Collaborative Research: Controls on Termination of Great Earthquakes in a Restraining Double-Bend of the Altyn Tagh Fault

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Fault segmentation, defined by geometric complexities such as restraining bends and stepovers, may play an important role in arresting earthquake rupture and thus limiting the maximum size of earthquakes. Understanding the mechanical processes that control fault slip within such complexities remains elusive. This study integrates field observations and rupture modeling to assess the conditions under which strike-slip earthquakes fail to break across the 200 km-long Aksay restraining double bend and stepover of the Altyn Tagh fault in northwestern China. This bend is flanked by, and transfers slip between, two fault strands within the left-lateral Altyn Tagh fault system. Because the Aksay bend is isolated from intersecting faults, its behavior is unlikely to be affected by stress-transfer from other parts of the active fault system, thus making it an ideal natural laboratory to compare model results against field observations. This research will test fault-based seismic hazard assessments that depend upon fault segmentation, and contribute understanding of how permanent deformation is divided among faults and the surrounding crust. To date, defining fault segmentation has been performed largely by expert assessment, with no site-specific physical basis. Numerical rupture simulations offer a much-needed physical foundation for fault segmentation, but with significant limitations. These limitations include that the pre-earthquake stress-state is unknown and unlikely to be smooth in zones of geometric complexity. Thus, the applicability of numerical rupture models to seismic hazard mapping remains untested. Multi-earthquake cycle rupture models that combine coseismic rupture with interseismic off-fault stress relaxation reveal patterns of stress-states, earthquake rupture sizes, and rupture extents in zones of structural complexity. This project will test these patterns against field observations including paleoseismology, fault slip-rates, and fault-slip direction. Simultaneously, this research will advance the state-of-the-art of these numerical models to include off-fault plasticity, and dipping, three-dimensional fault geometries.By integrating field observations and numerical rupture modeling, this research cuts across disciplinary boundaries in an effort to transform understanding of the earthquake rupture process and its manifestation in the geologic record. In so doing, this project builds upon an international collaboration with shared basic-science research objectives (to understand tectonic processes along faults) and shared societal need (to understand earthquakes). The outcomes of this research will directly affect understanding of hazards from great earthquakes on major faults. For example, the insights gained will be of direct relevance to understanding potential earthquakes and shaking hazards from rupture of the southernmost San Andreas fault.