Microseismicity and crustal deformation of the Kyparissiakos Gulf, south-western Hellenic Arc, using an "amphibious" seismic array and a 3D velocity model obtained from active seismic observations

In Fall 2006, in the frame of the European Community SEAHELLARC project, we deployed an "amphibious" seismic array consisting of 17 3-channel stand alone stations onshore and 17 4-channel ocean bottom seismographs in the Kyparissiakos Gulf and surrounding area of western Peloponnese. We observed the microseismic activity for a period of two months. The geometry and location of the array were designed to particularly observe the offshore activity and define the continuation of known onshore active faults offshore. Data evaluation and location of the seismic events were accomplished in three steps, successively improving the location accuracy. First we used the standard procedure for foci location with a 1D velocity model and the Hypoinverse algorithm. Locations were then improved by 3D passive tomography, using P and S arrivals. The optimum solution of the seismic foci location was finally achieved by using a 3D velocity model developed from active seismic observations and 3D gravity modelling. More than 3500 earthquakes over a threshold magnitude of 0.3 MD were identified, using arrivals from a minimum of six stations per event. Shallow seismicity is confined mainly onshore Zakynthos Island, at the continentocean transition to the west, and in the area of Pylos - Messinia. These zones are deforming by westward thrusting of the Alpine napes and collision of the continental backstop of pre-Apulia with the Ionian oceanic lithosphere at the Mediterranean Ridge. Deeper seismicity follows the subduction of the Ionian lithosphere below western Peloponnese with successively increasing hypocentre depths eastwards. A cluster of subcrustal events extending to 90 km depths is located in a pull apart basin NE of Strofades Island in the forearc. These events also generated T-phases. We attribute this "unusual" seismicity to lateral deformation and fracturing of the oceanic lithosphere during its subduction below a continental crust of laterally varying thickness.