**7. Conclusion**

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

Fig. 8. Induced changes in Coulomb failure stress (CFS) due to the main shock and its comparison to aftershock distribution. The CFS was computed for 6 reference depths: 1, 3, 5, 7, 9 and 11 km. The aftershocks were plotted according to the depth ranges. The assumed

main shock source plane is indicated in each figure.

The results obtained from the 1-D and 3-D inversions allow an insight of the crustal structure of the Azorean plateau region, primarily in the Central Group area between the islands of Faial and Pico. In terms of crustal thickness, the 1-D model points to a Moho located at 12.5 km depth while the 3-D model, though not allowing an evident definition in the velocities transitions, corroborates this suggestion with the Vp = 7.8 km/s isoline fluctuating around 14 km depth. Although this crustal thickness is higher than those determined in other studies, pointing to values around 8-11 km (Luis et al., 1998; Miranda et al., 1998; Luis & Neves, 2006), these works were performed at the plateau scale or in offshore areas while the models presented here necessarily include the structure of the islands (where the stations are located), thus justifying a further thickness increase of about 2-3 km. Additionally, a crustal thickness of 14 km fits better the estimated parameters for the largest instrumental earthquakes, both in terms of the sources estimated geometry and the correlation with the recorded magnitudes (Madeira et al., 2003).

The resulting distribution of seismicity indicates that for the main observable tectonic structures, the most active corresponds to the one of NNW-SSE orientation with the focal mechanisms indicating a dominant left strike-slip movement, with a secondary WSW-ENE direction of dominant right strike-slip. The NW-SE structures have a minor role, mainly confining seismicity.

The presence of seismic anisotropy was detected beneath the seismic stations (cf. Fig. 6), compatible with the EDA model, with a correspondence between the fracturing index and the positioning of each station. The estimated orientation for the direction of maximum horizontal stress from the S-wave polarization analysis, combined with single and composite focal mechanisms solutions, indicates a complex pattern in the crustal stress paths rotating from a general NW-SE direction to NE-SW in the eastern section of the island of Faial and extending into the sea. The comparison with the crustal stress distribution at the shallow layer of 1 km depth (cf. Fig. 8), shows that the measured direction of polarization of the first (fast) S-wave appear to follow the lobules shape, at least in northern Faial (RIBE, PCED, SAL stations) and NW Pico island (SET station). As for HOR station, in SE Faial island, is inside a low stress lobule and the polarization direction appears not to be correlated with the stress distribution although as it can be seen from Fig. 6, this station shows a bigger fluctuation in the measurements of the first polarization.

Comparing the distribution of seismicity and the rotation of SHmax direction (Fig. 5 and 7) with the tomographic model (Fig. 4), there is an apparent tectonic control by the highvelocity intrusion lying NE of Faial. It is essentially a crustal asseismic area enclosed by more active seismogenic areas, its form "conditioned" by the distribution of seismicity; in addition, the rotations in the directions of SHmax appeared to be sub-paralleled to the limits of this anomaly. Comparing with the Coulomb failure stress (cf. Fig. 8) planes coincident with the location of the main seismicity (layers between km depth) the SHmax directions show good correlation with the lobules shape, namely the rotation observed between NNW-SSE and WSW-ENE directions. The main exceptions appear to be the shallow seismicity observed inland in NE Faial island, in the area of Faial Graben.

The results shown here suggest a crustal thickening due to an increase in thickness of layer 3, the 1-D model presenting a total thickness of 9.5 km. This indicates that the accretion of the crust in the Azores plateau was essentially due to a relatively high magmatic supply rate, which cooled and crystallized at depth, with the rate of extruded volcanism though important playing a small role in the crustal build-up. The source of such high magmatism is still a matter under debate, maybe the result of the actual presence of a "hot-spot" in the Azores or, alternatively, be only the result of the condition of the plateau as a tectonic triple junction.

The mentioned presence of a high seismic waves velocity volume, interpreted as a crystalline intrusion of mafic (Gabbro) and ultra-mafic rocks, apparently constraining the observed seismicity and stress state, appears to corroborate the last assumption. Under the main volcanic edifices of Faial and Pico there is no clear signal of the presence of magmatic chambers, but this maybe result of to the poor coverage of seismic paths; the presence of a low-velocity anomaly is revealed, but this may be due only to the effect of a volcanic feeding system and not the presence of a magmatic chamber, something that concurs with the suggestion of Nunes et al. (2006) of a lack of magmatic chamber under Pico's volcano.

#### **8. Acknowledgments**

We would like to acknowledge the effort of all the people involved in the seismic monitoring campaign that took place in the Azores during July 1998, namely, C. Corela, A. Carvalho, and J. Simões. The figures were produced using the GMT software (Wessel and Smith 1991) and SEISAN software (Havskov and Ottemöller 1999).This work was supported by the Portuguese Fundação para a Ciência e Tecnologia, under projects MASHA (POCTI/CTA/39158/2001) and STAMINA (PDCTM1999MAR15255). Nuno A. Dias had a fellowship under the PRODEP III program, Action 5.3. This is a contribution to PEST-OE/CTE/LA0019/2011 – IDL.

#### **9. References**


volcanism though important playing a small role in the crustal build-up. The source of such high magmatism is still a matter under debate, maybe the result of the actual presence of a "hot-spot" in the Azores or, alternatively, be only the result of the condition

The mentioned presence of a high seismic waves velocity volume, interpreted as a crystalline intrusion of mafic (Gabbro) and ultra-mafic rocks, apparently constraining the observed seismicity and stress state, appears to corroborate the last assumption. Under the main volcanic edifices of Faial and Pico there is no clear signal of the presence of magmatic chambers, but this maybe result of to the poor coverage of seismic paths; the presence of a low-velocity anomaly is revealed, but this may be due only to the effect of a volcanic feeding system and not the presence of a magmatic chamber, something that concurs with the suggestion of Nunes et al. (2006) of a lack of magmatic chamber under Pico's volcano.

We would like to acknowledge the effort of all the people involved in the seismic monitoring campaign that took place in the Azores during July 1998, namely, C. Corela, A. Carvalho, and J. Simões. The figures were produced using the GMT software (Wessel and Smith 1991) and SEISAN software (Havskov and Ottemöller 1999).This work was supported by the Portuguese Fundação para a Ciência e Tecnologia, under projects MASHA (POCTI/CTA/39158/2001) and STAMINA (PDCTM1999MAR15255). Nuno A. Dias had a fellowship under the PRODEP III program, Action 5.3. This is a contribution to

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**8. Acknowledgments** 

**9. References** 

5811.

PEST-OE/CTE/LA0019/2011 – IDL.

