Microearthquake seismicity and active tectonics of northwestern Greece

We carried out a microearthquake survey lasting for six weeks in northwest Greece using 18 portable seismograph stations to examine a region in which normal and thrust faulting have been reported in close proximity to one another. With this array we located 148 events and determined fault plane solutions for eight events using only rays radiated upwards. The seismicity of the region is diffuse with events extending to depths of nearly 30 km, and there is a minimum in activity near a depth of 15 km. The fault plane solutions exhibit a wide spectrum of fault types and orientations and are not consistent with simple zones of shortening or extension. Neither tractions applied to the edges or bottom of the region nor deviatoric stresses that compensate for lateral variations in crustal thickness can account for the variety of fault plane solutions. We think that the complicated behavior is a manifestation of inhomogeneous deformation due, at least in part, to a pre-existing complicated juxtaposition of structures and formations.