This project aims to identify the physical processes underlying rock friction. It has strong implications for our understanding of earthquakes and associated hazards. Earthquakes occur periodically; their recurrence is due to the "stick-slip" behavior of large fractures in the Earth’s crust, called faults. A fault "sticks" in the time periods between earthquakes and "slips" during earthquakes. The stick-slip motion arises from the interaction of the elastic (spring-like) behavior of relatively cold rocks and the frictional behavior of faults. Rock friction has been extensively studied in the laboratory because of its relevance to earthquakes. Empirical equations – that is, derived from experimental data rather than based on known mechanisms – describe how friction varies with time and sliding velocity. Computer models use these equations to reproduce a wide range of earthquake-related phenomena. However, the physical and/or chemical processes underlying these equations are still largely unknown; identifying and quantifying them is critical for applying laboratory results to geological faults. Here, the research team investigates these processes at the microscale and nanoscale of the asperities (bumps) on the fault surface where fault rocks are in actual contact. They use atomic force microscopy and nanoindentation to mimic the behavior of single asperities and to measure their behavior at small scales. Feeding the results of experiments into computer simulations that incorporate larger scales, they model and predict the frictional behaviors of rock surfaces. Ultimately, the researchers aim to develop new equations that better capture the behavior of earthquake faults and improve hazard assessment. This project also provides support to two graduate students and a postdoctoral associate. It fosters training for undergraduate students and outreach to high-school students and teachers, notably from underrepresented groups in science.

Empirical rate-and-state friction laws, which describe the frictional sliding behavior of faults, are commonly used in earthquake models. Their physical basis is largely unknown, particularly for the equations that describe the evolution of the "state" of a frictional interface. This renders the extrapolation of laboratory results to geological faults fraught with uncertainty. A common explanation of frictional "state" is that it represents the true area of contact on a fault surface; this area evolves (increases) with time or slip due to asperity yielding and creep. An emerging alternative is that contacts strengthen due to chemical bonding at contact junctions. The team previously demonstrated – using single-asperity atomic force microscopy and coordinated with computer simulations, and with nanoindentation experiments - that both mechanisms may contribute to the increase of friction with time (or slip), an effect termed frictional aging. A unifying hypothesis is that asperity creep and chemical bonding occur simultaneously at asperity contacts, but that aging is due primarily to chemical bonding. In this scenario, contact area and chemical bonding are inextricably linked, with asperity yielding and creep providing the contact area upon which chemical bonding occurs. Here, the research team will conduct novel experiments and simulations at the nexus of geophysics, chemistry, materials science, and mechanics to unveil the physical basis for rate-and-state friction. Specifically, they seek to 1) elucidate the roles of yielding and creep of asperities in friction, 2) explore the influences of temperature and fluid chemistry on surface aging and 3) elucidate the roles of slip versus time in state evolution. They employ specimens made of silica and quartz and, for the first time, amorphous alumina, sapphire and feldspar. Results from single-asperity experiments are integrated into simulations which describe the behavior of rough rock surfaces in contact via multiscale modeling. The models incorporate asperity yielding and creep with chemical bonding effects for the first time, allowing new insights into rate-and-state friction behavior of rock surfaces and faults. This project may lead to a paradigm shift with transformative implications for understanding earthquake nucleation, and for the assessment of earthquake hazards and associated risks.

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.

National Science Foundation (NSF)
Division of Earth Sciences (EAR)
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Paul Raterron
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University of Wisconsin Madison
United States
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