The January 2012 earthquake sequence in the Cretan Basin, south of the Hellenic Volcanic Arc: Focal mechanisms, rupture directivity and slip models

Abstract

The January 2012 moderate-size (Mw5.3) sequence in Cretan Basin is studied, in an attempt to shed light to the deformation pattern along the central part of the southern Aegean Sea, south of the Hellenic Volcanic Arc. The two strongest events of the earthquake sequence, occurred on January 26 (UTC 04:24 Mw5.0) and on January 27 (UTC 01:33; Mw5.3), with epicenters lying 25km and 50km SW of the Christiani Isles and Santorini volcanic fields, respectively. Moment tensor inversions revealed the operation of pure strike–slip motions, along N–S and E–W trending vertical planes. The deviatoric moment tensor solutions, in a number of cases, involved more than 30% non-double couple component. The seismicity was confined in a vertical column of small (~5km) horizontal extent, whereas the best constrained depths were in the range of 5 to 14km. The good station coverage and the striking similarity of the waveforms permitted directivity effects to be resolved, from the shape and amplitude of the apparent Source Time Functions (STFs), obtained through an empirical Green's function approach. The rupture directivity towards south, resolved the N–S trending plane as the fault plane for both of the two strongest events. This sequence provided evidence that the WNW–ESE trench-parallel extension accommodated within the overriding Aegean plate, which so far was well documented in the western and eastern parts, it is also operating in the central part of the southern Aegean Sea. The slip models for the two strongest events were simple, single patched and the calculated static stress drop was of the order of 35 bars, in agreement with the average value in the back-arc Aegean Sea region.