Dynamic fault rupture process during the 1978 Thessaloniki earthquake, northern Greece

Abstract

Recent observations of strong earthquakes that occurred in the Aegean and surrounding areas during the last several years indicate rather low stress drop values. An interesting example is the 1978 Thessaloniki earthquake (northern Greece), with M s = 6.5. Values between 6.6 and 12 bars have been determined for this earthquake by assuming unilateral or bilateral rupture propagation and circular or rectangular fault models. These values are considerably lower than the average values calculated not only for intraplate earthquakes (about 60 bars) but also for interplate earthquakes (about 30 bars). In this paper an attempt is made to investigate the cause of this fact by studying the dynamic rupture process of this earthquake. A three-dimensional, spontaneous, shear crack model on a dipping fault in a layered medium was assumed, and a finite-difference approach has been adopted for numerically solving the wave equations in a three-dimensional space. It was found that such a low stress drop may be obtained in cases of small differences between ambient stress and fault strength, and between tectonic shear stress and dynamic frictional stress. Such physical conditions in the focal region may result in an average fault displacement which is close to the observed one for this earthquake.