NSFGEO-NERC: Earthquake nucleation versus episodic slow slip: what controls the mode of fault slip?

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This is a project that is jointly funded by the National Science Foundation’s Directorate of Geosciences (NSF/GEO) and the National Environment Research Council (NERC) of the United Kingdom (UK) via the NSF/GEO-NERC Lead Agency Agreement. This Agreement allows a single joint US/UK proposal to be submitted and peer-reviewed by the Agency whose investigator has the largest proportion of the budget. Upon successful joint determination of an award, each Agency funds the proportion of the budget and the investigators associated with its own investigators and component of the work.Earthquakes are typically generated by rapid slip – or dynamic rupture – on pre-existing tectonic faults. The faults are loaded by tectonic plate motions. Other forms of fault slip occur as well, including episodic slow slip events (SSEs). In that case, fault slip spontaneously accelerates but never reaches rapid earthquake slip speeds. Episodic SSEs can release the same amount of strain energy as earthquakes, but over days to weeks rather than seconds to minutes. SSEs are vital to understand as they relieve the stress buildup on faults and reduce seismic hazard. Yet they also transfer stress from one part of the fault to another, which can promote the nucleation and propagation of large destructive earthquakes. To date, the mechanisms underlying SSEs is not well understood. It is unclear what mechanisms slow down some slip instabilities (no earthquakes) yet allow others to turn into dynamic rupture (earthquakes). Here, an international team of scientists from the US and the UK explores the mechanisms underlying SSEs. The researchers use a combination of state-of-the-art laboratory experiments and numerical modeling. The modeling allows notably to extrapolate laboratory results, obtained on small specimens, to the scale of large tectonic faults. The project outcomes, which include physics-based simulations, improve earthquake hazard assessment in natural and induced seismicity. The project also provides support and training in an interdisciplinary context to several students and early career scientists. These include one graduate student and one postdoctoral associate at the California Institute of Technology. Here, the team tests three key hypotheses that may explain the stabilization of accelerating fault slip into episodic SSEs, rather than earthquake ruptures: 1) evolving rate dependence of friction, from velocity weakening to velocity strengthening, stabilizes the slip; 2) dilatant strengthening due to pore fluid effects stabilizes the slip; 3) spatial variations in fault properties contribute to determining the mode of fault slip. Key deliverables are constraints on the range of conditions and physics under which episodic slow slip, fault creep, or earthquakes can occur. The improved understanding of earthquake physics ultimately improves seismic hazard forecasting. The UK investigators conduct laboratory experiments for rock materials and fault conditions highly relevant to SSEs and earthquake nucleation, yet largely unexplored. They also carry out some numerical modeling of the experiments. The US investigators conduct additional detailed modeling of the experiments, to improve the constitutive laws governing fault slip. They also conduct numerical simulations of fault models governed by the improved friction laws to determine the implications of the experimental results for large-scale natural faults.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.