**4. Hypocentral relocation**

172 Earthquake Research and Analysis – Seismology, Seismotectonic and Earthquake Geology

Fig. 4. Horizontal depth planes of the 3-D Vp (left) and Vp/Vs (right) models, at selected depths. Triangles: seismic stations, crosses: grid nodes, dots: epicenters within 1 km vertical

range of the plane. Lateral XY coordinates according to the 3-D grid of Fig. 3.

The accurate determination of an earthquake hypocenter parameters, that is geographical coordinates, focal depth and time of occurrence, requires some knowledge on the crustal structure between the seismic focus and the receiving station. With the knowledge of such structure, usually parameterized in a seismic velocities model, the space-time coordinates of an event can be determined from the observed arrival moments of the seismic waves recorded at several stations (inverse problem). Depending on the specific structure of the used velocity model, the hypocentral location and especially the determination of the focal depth can undergo major changes.

Regarding the aftershocks of the July 9th 1998 earthquake, the preliminary location made by SIVISA (cf. Fig. 1) was carried out using model MAC (Hirn et al., 1980; Senos et al., 1980). MAC is a 1-D model developed with the purpose of locating all offshore seismicity in the Azores region, and reflects the average crustal structure of the plateau. The determination of the new 1-D model of the Faial-Pico-São Jorge area and of the tomographic 3-D model automatically entails a revision of the seismic hypocentral parameters used in the modelling.

In mathematical terms, the relocations by the new 1-D and 3-D models leads to substantial reduction of the data root mean square (RMS): relatively to the MAC model the new 1-D model allows for a reduction of 63% of the RMS, from 0,286 s to 0,103 s, while the use of the 3-D model leads to a further reduction of 32% of this parameter reaching 0,070 s (75.5% compared to the MAC model). In spatial terms, the changes entailed by the 1-D modelling generally do not exceed 2 km shifts of the epicentral position (cf. Fig. 5), with an additional 1 km in the tomographic inversion. The changes in focal depths are more relevant because they reduce the thickness of the fragile layer to the upper 15 km (22 km with the MAC model).

Fig. 5. Left: Comparison between the original epicenter locations (blue) of the 692 events with MAC model and the revised epicenter locations (red) with the new 1-D model FAIAL98. Right: Relocated events following the tomographic inversion; circles represent the final positions obtained with the 3-D model, line segments the position shift.

In terms of the seismicity spatial organization following the relocation (cf. Fig. 5), both models revealed two main alignments compatible with the two nodal planes of the focal mechanism of the main earthquake (cf. Fig. 1): one striking NNW-SSE, containing the majority of the aftershocks, and a secondary one with ENE-WSW direction. Most of the seismicity occurs in the Faial-Pico channel area, the inland events being less relevant both in terms of occurrence rates and magnitudes (Dias, 2005). This inland seismicity presents a NW-SE alignment, especially in Faial.

#### **5. Seismic anisotropy and crustal stress**

Seismic anisotropy is a three-dimensional phenomenon that takes different forms, corresponding to the variation in the wave's propagation velocity with azimuth and eventual splitting of an S-wave into several pulses (birefringence). The interpretation of the crustal seismic anisotropy is sometimes carried out according to the Extensive Dilatancy Anisotropy model (Crampin et al. 1984), which states that as result of a crustal stress field, a initially isotropic homogeneous medium undergoes micro-fracturing (or previously existing micro-fractures are reoriented), the assumed plane fractures adjusting to directions roughly parallel to the direction of maximum horizontal stress (and perpendicular to the direction of minimum horizontal stress). The presence of fluids in the crust (H2O, CO2) usually leads to the filling up of these micro-fractures, changing the propagation properties according to direction. An S-wave with sub-vertical incidence in such medium will split into two orthogonally polarized quasi-S-waves, propagating with different velocities due to the variations of the mechanical properties in the parallel and perpendicular directions to the micro-fractures. As a result seismic anisotropy will be observed, its level being evaluated from the measurement of the time difference between the arrivals of these two S-waves.

