Focal mechanisms of shallow and intermediate depth earthquakes along the Hellenic Arc

The active tectonics of the southern Aegean Sea, a region of intense shallow and intermediate depth seismicity, is studied based on well-constrained focal mechanisms. We use the focal mechanisms of 28 earthquakes (with moment magnitudes M > 4.9) of the period 1977-2002 whose: source parameters we determined from inversion of body waves and the focal mechanisms of 85 other earthquakes to re-examine the style of faulting in the shallow and deeper parts of the subducting slab. The deepest event that we studied has depth of similar to160 km. The combined data set of focal mechanisms, indicate the following, starting from south to north: (a) NW-SE compression with thrusts along the outer central Mediterranean Rise; (b) reverse and low angle thrust faulting at depths up to 40 km with P axis having a constant direction which is almost perpendicular to the strike of the arc, in the western and central parts and parallel to the arc in the eastern part; (c) normal faulting with E-W trending T-axes showing along-arc extension at depths up to 40 km; and (d) strike-slip faulting with P-axes parallel to the strike of the arc for events deeper than 40 km. Of special interest is the area between the island of Zante and the western coasts of Peloponnese where shallow and deeper events are of strike-slip faulting. The eastern part of the Hellenic arc exhibits a different behaviour than the western and central part. In the eastern part P-axes for both shallow and deeper events have the same NE-SW parallel to the arc trend. This is not the case in the western and central part of the are where there is a clear discrimination between shallow and deeper events. In the eastern part the angle of the subducting plate is steeper than the other parts and the T - axes are well aligned along dip of the subducting plate. If for the deeper than 40 km events the similar toE-W trending planes are assumed as the fault planes, then the western parts of the arc (Kythira, Milos islands) are connected to dextral strike-slip motions while the eastern parts (near Rodos Island) are connected to sinistral strike-slip motions. (C) 2004 Elsevier Ltd. All rights reserved.