Shear wave splitting and V^P/V^S variations before and after the Efpalio earthquake sequence, western Gulf of Corinth, Greece

On 2010 January 18 and 22, two earthquakes ofMW 5.3 and 5.2, respectively, occurred near the town of Efpalio on the western Gulf of Corinth. We performed a shear wave splitting analysis using the cross-correlation method and calculated V^P/V^S ratios for events that occurred in the epicentral area of the Efpalio earthquakes, between 2009 January and 2010 December. The data analysis revealed the presence of shear wave splitting in the study area, as well as variations of the splitting parameters and V^P/V^S ratios. The average values of time-delay, fast polarization direction and V^P/V^S ratio for the time period before the Efpalio earthquakes, were calculated at 2.9 ± 0.4ms km^-1, 92° ± 10° and 1.76 ± 0.04, respectively, while after the occurrence of the earthquakes, including the aftershock sequence, they were calculated at 5.5 ± 0.5ms km^-1, 82° ± 9° and 1.88 ± 0.04. A few months after the occurrence of the Efpalio earthquakes, the mentioned splitting parameters were calculated at 3.6±0.4ms km^-1 and 83° ±9°. V^P/V^S ratio exhibited a mean value of 1.87 ± 0.04. The mean fast polarization directions were in general consistent with the regional stress field, almost perpendicular to the direction of the extension of the Gulf of Corinth. The observed increase in the time-delays and V^P/V^S ratios after the Efpalio earthquakes indicates changes in the crustal properties, which possibly resulted from variations in the pre-existing microcrack system characteristics. We suggest that a migration of fluids in the form of overpressured liquids, which are likely originated from dehydration reactions within the crust, was triggered by the Efpalio earthquakes and caused the observed variations. The findings of this work are consistent with those of previous studies that have indicated the presence of fluids of crustal origin in the study area.