Rates of active deformation in the Aegean Sea and surrounding regions

Abstract Average strain rates are calculated from earthquakes in the period 1908-81 that occurred in the Aegean Sea extensional region, and in the convergent zone associated with the Hellenic Trench. In spite of large uncertainties resulting from the use of an M_S : M_o relationship, seismic N-S extensional rates in the Aegean are in the region 20–60 mm yr^-1 whereas seismic shortening rates in the Hellenic Trench are less than about 15 mm yr^-1. This is surprising because Africa and Eurasia are known to be converging, not separating. This apparent anomaly is caused by most of the convergence in the Hellenic Trench occurring aseismically. By contrast, the seismic extensional rates in the Aegean agree quite well with those expected from other arguments. The present day extensional rates are sufficiently high for McKenzie's instantaneous stretching model to be applicable. There is some evidence that these high extensional rates have operated throughout the last 5 Myr.