We propose to study rheological behavior of the crust and faults of the Tibetan plateau using geodetic data measured following large earthquakes. Two large earthquakes occurred in the Tibetan plateau region in recent years: the 2001 Mw 7.8 Kokoxili earthquake occurred on the western section of the East Kunlun fault in northern plateau and the 2008 Mw 7.9 Wenchuan earthquake occurred on the Longmen Shan fault at the eastern rim of the plateau, respectively. These large quakes provide unique opportunities to study stress changes that result from their occurrences, triggering a variety of processes in the fault zone and surrounding materials that in turn relax the stress over time. The proposed work will help gain understanding about stress/strain evolution within an earthquake cycle, and how the stress coupling between the upper and lower crust and between the stresses inside and outside of a fault zone drives the system to failure. Such knowledge will also shed lights on continental dynamic processes, and be useful for seismic hazard mitigation and reduction. The proposed work will develop new techniques and computer code for modeling visco-elastic deformation in a lateral inhomogeneous lithosphere.
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. It has been debated whether viscous crust channel flow exists underneath certain parts of the plateau, and if so, how that drives deformation in and around the plateau, particularly the eastward expansion/extrusion at its eastern rim, and earthquakes occurred during the tectonic process. To tackle these issues we plan to develop visco-elastic deformation models and simulate postseismic deformation, using space geodetic, particularly GPS, observations as constraints. To precisely interpret the data we will modify a code to model visco-elastic deformation in a layered media, and incorporate a boundary element approach to allow simulating deformation in a layered media with lateral property change across faults. GPS, InSAR, geological, and seismological data will be used to invert for the deformation source and structure of the model, to better understand the rheologies of the fault zone and the crust and upper mantle in northern and eastern rim of Tibet. Given the spatial proximity of the two earthquakes, their joint study has some advantages: Precisely monitored post-Wenchuan deformation will provide tight constraints on the initial phase, and the less precisely monitored post-Kokoxili deformation will provide constraints on a longer time-span of the model, respectively. We will compare the modeling results between the 2001 Kokoxili and the 2002 Mw 7.9 Denali, Alaska (both predominantly strike-slip) earthquakes, and between the 2008 Wenchuan and the 1999 Mw 7.6 Chichi, Taiwan (both predominantly reverse-faulting) earthquakes. Similarities in the earthquake sizes and faulting mechanisms and differences in the Earth structures will shed light on the seismogenic processes for the earthquake pairs, and lithospheric rheologies of Tibet, Alaska, and Taiwan.