Following the method of Bouin et al. (1996) 438 events were selected, the analysis of their records suggesting signs of anisotropy. Although the 3D revised hypocenters solutions are more accurate, the error involved in the measurement of the splitting directions allows using the simpler 1D approach: the already referred sampling rate of 62.5 Hz, for the data stored for the majority of the digital temporary stations, coupled with the difficulties in the north alignment of the seismic sensors in basaltic oceanic islands, outmanoeuvre the accuracy of using 3D locations instead of 1D. On the other hand, most of the programs available for crustal stress modelling are 1D based (Robinson & McGinty, 2000).

The detection and quantification of such anisotropy was possible only in some of the digital stations located on the islands of Faial and Pico (cf. Fig. 6). In each station, the presence of anisotropy in the S-wave window records was shown by a systematic polarization of the first pulse of this kind of wave, sometimes followed by a second pulse showing a roughly orthogonal polarization. Figure 6 represents the results obtained for each station, with the rose diagrams representing the statistical direction of polarization of the first (blue) and second (red) S-waves. In a general, the observed polarization is very stable and independent of the epicentral distribution, ranging from approximately NW-SE direction in the northern Faial and north-western Pico to a significantly orthogonal WSW-ENE direction in the eastern area of Faial island.

In terms of the seismicity spatial organization following the relocation (cf. Fig. 5), both models revealed two main alignments compatible with the two nodal planes of the focal mechanism of the main earthquake (cf. Fig. 1): one striking NNW-SSE, containing the majority of the aftershocks, and a secondary one with ENE-WSW direction. Most of the seismicity occurs in the Faial-Pico channel area, the inland events being less relevant both in terms of occurrence rates and magnitudes (Dias, 2005). This inland seismicity presents a

Seismic anisotropy is a three-dimensional phenomenon that takes different forms, corresponding to the variation in the wave's propagation velocity with azimuth and eventual splitting of an S-wave into several pulses (birefringence). The interpretation of the crustal seismic anisotropy is sometimes carried out according to the Extensive Dilatancy Anisotropy model (Crampin et al. 1984), which states that as result of a crustal stress field, a initially isotropic homogeneous medium undergoes micro-fracturing (or previously existing micro-fractures are reoriented), the assumed plane fractures adjusting to directions roughly parallel to the direction of maximum horizontal stress (and perpendicular to the direction of minimum horizontal stress). The presence of fluids in the crust (H2O, CO2) usually leads to the filling up of these micro-fractures, changing the propagation properties according to direction. An S-wave with sub-vertical incidence in such medium will split into two orthogonally polarized quasi-S-waves, propagating with different velocities due to the variations of the mechanical properties in the parallel and perpendicular directions to the micro-fractures. As a result seismic anisotropy will be observed, its level being evaluated from the measurement of the time difference between

Following the method of Bouin et al. (1996) 438 events were selected, the analysis of their records suggesting signs of anisotropy. Although the 3D revised hypocenters solutions are more accurate, the error involved in the measurement of the splitting directions allows using the simpler 1D approach: the already referred sampling rate of 62.5 Hz, for the data stored for the majority of the digital temporary stations, coupled with the difficulties in the north alignment of the seismic sensors in basaltic oceanic islands, outmanoeuvre the accuracy of using 3D locations instead of 1D. On the other hand, most of the programs available for crustal stress modelling are 1D based (Robinson &

The detection and quantification of such anisotropy was possible only in some of the digital stations located on the islands of Faial and Pico (cf. Fig. 6). In each station, the presence of anisotropy in the S-wave window records was shown by a systematic polarization of the first pulse of this kind of wave, sometimes followed by a second pulse showing a roughly orthogonal polarization. Figure 6 represents the results obtained for each station, with the rose diagrams representing the statistical direction of polarization of the first (blue) and second (red) S-waves. In a general, the observed polarization is very stable and independent of the epicentral distribution, ranging from approximately NW-SE direction in the northern Faial and north-western Pico to a significantly orthogonal WSW-ENE direction in the

NW-SE alignment, especially in Faial.

the arrivals of these two S-waves.

