Earthquake mechanisms in the Hellenic Trench near Crete

In the Hellenic Trench south of Crete convergence between the southern Aegean Sea and Africa occurs at a rate of at least 60 mm yr−1. Previously published first motion fault plane solutions show a variety of different fault orientations and types, but are not well constrained. Furthermore, the lack of reliable focal depths for these earthquakes has obscured any simple pattern of deformation that might exist. Nonetheless, the mechanisms of these earthquakes have strongly influenced views of the tectonics in the Hellenic Trench. We have improved estimates of the fault parameters and focal depths for 14 of these earthquakes, using long-period P- and SH-waveforms. The earthquake mechanisms fall into four groups: (a) normal faults with a N–S strike in the over-riding material above the subduction zone; (b) low-angle thrusts with an E–W strike at a depth of about 40 km; (c) high-angle reverse faults with the same strike but shallower focal depths than (b); (d) events within the suducting lithosphere with approximately E–W P axes. The thrusting in groups (b) and (c) is probably the mechanism by which the sediments of the Mediterranean Sea underplate and uplift Crete. These events have slip vectors in the direction 025 ± 12° which represents the convergence direction between Crete and Africa along the SW-facing boundary of the Hellenic Trench. One of the events in group (d) occurred beneath the Mediterranean Ridge and involved high-angle reverse faulting with a WNW–ESE P axis: almost perpendicular to the direction of shortening deduced from folds at the surface. The Mediterranean Sea floor in this region appears to be in a state of E–W compression, for reasons that are not clear.