3-D Crustal Velocity Structure from Inversion of Local Earthquake Data in Attiki (Central Greece) Region

Three-dimensional velocity structure of the upper crust was determined by inversionof P-wave travel times in the region of Attiki Prefecture (Greece), located at the eastern part of the Greek mainland, which is traditionally considered as an area of low seismicity. An earthquake of Ms = 5.9 occurred there, on September 7, 1999, causing extensive damage in the metropolitan area of Athens. A two-step tomographic procedure was applied to investigate the 3-D crustal velocity structure. The data set consisted of travel time residuals of 466 very well located earthquakes. In order to improve the initial velocity model, before the inversion of the data, a `minimum 1-D' initial velocity model was obtained and, therefore, for the first time a reliable velocity model for the region was derived. Theresults show that the velocity distribution is influenced rather from the geology than the seismotectonic regime of the region. At shallow depths (less than 4 km) the velocity increases gradually with respect to the depth without any sharp variation. On the contrary, the lateral variation of velocity seems to be affected from the geological regime of the region. The low velocities at this depth seem to be typical for sedimentary basins, likethose of the investigated region. At deeper layers (larger than 5 km) a different picture is obtained where both the lateral and vertical velocity variations are sharply pronounced. At the depth of 8.5 km, the aftershock area is covered by relatively low velocities, while a region of higher velocity covers the central part of Attiki almost coinciding with the transition zone between the Pelagonian and Attico-Cycladic massifs. Finally, the influence of the geological regime is well expressed by a high velocity anomaly, which is predominant beneath the Palaeozoic and Mesozoic metamorphic basement of the investigated region.