The tectonic setting and earthquake ground hazards of the 1993 Pyrgos earthquake, Peloponnese, Greece

Abstract

The Pyrgos earthquake in western Greece (26 March 1993) ( M L = 5.2) was caused by oblique-normal slip on a north-dipping and WNW-trending fault, and produced significant damage in the town of Pyrgos. Several lines of evidence from the epicentral area suggest that faults within two active graben in the Pyrgos basin, the Grape and Alfios grabens, were reactivated during this earthquake and subsequent aftershocks. Evaluation of morphotectonic data, the fault slip-rate between 0.05 and 0.35 mm a −1 and seismic activity associated with the 1993 earthquake, suggest that the Alfios graben is the most active structure in the basin. This graben, which is 25 km long and crosses the Pyrgos basin, is segmented along its strike by transfer zones. Analysis of the seismic activity in this graben shows that transfer zones are the focus for the concentration of seismic activity and thus the concept of transfer zones is important for understanding the Pyrgos earthquake. The 1993 earthquake caused landslides at 47 locations and liquefaction phenomena at 7 localities within an area of 145 km 2 . The lower threshold level to induce landslides was M L = 4.0 and to induce liquefaction was 5.0. The majority of landslides occurred along fault scarps and either caused the slope to retreat and/or acted to protect the slope gradient.