Pergamon OBSERVATIONS ON THE 3-D CRUSTAL VELOCITY STRUCTURE IN THE KOZANI-GREVENA (NW GREECE) AREA

Abstract-Three-dimensional velocity structure of the upper crust was determined by inver-sion of P-waves travel time in the area of the Aliakmon river basin, where a strong earthquake of MS = 6.6 occurred on May 13, 1995. The area is located at the northwestern part of the Greek mainland. To investigate the 3-D crustal velocity structure of the region, a two-step tomographic procedure has been applied. The data set consists of the travel time residuals of 366 very well located earthquakes, recorded by at least seven stations of a portable network, which was installed at the Kozani-Grevena broader region by the Institute of Geodynamics, immediately after the shock of 13 May, 1995. In order to improve the initial (reference) velocity model, before the inversion of the travel times, the “minimum 1-D ” initial velocity model was obtained. The results show that at shallow depths low velocities are predominant in the whole region and there is no sharp horizontal velocity variation. The velocity at shallow depths seems to be affected from the surface geology. Despite the absence of large scale seis-motectonics features in the region there is a remarkable vertical increase of the velocity below the depth of 15 Km, supporting the existence of a second order discontinuity in th