A COMPREHENSIVE EVALUATION OF EARTHQUAKE SOURCE MECHANISMS WITHIN THE MIDDLE EAST

The scarcity of reliable earthquake data and the structurally complex nature within the Middle East region requires a thorough examination of obtained earthquake source mechanism solutions and corresponding depths. Localized regions within Iran, the South Caspian basin, Tian Shan, and Hindu Kush represent complex areas of heterogenous continental collision dynamics where small scale faults account for the accommodation of stress within the region and complicate tectonic interpretations. To better address these complexities, we present an accurate and robust earthquake catalog with error assessment to provide a dependable resource for tectonic interpretations and update present wave speed models within the Middle East region.

Earthquake source mechanisms and depths were obtained for 270 events from 1990-2007 with magnitudes greater than 5.5. Using both 1- and 3- dimensional wave speed models, a full-waveform moment tensor inversion was calculated for each event analyzed at periods of 25-400 seconds and 80-400 seconds. Error was assessed using a bootstrap method to directly compare the results between models and period ranges as well as to assess the method’s stability. Using results obtained with a 3D model at 25 seconds we observe a 10-40% reduction in variance from initial solutions provided by globalCMT.org and an average cross correlation value of 0.86. The results presented here offer a significant improvement over current velocity models within the study region and provide more accurate grounds for further tectonic interpretation.

Calculated source mechanisms correspond favorably with broad tectonic features within the study region; however due to the nature of localized faults, many earthquakes may reflect motion on either blind faults or the combination of movement on multiple faults with different orientations. Consistent with broad regional plate motions within the Zagros Mountains and northern Turkey, we observe shallow right-lateral slip corresponding to the expected motion along the Main Recent Fault and North Anatolian Fault. NW-SE striking mechanisms suggest regional NE compression and localized tensile stress which trace the locations of numerous interconnected thrust faults traversing though the Caucasus and Apsheron-Balkhan sill, into the Kopeh Dagh range. General NW-SE extension is observed in the Gulf of Aden, with components of left-lateral slip that accommodate the curvature and joining of the mid-ocean ridge system in the Indian Ocean. Two normal faults located in the Makran, on the border of southern Iran and Pakistan, match up to a slow velocity region observed at 50km within the current 3D wave speed model (s29ea) and may be related to present day subduction. Complex normal and reverse faulting is seen in the western syntaxis region of Tibet, where strikes vary nearly 180◦ trending from the NW to NE.