McGinty, 2000).

eastern area of Faial island.

**5. Seismic anisotropy and crustal stress** 

Fig. 6. Epicenters of the 438 selected events together with rose diagrams (10º intervals) of the polarizations directions, as measured in each station, of the fast (blue) and slow (red) Swaves. The size of each station symbol is proportional to the respective quantified anisotropy level, with the mean polarization direction of the fast S-wave projected over each station.

To relate the crustal stress state in the area affected by the seismic crisis, there are two seismologic indicators to determine the direction of maximum horizontal stress, SHmax or σ1: the analysis of the polarization of the S-wave and the computation of focal mechanisms. The markers associated with focal mechanisms (usually the direction of the P axis) reflect the state of stress in the source area, while the direction of polarization of the first wave associated with the birefringence of S-waves reflects the direction of maximum horizontal stress in the shallower structure located beneath the station. In case of focal mechanisms, single or composed, the estimate of SHmax is made according to the criteria defined by Zoback (1992), which relates the orientation of tension T or pressure P axis with the direction of SHmax.

To calculate single focal mechanisms, 18 events were selected with a minimum of 11 polarities for the P-wave, the hypocentral location used in calculating the focal parameters obtained from the 1-D model FAIAL98 (Matias et al., 2007). In the case of composite focal mechanisms, a similarity analysis of the recorded waveforms was performed by crosscorrelation, which established a classification of "similar" earthquake clusters; subsequently, the joint focal mechanism was calculated for the 16 more numerous and stable clusters (Dias, 2005). The compilation of these results is represented in Figure 7 together with the estimated SHmax direction for both indicators.

Fig. 7. Left: Aftershocks distribution of 9-7-1998 main shock, for events recorded by at least - 4 stations readings and relocated with the 1-D model FAIAL98; single (black) and composite (grey) focal mechanisms, together with the Harvard-CMT solution for the main shock. Right: maximum horizontal stress (SHmax) directions obtained from focal mechanisms and S-waves polarization analysis. The segments length is proportional to the quality of the stress measurement following Zoback (1992). The regime indicates the focal mechanism type: FN - normal fault, FD - stryke-slip fault, FI – inverse fault, DI/DN –oblique fault with inverse/normal component. Batimetry of Lourenço et al. (1998).

Figure 7 shows two dominant almost orthogonal general orientations for the maximum horizontal stress, N220ºE -N260°E and N90ºE-N130°E, limited to two distinct areas, with an apparent sharp transition of SHmax between them. As this indicator corresponds to the horizontal projection (i.e. two-dimensional) of a three-dimensional crustal stress vector, this sharp transition may be apparent, since the horizontal projection of T and P axis of the focal mechanisms suggests a continuous rotation (albeit fast) in the orientation of these axis (Dias, 2005).

The orientations obtained for the dominant polarization directions show some correlation with the tectonic alignments of Faial and Pico, which could suggest a tectonic control of the crustal fracturing near the stations. The observed anisotropy is consistent with the presence beneath the stations of anisotropy of the type envisaged by the EDA model, with the crack planes parallel to the direction of maximum horizontal stress. The major uncertainty is related with the depth extension of the anisotropic structures. The estimated directions for the maximum horizontal stress (SHmax), obtained from single and composed focal mechanisms solutions, is around 50º-80º in the NE zone of the Faial island, and a SHmax direction of around 130º-140º for the NW area of Pico island. The different orientations in the polarization direction obtained for the stations located in the north of Faial appears to be also related with the effect observed in the macroseismic effects (substantial mitigation of intensities) and the blocking of the progression of seismic activity to NW (Matias et al., 2007).
