The January 2010 Efpalion earthquakes (Gulf of Corinth, Central Greece): earthquake interactions and blind normal faulting

On 18 January 2010, 15:56 UTC, a M w = 5.1 (National Observatory of Athens; NOA) earthquake occurred near the town of Efpalion (western Gulf of Corinth, Greece), about 10 km to the east of Nafpaktos, along the north coast of the Gulf. Another strong event occurred on 22 January 2010, 00:46 UTC with M w = 5.1 (NOA) approximately 3 km to the NE of the first event. We processed the seismological and geodetic data to examine fault plane geometry, dip direction, and earthquake interactions at the western tip of the Corinth rift. Our data include relocated epicenters of 1,760 events for the period January–June 2010 and daily global positioning system observations from the Efpalio station for the period 1 December 2009–1 March 2010. We suggest that the first event ruptured a blind, north-dipping fault, accommodating north–south extension of the Western Gulf of Corinth. The dip direction of the second event is rather unclear, although a south dip plane is weakly imaged in the post-22 January 2010 aftershock distribution. A Coulomb stress model based on homogeneous slip distribution of the first event showed static stress triggering of the second event of the order of 22–34 KPa that was transferred along the plane of failure. We also point out the existence of north dipping, high-angle faults at 10–15 km depths, which were reactivated because of Coulomb stress transfer, to the west and south of Efpalion. The January 2010 earthquakes ended a 15-year-old quiescence in that area of the Gulf. The crustal volume near Efpalion was also characterized by b values in the range 0.6–0.8 (1970–2010 period).