A detailed analysis of the aftershock sequence of the September 7, 1999 Athens earthquake was performed in order to define the fault planes activated during this sequence and study the tectonic regime of the area. Calculated fault plane solutions were verified by the composite solutions and the application of the principal parameters method. The combined results indicate a uniform tectonic status in the western part of the aftershock area, with normal faulting of WNW-ESE trend and an average dip of 60Ί and a more complex one in the eastern part, where the azimuths of the activated fault planes vary and a transverse antithetic fault is also active. This variation could possibly imply a variation of the local stress field.

Seismic anisotropy, deduced from SKS splitting measured at 25 stations installed in the Aegean, does not show a homogeneous pattern. It is not restricted to the North Anatolian Fault but is distributed over a region several hundreds kilometers wide. Little anisotropy is observed in continental Greece or along the Hellenic arc; however, significant anisotropy is observed in the north Aegean Sea. Large values of delay times suggest that anisotropy is due to a long path within the upper mantle and to strong intrinsic anisotropy. Our results, both in fast polarization directions and in values of delay time, do not support the idea that anisotropy is associated with inherited tectonic fabric nor are they consistent with the present-day Aegean motion relative to an absolute frame. In contrast, the direction of fast polarization and the magnitude of delay times correlate well with the present-day strain rate observed at the surface deduced from both geodetic measurements and seismicity. This anisotropy is not horizontally restricted to major surface faults but is spread over a wide region.