Episodic occurrence of strong (Mw ≥ 6.2) earthquakes in Thessalia area (central Greece)

Between 1954 and 1957, three seismic sequences struck the southern margin of Thessalia basin (central Greece). The proximity in time (similar to 3 yr) and space (similar to 100 km) between these three seismic manifestations suggests a possible link between them. Their interconnection is then investigated by resolving changes of Coulomb failure function (DeltaCFF) during the 20th century. Coulomb stress changes were calculated assuming that earthquakes can be modelled as static dislocations in an elastic half-space, and taking into account both the coseismic slip in strong (M-w greater than or equal to 6.2) earthquakes and the slow tectonic stress buildup associated with major fault segments. The study area constitutes one of the most active fault zones of the Greek mainland where several strong events have occurred in the past as it is inferred from both historical and instrumental information. However, no earthquake with M-w greater than or equal to 6.2 occurred there for about two centuries before the last activity. This fact led us to investigate the occurrence mode of strong events in the Thessalia area since the 16th century, when adequately reliable historical information exists for the stronger events. Forasmuch as they occur in clusters both in space and time, as it is the case during the 20th century, the episodic occurrence of strong events in this area is substantiated, when these episodes are separated by longer quiescent periods. Since during the 20th century all the earthquakes have occurred in stress-enhanced areas, by extending the stress calculations to 2025 and, provided that no additional large (M-w greater than or equal to 6.2) earthquake occurs between 2002 and 2025, assessment for the future seismic hazard is given. (C) 2003 Elsevier B.V. All rights reserved.