Mode of Strong Earthquake Recurrence in the Central Ionian Islands (Greece): Possible Triggering due to Coulomb Stress Changes Generated by the Occurrence of Previous Strong Shocks

Abstract

The spatiotemporal distribution of shallow strong (M 6.3) earthquakes occurring in the area of central Ionian Islands is analyzed. These shocks are generated on two adjacent fault segments with different strike, but both segments are associated with strike-slip faulting, constituting the boundary between continental collision to the north and oceanic subduction to the south. Seismic activity is concentrated in short time intervals alternating with much longer relatively quiescent periods. Each active period consists of a relatively large event or series (two to four) of events occurring closely both in space and time. This alteration was observed to happen four times since 1867, the period when complete data exist for the study area. Since the phenomenon is not strictly periodic, and during each active period multiple events occurred, I attempted to interpret the seismic behavior on the basis of possible trig- gering. I then investigated how changes in Coulomb failure function (DCFF) asso- ciated with one or more earthquakes may trigger subsequent events. Both the co- seismic slip due to the generation of the strong earthquakes and stress buildup associated with the two major fault segments were taken into account for the DCFF calculation. Earthquakes can be modeled as static dislocations in elastic half-space, and the stress pattern has been calculated according to the geometry and slip of each of the faults that ruptured in the chain of events. These calculations show that 13 out of 14 earthquakes of M 6.3 were preceded by a static stress change that encouraged failure. The magnitude of the stress increases transferred from one earthquake to another ranged from 0.01 MPa (0.1 bar) to over 0.1 MPa (1 bar), showing that stress transfer adequately explains the occurrence pattern of these events. Maps of current DCFF provide additional information to long-term earthquake prediction. Areas of positive DCFF have been identified at two sites in Kefalonia and Lefkada faults, respectively, where the next strong events are expected to occur, a fact that will substantiate the hypothesis elaborated in this article.