Frictional strength evolution and earthquake nucleation

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Earthquakes are generated by the rupture of geologic faults. Understanding these hazardous events requires developing mathematical model descriptions of sliding friction and fault rupture. Earthquake forecasting relies on these models, which are constrained by laboratory data and assume phenomenological laws. However, model predictions are highly sensitive to the chosen laws. The PI recently developed a new analytical approach to examine such sensitivities. He demonstrated that existing models of earthquake nucleation and slip may preclude a minimum nucleation size (or minimum earthquake size). Such a minimum size was thought to be required in prior fault models. Here, the PI and his team extend this work to unify existing phenomenological laws for fault friction and the understanding of dynamic rupture nucleation. The expected solution can realign the theoretical framework of earthquake physics with growing observational evidence that threatens the paradigm of a minimum earthquake size. Consequently, this project has critical implications for earthquake modeling and forecasting. It also provides support for a graduate student and a postdoctoral associate. This project addresses the fundamentals of the nucleation and propagation of an earthquake-generating fault rupture. Theoretical models of fault rupture are often based on so-called rate- and state-dependent friction laws, where fault strength depends on the slip rate of the host rock as well as its sliding history, or state. Our understanding of the seismological implications of earthquake nucleation and sliding friction relies on these phenomenological laws. Traditionally, these laws are thought to imply that a minimum fault size is necessary to nucleate an earthquake. An increasing body of experimental friction data has led to several formulations for frictional strength evolution. However, these different formulations produce disparate model predictions for earthquake nucleation. The PI recently developed a new analytical perspective in which the occurrence of slip instability and earthquake nucleation in previous formulations are end-members of a non-linear solution. This approach moves past the classic linear perturbation analysis and eliminates the need for a minimum nucleation size. Here, the PI and his team extend this work to a generalized form of the rate- and state-dependent friction law that advances and unifies the understanding of the so-called aging and slip laws. In this effort, the team examines previous and new descriptions of sliding friction, their role in models of the earthquake process and their seismological implications.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.