Strength of continental lithosphere in western China from seismic body wave studies

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This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).One of the basic tenets of plate tectonics is that quasi-rigid plates traverse the surface of the Earth, with significant deformations confined to regions near plate boundaries. While this idea works well in the oceans, it has had to be modified substantially in the continents, where active mountain building and seismicity related to the continental collision can be located thousands of kilometers away from the plate boundary. The present-day continental collision between India and Eurasia is arguably the best natural laboratory were we can investigate relationships between stresses applied to the lithosphere and resulting deformation at depth. We will investigate seismic activity within and around the Tarim basin ? a largely un-deformed area between the highly deformed Tibetan Plateau and the Tien Shan mountains. Where earthquakes are (and are not) taking place in this region will help us understand how the body of the continent is responding to the stress applied.This work will be done in collaboration with the Provincial Bureau of Seismology in Xinjiang (China), and will hopefully lead to further scientific collaboration between US and Chinese researchers. Our findings will advance our understanding of the lithospheric deformation in continental regions. Since this deformation is often accomplished through destructive earthquakes, our work will have direct implications for understanding of seismic activity in continental regions. Specific results will directly benefit the seismic hazard monitoring and mitigation needs of the Xinjiang region of China.Seismic activity represents a response of the lithosphere to stress, and regions of intense deformation of continental lithosphere are the best places to find observational evidence for the strength distribution within it. Arguments in favor of locating continental lithosphere?s strength in both the upper crust and the uppermost mantle rely, in part, on the rare but significant occurrence of subcrustal earthquakes in the continental lithosphere. However, whether earthquakes do indeed occur beneath the crust of the continental lithosphere is not universally accepted. Arguments for and against the notion of subcrustal lithosphere being strong enough to permit brittle failure hinge on identifying earthquake hypocenters located within it. The difficulty arises from uncertainties in both earthquake locations and crustal thicknesses in areas where sub-crustal earthquakes have been previously reported. One such area is the Tarim basin, and the surrounding mountainous regions (Kunlun and Tien Shan). Data from the regional seismic monitoring efforts, previously not available for study, will enable us to develop highly accurate hypocentral parameters for earthquakes in the region, and also to establish with high fidelity the depth of the crust-mantle transition. We will perform, in a coordinated manner, analyses of crustal structure (using teleseismic body wave methods) and hypocentral parameters of local earthquakes (using regional centroid moment tensor inversions). We expect to resolve the key uncertainty (whether there are earthquakes below the crust mantle transition) and to develop a nuanced understanding of the way regional lithosphere is deforming at depth.