LAUSR

Abstract An M 7.4 earthquake occurred near the town of Izmit on August 17, 1999 causing enormous casualties and economic losses: over 15 000 victims and several thousand irreparably damaged buildings and structures. The earthquake produced a set of fractures and fissures in the ground extending for more than 70 km from the Gulf of Izmit, with a new tectonic fault being formed that is more than 150-km long. This paper reports results from mathematical modeling of the state of stress and strain in the epicentral zone before and after the Izmit earthquake along with seismological observations. The modeling used a method for calculating the state of stress and strain in a blocky elastic isotropic heterogeneous medium that has been disturbed by a system of faults caused by an external tectonic stress field, with the boundary conditions being specified from seismological data. We show that the epicenters of large crustal earthquakes tend to occur in areas of highly concentrated stress intensity under a definite relationship between the principal stresses. The aftershock process is due to the release of stresses produced by a new rupture, while the resulting aftershock clusters correlate spatially with the zone of released stress intensity. The tectonic rupture is assumed to propagate toward zones of high stress intensity. The results obtained in this study can be useful in a deterministic approach to the assessment and prediction of earthquake hazard, as well as the length and area of earthquake rupture.