Reassessment of 50 years of seismicity in Simav-Gediz grabens (Western Turkey), based on earthquake relocations

Western Turkey is a rapidly deforming region with a long history of high-magnitude normal faulting earthquakes. However, the locations and slip rates of the responsible faults are poorly constrained. Here, we reassess a series of large instrumental earthquakes in the Simav-Gediz region, an area exhibiting a strong E-W gradient in N-S extension rates, from low rates bordering the Anatolian Plateau to much higher rates in the west. We start with investigating a recent Mw 5.9 earthquake at Simav (19 May 2011) using InSAR, teleseismic body-wave modeling and field observations. Next, we exploit the small but clear InSAR signal to relocate a series of older, larger earthquakes, using a calibrated earthquake relocation method which is based on the hypocentroidial decomposition (HDC) method for multiple event relocation. These improved locations in turn provide an opportunity to reassess the regional style of deformation. One interesting aspect of these earthquakes is that the largest (the Mw 7.2 Gediz earthquake, March 1970) occurred in an area of slow extension and indistinct surface faulting, whilst the well-defined and more rapidly extending Simav graben has ruptured in several smaller, Mw 6 events. However, our relocations highlight the existence of a significant gap in instrumental earthquakes along the central Simav graben, which, if it ruptured in a single event, could equal ~Mw 7. We were unable to identify fault scarps along this section due to dense vegetation and human modification, and we suggest that acquiring LiDAR data in this area should be a high priority in order to properly investigate earthquake hazard in the Simav graben.