- ItemOpen AccessThe crustal anisotropy pattern in the epicentral area of the 2008 Mw 6.4 earthquake in northwest Peloponnese, GreeceOn June 8, 2008, a Mw 6.4 earthquake occurred on a strike-slip fault in north-west Peloponnese, Greece, 35 km southwest of the city of Patras. We analysed part of the aftershock sequence of the 2008 earthquake, recorded by a portable network of six stations and a permanent station of the Hellenic Unified Seismological Network, in order to perform a shear wave splitting analysis. We determined fast polarization directions φ, and normalized time-delays dt between the fast and slow components. The average value of dt was calculated at 1.7 ±0.5 ms/km, while φ values varied between 155 deg ±8 deg and 11deg ±9 deg , with an average of 170 deg ±9 deg . The observed φ values at all stations were inconsistent with the regional stress field, which is characterized-by a general E-W orientation of the maximum horizontal compressive stress. On the contrary, the observed fast directions appear to intersect the strike of the causative fault (210o) in smaller angles than the regional principal compressive stress axis. The findings of this work are consistent with those of previous studies in the area, reflecting possibly the presence of a locally rotated principal stress axis to more favorable angles to the strike of the fault of the 2008 strong earthquake.
- ItemOpen AccessShear wave anisotropy measurements above small earthquakes in Trichonis lake, GreeceOn April 2007, an earthquake swarm occurred in the vicinity of Trichonis Lake, western Greece. The seismic activity started on April 10th, 2007 after the occurrence of three moderate size earthquakes Mw 5.0-5.2. We performed shear wave splitting measurements on seismic events recorded during the first week of the seismic activity by a portable network of 8 stations that was installed in the area by the University of Patras Seismological Laboratory. We were able to take measurements from 5 stations as the seismicity was concentrated mostly at the eastern part of the lake. Two splitting parameters were measured through the data processing, the time-delays between the fast and slow shear wave components and the polarization directions of the fast components. In general, the data analysis revealed the presence of shear wave anisotropy in the study area. The average value of normalized time-delays was calculated at 6.9 ±1.1 ms/km while the fast polarization directions had an average of 130 deg ±14 deg. The mean fast polarization directions were consistent with what is expected concerning the local stress field, as it was sub-parallel to the strike of the major faults at the eastern part of the lake and almost perpendicular to the direction of extension. Therefore, the findings can be interpreted by an anisotropic volume of stress-aligned micro-cracks within the upper crust according to the extensive dilatancy anisotropy model.