Stochastic Strong Ground-Motion Simulation of the 7 September 1999 Athens (Greece) Earthquake

Abstract

The stochastic method for finite faults is applied to simulate strong ground motion from the 7 September 1999, moment magnitude M 5.9 Athens earthquake. The method includes descritization of the fault plane into a certain number of subfaults, each of which is assigned an ω -2 spectrum. A slip-distribution model, derived from previous studies of this earthquake, is used to specifically account for the source effect. Contributions from all subfaults are then empirically attenuated to the observation sites, where they are summed to produce the synthetic acceleration time history. The method is first calibrated against its ability of reproducing the recordings at 19 strong-motion stations, at epicentral distances ranging from 16 to 61 km. The calibrated model is then applied to calculate synthetics at a large number of grid points covering the area around the fault plane. Simulated peak values are subsequently used to produce synthetic peak ground acceleration and spectral acceleration maps at hard rock. Both peak ground acceleration and spectral acceleration maps imply energy directivity toward the east, where most of the damage was concentrated. The directivity effect is most prominent at large periods (>2 sec) and in the period range 0.2 to 0.3 sec. Independent geotechnical studies showed considerable site effect at periods <0.5 sec within the meizoseismal area. This result, coupled with the results of the present study, imply that the damage distribution pattern of the 1999 Athens earthquake can be explained by the destructive combination of two factors: the source directivity and the site effect.