Focal mechanisms of earthquake multiplets in the western part of the Corinth Rift (Greece): influence of the velocity model and constraints on the geometry of the active faults

The composite fault plane solutions for 24 large multiplets recorded in the western part of the Corinth Rift between 2000 and 2007 are computed by jointly inverting P polarities and Sv/P, Sh/P, Sv/Sh amplitude ratios of the direct waves. The fault plane solutions are determined using 1-D and 3-D velocity models. Solutions computed with the 3-D velocity model are preferred to the ones computed with the 1-D model because overall, 3-D solutions have a better score function. They correspond essentially to E–NE/W–SW and W–NW/E–SE striking normal faults, which is consistent with the N–S extensional/vertical shortening tectonic regime of the area. For 15 multiplets, one of the nodal planes is similar to the plane delineated by the earthquakes. It is then possible to determine which nodal plane is the fault plane. The analysis of the fault plane solutions highlights a clear decrease of their dip with depth and towards the north. Several multiplets with steeply dipping fault planes (50°–60°) located at depths of 7–8 km are clearly located at the base of onshore and offshore faults that crop out close to the south border of the Corinth Gulf, indicating that these faults are steep down to 7–8 km depth. To the north, multiplets underline a low angle north-dipping structure (20°–30°) on which steep north-dipping faults could take root.