PREEVENTS Track 2: 3D Nonlinear Simulation of Large Earthquakes on the Southern San Andreas Fault

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The latest official earthquake rupture forecast for California (UCERF3) predicts a probability of 22% within the next 30 years for a magnitude 7.7 or larger earthquake in southern California, with the San Andreas fault as a likely causative fault. The population (exceeding 13 million) and infrastructure in greater Los Angeles are highly vulnerable to such an event. The benefits of previous ground motion predictions for large earthquake scenarios on the southern San Andreas fault are limited by deficiencies in the underlying simulation techniques as well as bandwidth. This project will use recent advances in available supercomputing resources and sophistication of numerical modeling codes to bring the simulations to a level that can be used for engineering design. The resulting ground motion models will allow improved seismic hazard analysis with realistic model features deemed to be important at the higher frequencies. The project will provide a first step toward significantly improved seismic hazard estimation, initially for a large event on the San Andreas fault in southern California. However, the results can be used in other areas where large earthquakes are possible in the future (e.g., northern California, western Washington, Wasatch Front, New Madrid Seismic Zone). The research is expected to enable refined predictions of peak ground motions for extreme events in the future, including the near-fault area where observations are sparse. If properly used, these results could affect current hazard maps and engineering design, and mitigate the loss of life and property in future large earthquakes.Large-scale computational efforts for large earthquake scenarios on the southern San Andreas fault have shown significant variability of the resulting long-period (longer than 0.5 s) ground motion levels due to 3D basin effects. However, the long-period ground motions predicted from these studies have limited use for practical purposes, due to simplifications in the underlying ground motion modeling as well as computationally-imposed constraints on the frequency content. This project will bring the simulations to a sophistication useable for engineering design and seismic hazard analysis. The maximum frequency of previous ground motion models for large southern San Andreas fault events will be increased to 5 Hz with realistic near-surface velocities and include new model features deemed to be important at the higher frequencies, such as small-scale source and media heterogeneity, frequency-dependent viscoelastic attenuation, and site effects. The current engineering practice of site-specific hazard assessment still relies on 1D nonlinear (or equivalent linear) simulations to predict the response of soils. The project will develop models and computational strategies that integrate nonlinear soil response into 3D simulations, validate them using borehole array data, and apply the models predictively to large San Andreas fault scenarios. In addition, the research will investigate how surface waves contribute to amplification, and how this amplification is affected by nonlinearity and near-surface anelastic attenuation. The highly scalable GPU-based finite-difference code AWP and available supercomputing resources provide the foundation for the challenging computational aspects of the research. The project will complete on-going work on a discontinuous mesh capability to allow for lower near-surface velocities and higher frequencies in the simulations, accurate source insertion in a nonlinear medium, and refinement of 3D nonlinear rheology.