**6. Coulomb stress variation**

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

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

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

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

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,

shock. Right: maximum horizontal stress (SHmax) directions obtained from focal

fault with inverse/normal component. Batimetry of Lourenço et al. (1998).

2005).

estimated SHmax direction for both indicators.

The aftershock distribution (Fig. 5) presents an odd distribution, with events aligned along roughly perpendicular directions, and the area surrounding the main shock seems to be devoided of significant aftershock activity. Furthermore, the focal mechanisms obtained from the best-constrained aftershocks indicate both strike-slip and normal faulting, suggesting that different faults have ruptured after the main shock.

Following the revision by Das & Henry (2003) on the relationship between the main earthquake slip and its aftershock distribution, the occurrence of low magnitude events in the high-slip regions of the fault that ruptures is rare. Instead, the authors have found that, in most cases, the aftershocks occur in nearby faults that are reactivated by a favourably oriented post-seismic static stress.

To investigate this possibility we have used the method implemented in the program GNStress (Robinson & McGinty, 2000), that calculates the crustal strains and their derivatives in a homogeneous half-space due to slip in a rectangular fault. To convert strains to stresses in the half-space a constant rigidity of 2.68x1010 Pa was used. A regional stress field with 1 (maximum compressive stress) oriented N135E, was assumed according to the transtensional tectonic regime deduced for this area of the Azores archipelago from neotectonic investigation (Madeira & Ribeiro, 1990). For the main shock source description we used the parameters derived from GPS data (Fernandes et al., 2002), namely the solution of a rectangular fault 9 km long x 4.5 km wide, oriented N165E, with the top of the fault plane lying 2 km below the topographic zero. A detailed discussion of the procedure can be found in Matias et al. (2007).

The calculated distribution of the Coulomb failure stress is presented in Fig. 8. Looking at the depth distribution of the variation in the Coulomb failure stress, and its relationship with aftershock distribution, we conclude that the main shock ruptured a very shallow fault, from a depth of 2 to 7 km, and then most of the aftershocks occurred below this depth along faults that were favourably orientated in relation to the post-seismic stress field. This very simple model doesn't explain the shallowest activity, above 7 km depth. It may be a consequence of the simple half-space model assumed for the stress computations.

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.
