MICROSEISMICITY OF THE FRONTAL HELLENIC ARC ACCORDING TO OBS OBSERVATIONS

Five ocean bottom seismographs (OBS) of the pop-up and tethered types were installed southeast of the island of Crete at depth ranging from 1700 to 4000 m, and worked continuously for one week (July 2-9, 1988). Some one thousand microearthquakes with local magnitude M(L) from approximately 0 to 1.9 were registered. A total of 105 quakes were recorded by three or more instruments. Foci of these microearthquakes in the earth's crust and upper mantle are concentrated off the eastern flank of Crete along the contact between two blocks of the Hellenic arc. The en-echelon arrangement of the Pliny and Strabo deep-water trenches to the south of Crete is accompanied by a twofold Benioff zone. The seismic zones dip under the Hellenic arc with the same angle of 60-degrees and are also en-echelon. The Strabo zone is probably the younger with a maximum focal depth of 70 km against 120 km for Pliny zone. Seismic activity in the crust beneath the island is revealed to extend along the entire slope of the Pliny trench, but there is a principal maximum of activity at +/- 5 km, a minimum at 15-20 km, and a second maximum at 20-30 km. These findings are in agreement with results from OBS observations made to north of Crete in 1987.