COSEISMIC CRUSTAL DEFORMATION FROM MICROSEISMICITY IN THE PATRAS AREA, WESTERN GREECE

The Patras area lies in the western part of central Greece. It is an area characterized by high seismicity and complex neotectonics. Several devastating earthquakes have occurred in the region since 600 BC. Contemporary crustal deformation is examined in this area using microearthquake data recorded over a lengthy period, during 1983-84, by the Patras Seismic Network, principally, and to a lesser extent by the Volos Seismic Network. The microseismicity (1.8-3.9 M(L)) defines a zone deepening to the NE, which justifies a possible extension of the Gulf of Corinth major graben towards the Trikhonis Lake to the NW. Spectra of 108 well-located microearthquakes are estimated, using P-waves obtained by selective windowing designed to include only the P-phase; seismic moments in the range 0.3-45.7 x 10(12) N m are obtained, accompanied by estimates of seismotectonic source parameters including source radii, average stress drop and average coseismic slip. Poor correlation is found between seismic moment and magnitude, and the likely reason is the complex nature of the neotectonic regime existing in the area. Two zones differing in crustal deformation characteristics are observed. The Corinth-Trikhonis zone reveals two sets of characteristic faults. The first set is represented by microearthquakes showing distinctive and relatively higher seismic moments in conjunction with lower stress drops and seismic slips. This set of faults shows greater source radius than the second set, and therefore the faults are longer. The second set is characterized by an almost constant source radius within the range of uncertainty, and a wide range of seismic moments, stress drops and seismic slips. The Rio zone is characterized by low seismic slip, stress drop and fault radii, with the exception of the locality south of the city of Patras, where relatively higher seismic slip, stress drop and fault radius are observed.