The 2017 July 20 Mw 6.6 Bodrum-Kos earthquake illuminates active faulting in the Gulf of Gökova, SW Turkey

The 2017 July 20 Bodrum-Kos earthquake (M_w 6.6) is the largest instrumentally recorded earthquake in the Gökova graben, one of the primary physiographic features of SW Turkey. Using seismology and satellite geodesy, we investigate its source characteristics, aftershock distribution, relationship with earlier instrumental seismicity, and association with known surface faulting. We show that the earthquake ruptured a planar (non-listric) normal fault that dips gently (∼37°) northwards beneath the northern Gulf of Gökova coastline, initiating at a depth of ∼11 km and rupturing upwards and bilaterally. Aftershocks concentrate around (but not necessarily on) the western, eastern and downdip edges of the ∼25 km-long rupture plane, and have maximum focal depths of ∼15 km. The main shock surface trace bounds a bathymetric ridge east of Kos island, and may be one of the several faults imaged previously in this area using seismic profiling and multibeam sonar. The fault thus lies within the hangingwall of the lower-angle (∼20° N-dipping) South Datça fault, which it presumably cross-cuts at depth. Through calibrated relocations, we confirm that sequences of moderate (M_w 5-5.5) earthquakes in 1989, 2004 and 2005 occurred in the eastern and central Gulf of Gökova, many of them likely within the hangingwall of the S-dipping Gökova fault. Overall, our results indicate a switch from dominant S-dipping normal faulting in the eastern graben to dominant N-dipping faulting in the west, but we find no support for a proposed NE-SW-trending left-lateral fault in the central Gulf; most colocated focal mechanisms involve ∼E-W normal faulting. Finally, the Bodrum-Kos main shock adds to growing set of examples from across the Aegean region of large normal faulting earthquakes that cut the seismogenic layer as simple planar structures.