Collaborative Research: Earthquake Gates: Linking Earthquake Rupture Length to the Dynamics of Restraining Double Bends on the Altyn Tagh Fault

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The length of a fault rupture generally controls the magnitude of a large earthquake. Unusually large, rare, and unexpected earthquakes overwhelm mitigation measures and the societal capacity to respond, leading to a cascade of disastrous effects. In order to assess the potential for such rare events, this study calibrates how effectively geometrical complexities of a fault impede earthquake rupture propagation. The project, carried out in close collaboration with Chinese researchers, integrates field observations of fault geometry and slip behavior of the Altyn Tagh fault in China, one of the longest active strike-slip faults on Earth, with numerical rupture simulations to predict the range of potential earthquake sizes along a major intracontinental strike-slip fault. The outcome of this research will be a means to assess the likelihood of rare, unusually large events. The project will advance desired societal outcomes through: (1) full participation of women and underrepresented minorities in STEM; (2) improved well-being of individuals in society through a new understanding of earthquake rupture processes; (3) development of a diverse, globally competitive STEM workforce through training of graduate and undergraduate students; and (4) increased partnerships through a strong international collaboration with Chinese scientists and international research experiences for students. The project is supported by the Tectonics Program and NSF's International Science and Engineering program.The research project will develop and apply techniques to integrate field observations of fault geometry, kinematics, and slip behavior with numerical rupture simulations to predict the range of potential earthquake sizes along the central Altyn Tagh fault over a length (800 km) that well exceeds the longest recorded continental strike-slip earthquake (420-450 km). The central Altyn Tagh fault fault is divided into segments by four restraining double bends (Aksay, Pingding Shan, Akato Tagh, and Sulamu Tagh) that are each hypothesized, based on their geometry, to stop most, but not all earthquake ruptures. Multi-cycle spontaneous dynamic rupture models show that these earthquake gates may be open or closed to a particular direction of rupture propagation depending upon fault geometry and stress conditions inherited from prior earthquakes. Prior research showed that dynamic rupture effects (resulting from seismic wave propagation from the rupture front) and interseismic stress relaxation (off-fault deformation) both contribute to geologically testable patterns of along-strike earthquake slip and cumulative slip-rate gradients. This project will apply these rupture models and geologic tests to the Pingding Shan double restraining bend, which appears to be a relatively nascent structure along the Altyn Tagh fault. A thorough field campaign will collect new slip rate, slip-per-event, fault kinematic, and structural data to constrain a multi-cycle rupture model for the Pingding Shan restraining double bend. Models will be developed that couple all four of the major restraining double bends of the central Altyn Tagh fault, and the ensemble behavior of this geologically-calibrated model system will be investigated to determine the likelihood of rare, exceptionally long earthquake ruptures.