The January 2010 Efpalio earthquake sequence in the western Corinth Gulf (Greece)

Abstract

The January 2010 Efpalio earthquake sequence provides some key elements to enhance our view on the western Corinth Gulf tectonics. The sequence lasted almost six months, and included two Mw>5 strong events, both exhibiting normal faulting along ~E–W trending planes. This paper attempts to construct a unified seismotectonic model of the sequence jointly interpreting earthquake locations, moment-tensors and slip inversions in terms of the possible activated fault planes. Previous studies have connected the prevailing microseismic activity to a major low-angle, north-dipping structure under the Corinth Gulf and the Efpalio sequence favors such a general trend. Moreover, it clearly shows the significance of the shallow activity, so far less recognized, and possibly connected to the relatively steep faults outcropping on the northern coast. The first 18 January 2010 Mw>5 event had almost no on-fault aftershocks and most likely it occurred on a 55° south-dipping nodal plane. The early off-fault aftershocks formed two clusters roughly E–W trending, both of which are connected with normal faulting. Cross-sections revealed that the northernmost cluster is connected with a north-dipping structure, where the second 22 January 2010 Mw>5 event occurred. In addition, the very shallow parts of the faults (<4km) were mainly aseismic, probably due to their creeping behavior. Interestingly, both clusters, at their western and eastern ends, are bounded by NE-SW trending strike-slip faults, a pattern previously observed in normal-faulting structures in Greece. This observation further invokes the role of transfer faults in the western termination of Corinth Gulf, which provide the link with regional structures, such as the Trichonis and Rion-Patras fault systems. Most of all the 2010 Efpalio sequence enhanced the complex mechanical interactions within the Corinth Gulf fault network, with many earthquake generating cluster centers, an observation which has strong implications for the seismic hazard of this densely populated region.