UNKNOWN ACTIVE FAULT REVEALED BY MODERATE EARTHQUAKES AND AFTERSHOCKS NNE OF KARABURUN, WEST OF CITY OF IZMIR, TURKEY

The Aegean Sea and its surrounding area (Greece and western Turkey) comprise one of the most seismically active plate boundary regions. We used the Cepstral Stacking Method (CSM), developed by S.S. Alexander, to determine focal depths in Izmir (north of the Karaburun shoreline). Focal depth and the source mechanism that are important for active fault identification were provided by three 24 May 1994 earthquakes, all close to magnitude 5, that occurred in northern Karaburun shoreline area, west of the city of Izmir. The focal mechanism of the first of these three events was reported by the International Seismological Center bulletin and relative locations of these events are well constrained by the large number of observing stations. Since a focal mechanism solution gives two possible fault plane orientations, the spatial distribution of aftershocks associated with the events or other evidence has to be used to infer the correct strike direction and fault plane of the active fault. Because depth was poorly constrained or could not be determined in the standard hypocenter determinations, arbitrary depths were previously assigned. In addition, the estimated depths of other events in the area may not be accurate, although they are mostly within the crust. Such poor depth estimations adversely affect active fault identification, particularly for dipping faults like those in the northern Karaburun shoreline area, and, therefore, seismic hazard and risk assessments. We have now identified an active fault located in northern Karaburun, Aegean Sea (west of Izmir Bay) by applying the CSM method for 3 earthquakes that occurred on May 24, 1994. In the same area, a recent earthquake that occurred on May 1, 2012 (M=5) with many aftershocks validates our early interpretations of the predicted active fault geometry NNE of the Karaburun shoreline area; these events are located on a WNW-striking normal fault dipping SW whose surface trace lies further north under the water where it was never mapped before. This result is also supported by the seismicity pattern because of its termination approximately where the fault plane projects to the surface.