Three-Dimensional P-Wave Crustal Velocity Structure beneath Athens Region (Greece) Using Micro-Earthquake Data

Abstract

Three-dimensional P-velocity structure of the upper crust was determined by inversion of P-wave travel times in the Athens broader area (Greece), where a destructive earthquake of 5.9 Ms magnitude occurred on September 7, 1999. The investigated area is located at the eastern part of Greek mainland and was previously considered as a region of low seismicity. To investigate the 3-D crustal structure of the region a two-step tomography procedure has been applied. The selected data set consists of 240 located earthquakes, recorded by at least eight stations of a portable ten station network, which was installed in the area by the Institute of Geodynamics, immediately after the main event. In order to improve the initial 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 investigated region, without any sharp horizontal velocity variation. The low velocities at this depth seem to be typical for sedimentary basins, like those of the investigated region. The most predominant feature is a low velocity layer at a depth of 6—8 km. At the same depth, few aftershocks have been recorded by the local network. At deeper layers, higher velocities cover the central part of Attiki region, almost coinciding with the transition zone between the Pelagonian and Attico-Cycladic massifs.