Earthquake mechanics on faults that operate at low average levels of prestress

越来越多的观测证据表明，倾向于产生大的破坏性地震的主要成熟断层是“弱”的，即在平均应力水平上产生的地震远低于基于典型静摩擦系数0.6 - 0.8和断层-正应力-覆盖层-静水孔隙压力数量级的预期。已经提出了两类模型来解释这种低应力行为：(I)长期弱断层，在缓慢（准静态）和快速（地震）滑动期间；（II）准静态强但同震弱断层，其静态阻力平均很高，但在地震滑动率下大大减弱。在本项目中，研究者建议建立代表两类低应力断层的断层模型，模拟包括地震序列在内的这类断层的长期滑动历史，并探讨两类模型之间以及模型与自然断层之间在地震和地震行为方面的异同。该项目旨在确定一组模型，这些模型可以再现各种环境下成熟断层的一系列可靠观测结果。作为研究的一部分，他们计划调查地震成核在整体断层行为中的作用，探索地震与长期行为之间的关系，并测试简化的准动态地震模拟方法捕捉全动态模型结果的能力。该项目将有助于更好地了解成熟断层的潜在地震情况，包括成核和非均质性的作用。这种理解将有助于开发早期预警算法，并为地震工程中使用的情景地震制定套件。结合波传播规范、场地和建筑物反应规范，模拟地震序列的方法可以成为人口密集地区地震灾害分析的基于物理的模拟能力的重要组成部分。

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.