Seismicity at the convergent plate boundary offshore Crete,Greece, observed by an amphibian network

We investigate microseismic activity at the convergent plate boundary of the Hellenic subduction zone on- and offshore south-eastern Crete with unprecedented precision using recordings from an amphibian seismic network. The network configuration consisted of up to eight ocean bottom seismometers as well as five temporary short-period and six permanent broadband stations on Crete and surrounding islands. More than 2,500 local and regional events with magnitudes up to M _L = 4.5 were recorded during the time period July 2003–June 2004. The magnitude of completeness varies between 1.5 on Crete and adjacent areas and increases to 2.5 in the vicinity of the Strabo trench 100 km south of Crete. Tests with different localization schemes and velocity models showed that the best results were obtained from a probabilistic earthquake localization using a 1-D velocity model and corresponding station corrections obtained by simultaneous inversion. Most of the seismic activity is located offshore of central and eastern Crete and interpreted to be associated with the intracrustal graben system (Ptolemy and Pliny trenches). Furthermore, a significant portion of events represents interplate seismicity along the NNE-ward dipping plate interface. The concentration of seismicity along the Ptolemy and Pliny trenches extends from shallow depths down to the plate interface and indicates active movement. We propose that both trenches form transtensional structures within the Aegean plate. The Aegean continental crust between these two trenches is interpreted as a forearc sliver as it exhibits only low microseismic activity during the observation period and little or no internal deformation. Interplate seismicity between the Aegean and African plates forms a 100-km wide zone along dip from the Strabo trench in the south to the southern shore-line of Crete in the north. The seismicity at the plate contact is randomly distributed and no indications for locked zones were observed. The plate contact below and north of Crete shows no microseismic activity and seems to be decoupled. The crustal seismicity of the Aegean plate in this area is generally confined to the upper 20 km in agreement with the idea of a ductile deformation of the lower crust caused by a rapid return flow of metamorphic rocks that spread out below the forearc. In the region of the Messara half-graben at the south coast of central Crete, a southward dipping seismogenic structure is found that coalesces with the seismicity of the Ptolemy trench at a depth of about 20 km. The accretionary prism south of Crete indicated by the Mediterranean Ridge showed no seismic activity during the observation period and seems to be deforming aseismically.