Preliminary results of P-wave tompgraphy determined from selected aftershocks of the Egion 1995 earthquake, Gulf of Corinth, Greece

Aftershocks which followed the 1995 Egion earthquake of magnitude ML=6.2 were used to detrminea preliminary P-wave velocity structure of the upper crust in a region of the western part of the Gulf of Corinth, western Greece. 680 selected P-wave arrival times from 108 aftershocks recorded by the seismic network PATENT of the University of Patras and the temporary EGION network were inverted for the P-wave velocity distribution with respect to two classes of models: horizontal homogenous layers and a single laterly heterogeneous half a space. Travel time data were inverted both in a standard way using the damped least-square approach and using Cauchyrobust norm and genetic algorithm optimization method in the framework of the Bayesian inversion scheme. Although the main features of P-wave distribution obtained by both approaches are similar, the differences were found to be up to a few percents even in a well resolved area. Generally, the models obtained by genetic algorithm provide smaller residua then those calculated by means of the damped least-squares. It has also been found that the main shock of 15 June and most of considered aftershocks occurred in the region of an increased velocity (about 0.4km/s) as compared to the average velocity of an adjacent area.