This is a collaborative project between US and Chinese scientists to study the deformation process of the November 14, 2001 MW 7.8 Kokoxili earthquake in western China. Large earthquakes provide a unique opportunity to learn about the rheology of fault zones and the crust and mantle. The stress changes that result from large earthquakes trigger a variety of processes in the fault zone and surrounding material that in turn relax the stress over time. The time dependence of these processes is related directly to the stress evolution in the fault zone and the material properties. Previous studies have debated which mechanism dominates the postseismic deformation: afterslip on the fault plane, particularly in the transition depth between the brittle and ductile layers, or visco-elastic relaxation in the lower crust and upper mantle. GPS data have been collected before and after the earthquake. The initial postseismic result shows rather unique deformation features, such as strong asymmetry across the Kunlun fault, fast temporal and slow spatial decaying rate, which can be directly modeled to differentiate the two deformation mechanisms mentioned above. Preliminary model result shows that it requires a combination of the two mechanisms to explain the data. Detailed studies require a multi-disciplinary approach to solve for the coseismic rupture that drives the postseismic deformation. They also require advanced modeling methods and programs to properly model the stress evolution and deformation in the crust and upper mantle with complex rheology. In this project a boundary element code is developed as the inversion tool. GPS, InSAR, geological, and seismological data are analyzed and inverted for both the source and structure, to better understand the rheology of the fault zone and the crust and mantle in north Tibet. A comparative study is performed between the Kokoxili earthquake and the November 3, 2002 MW 7.9 Denali fault earthquake in Alaska. Similarities in the earthquake sizes and faulting mechanisms and differences in the Earth structures shed light on the seismogenic processes for both earthquakes, and the rheologies of Tibet and Alaska. This study also helps solve a decades long debate about secular deformation of the Tibetan plateau: Is the deformation from the collision with India broadly distributed or block-like. Much of the debate has been focused on the fault slip rates along major strike slip faults and the rheology of the fault and surrounding crust and mantle. Comparison between the pre-quake and post-quake slip rates helps better understand the stress/strain evolution and fault slip rate change over an earthquake cycle. Knowledge of fault zone rheology also helps, because a weak fault zone surrounded by strong ambient crust usually deforms in a block-like manner, while a highly ductile lower crust tends to result in broadly distributed deformation.
