Accumulating observational evidence suggests that major mature faults, which tend to produce large, destructive earthquakes, are "weak", i.e. produce earthquakes at average stress levels much lower than expected based on typical static friction coefficients of 0.6 to 0.8 and fault-normal stresses of the order of overburden minus hydrostatic pore pressure. Two classes of models have been proposed to explain such low-stress behavior: (I) chronically weak faults, both during slow (quasi-static) and fast (seismic) slip, and (II) quasi-statically strong but co-seismically weak faults, for which static resistance is high on average, but weakens substantially at seismic slip rates. In this project, the investigator proposes to build fault models representative of the two classes of low-stressed faults, simulate long-term slip histories on such faults that include sequences of earthquakes, and explore similarities and differences in both seismic and aseismic behavior between the two classes of models, and between the models and natural faults. The project aims to determine the set of models that can reproduce a range of robust observations about mature faults in various settings. As a part of the study, they plan to investigate the role of earthquake nucleation in overall fault behavior, explore the relation between seismic and long-term behavior, and test the ability of simplified, quasi-dynamic, methods of earthquake simulation to capture the results of the fully dynamic models.
The project will lead to better understanding of potential earthquake scenarios on mature faults, including the role of nucleation and heterogeneity. Such understanding would contribute to developing algorithms for early warning and formulating suites of scenario earthquakes for use in earthquake engineering. Combined with codes for wave propagation and site and building response, the methodology for simulating earthquake sequences can become a vital ingredient of physics-based simulation capability for seismic hazard analysis in densely populated areas.
