Seismotectonics of the Gulf of Patras, Central Greece: New insights from earthquake relocation, focal mechanisms and stress inversion

The Gulf of Patras lies at the western-end of the Corinth Rift (Greece), where the crust accommodates the kinematic transition from extension, along the rift, to oblique compression, along the western end of the Hellenic Subduction System (HSS). Nevertheless, the mechanism by which this transition occurs over earthquake timescales, is unclear. In this study, we analyze eleven years of seismicity within the Gulf of Patras by performing double-difference relocation and stress inversion. The distribution of epicenters shows spatially distinct seismicity patterns with earthquakes being dense and shallow (<20 km) in the northeastern gulf, diffuse and distributed over a broad range of depths (25–40 km) in the center, and sparse in the western gulf. Shallow microseismicity terminates abruptly at the junction between the gulfs of Corinth and Patras. Focal mechanism solutions suggest that shallow events are associated with E-W trending normal faulting, while deeper seismicity is linked to strike-slip faulting. The Achaia-Elia Fault System (AEFS), that traverses Peloponnese in a NE-SW direction and intersects the rift at high angles, is clearly delineated by the relocated epicenters, to step northeastward toward the city of Patras through a series of ‘en echelon’ earthquake clusters. Well-formed clusters of relocated earthquakes delineate three parallel NW-SE trending faults that extend across the Gulf of Patras, possibly linking active faults on either side of the gulf, a finding that may have implications for the seismic hazard of the city of Patras. Stress tensor analysis derived from moment tensors and focal mechanisms, indicates simultaneous strike-slip and normal faulting operating at different depths within the Gulf of Patras.