Western Hellenic subduction and Cephalonia Transform: local earthquakes and plate transport and strain

Focal parameters of local earthquakes in the region of the Ionian Islands o f western Greece are constrained with a temporary dense array of three-comp onent seismographs operated jointly offshore and onshore. Seismic deformati on is documented to be confined to the east of the N20 degrees E-striking s teep continental slope west of Cephalonia island, the right-lateral Cephalo nia Transform Fault, CTF, inferred from large earthquakes. The pre-Apulian continental material appears to be only deforming east of the transform fau lt, where it is in upper plate position to the Hellenic subduction. East of the transform fault, the transmission velocity tomography from local earth quakes, compared in depth-section with a previous marine reflection profile , provides evidence in support of a shallow landward dipping boundary aroun d 12 km deep under the Ionian Islands along which they may override the low er plate. On either side of this interface local earthquakes occur with dif ferent focal mechanisms, in support with its interpretation as the interpla te. Under Cephalonia island, reverse-faulting deforms the upper plate along NW-SE structures, which may also be affected by left-lateral bookshelf-fau lting. Small earthquakes show normal faulting along the western coast of Ce phalonia and its extension 20 km SSW, the trace of the CTF as inferred from the occurrence of the large strike-slip earthquakes. Another group of norm al-fault earthquakes locates in the lower plate from under Cephalonia to Za nte, just outboard of a possible change of interplate dip suggested from re flection seismics landward under the islands. These normal-fault earthquake s appear to coincide in position with that of the load imposed by the upper plate transported over them, rather than occurring in an outer rise, outbo ard the plate boundary and trench, as observed in other subductions and att ributed to the control by the flexural bending of the lower plate under the pull of the sinking slab. Interpretation has to consider several peculiar features of plate interaction in western Greece with respect to a steady-st ate model for major subduction zones, in particular: a fast deformation of the upper plate in front of an orogenically overthickened crust and of the southwestward push of extruding Anatolia; its transport, which is the cause of the migration of the plate boundary rather than the roll-back of a slab which has been proposed to be detached; possibly a flat and ramp shape of the interplate; the geometrical complexity of the shear limit across the CT F between subduction and collision, and the nearby variation of the nature of the foreland crust. (C) 2000 Elsevier Science B.V. All rights reserved.