Observations of shear-wave splitting parameters in the Western Gulf of Corinth focusing on the 2014 Mw = 5.0 earthquake

The Western Gulf of Corinth, located in Central Greece, is an active neotectonic graben that features intense seismicity. Events recorded by dense local networks during 2014 (including those belonging to an offshore swarm between Nafpaktos and Psathopyrgos) were used to perform a shear-wave splitting study, with recordings that fulfill strict criteria. The splitting parameters, determined by visually inspecting polarigrams and hodograms, are the polarization direction of the fast shear-wave and the time-delay between the two-split shear-waves. Over 960 observations were obtained for 13 stations in the area. Temporal variations of normalized time-delays were investigated to extract patterns that indicate stress accumulation and release. Such patterns were identified in 5 stations, where linear trends of increase, followed by a decrease a couple of weeks before the Mw = 5.0 7th November 2014 earthquake, were identified. These observations highlight similarities with behavior of time-delays before the occurrence of earthquakes in other areas. Nevertheless, scattering of measurements poses a significant challenge in interpreting these variations. The mean polarization direction of the fast shear-wave in each station is consistent with the known regional maximum horizontal stress component (i.e. WNW-ESE), except for three stations located in the northern shore of the gulf, which present a general NE-SW polarization direction. The former is interpreted with the Anisotropic Poro-Elasticity model, where fluids within microcracks, aligned with the regional stress regime, affect shear-wave splitting. The latter are located in an area where faults of similar direction have been mapped, which could indicate the presence of secondary local stress states.