SEISMIC VELOCITY CONSTRAINTS IN THE THESSALONIKI AND CHALKIDIKI AREAS(NORTHERN GREECE) FROM A 3-D TOMOGRAPHIC STUDY

Three-dimensional tomographic inversion of P-wave travel-time data is used to investigate the seismic velocity structure of the crust in The ssaloniki and Chalkidiki, northern Greece. Local earthquakes, recorded by two networks operating in the area, are used as natural seismic so urces. Two different target volumes, defined on the surface by 39 degr ees 50'-41 degrees 50'N and 21 degrees 25'-24 degrees 20'E, and 40 deg rees 10'-41 degrees 00'N and 22 degrees 45'-23 degrees 50/E, are inves tigated. The first dataset is recorded by 12 stations and the second b y 29. The size of the blocks used to parameterize the areas is 10 x 10 km and 3 x 3 km in the horizontal, respectively, with varying depth l ayering. The major seismic velocity anomalies within the crust, obtain ed by the tomographic inversion, are resolved with a horizontal spatia l resolution of about 20 and 7 km for the first and second target volu me, respectively. Our particular interest is to illuminate velocity an omalies and more detailed characteristics of the two main Neogene-Quat ernary basins in the area (Vardar-Axios and Struma-Strymon). These bas ins are identified as low-velocity features overlying relatively highe r P-wave velocity structures in the lower crust. The complex Mygdonian area reveals a similar pattern of low-velocity basin overlying higher -velocity basement. Overall, the velocity patterns correlate well with the location and strike of the main geological and tectonic units in the area, as well as the basic assumptions on basin development. This highlights the utility of local tomography to illuminate structural, t ectonic and theological properties within the crust.