be neglected for a correct planning of edifices and man-made structures in order to reduce the potential risk of building damage as a result of ground motions.
