Department of Physics & Astronomy
University of New Mexico

Physics and Astronomy Colloquium

Rock friction, from the lab to Discrete Element Method simulations to earthquakes

Presented by Allan Rubin, Princeton

If one wants to simulate slip on geological faults, a constitutive equation for rock friction is required. For many decades the state-of-the-art has fallen under the general heading of "rate- and state-dependent friction", the idea that resistance to sliding depends upon both the sliding rate, and a more nebulous fault property ("state") that evolves with slip history. Despite the wide variety of materials exhibiting the rate-state phenomenology, the equations used to describe it are largely empirical, and none faithfully reproduce the full range of laboratory experiments. And these equations matter, in the sense that differences between those that have been proposed can make the difference, in numerical simulations, between earthquake nucleation zones that could be potentially detectable by instruments at the Earth's surface, and those that cannot.

To the extent that the rate-state equations are not entirely empirical, they embody the notion that near-stationary interfaces strengthen logarithmically with time, because of time-dependent plastic flow or chemical bonding at contacting asperities. However, a re-analysis of old lab experiments, and new experiments by our collaborators, show that the equations that embody this log-time strengthening describe surprisingly little rock friction data. Instead, an old empirical equation in which state evolves only with slip, rather than with elapsed time, does much better. I'll give a broad introduction to rate-state friction, motivate why seismologists care about this rather esoteric subject, and describe these perplexing laboratory results. I'll also describe Discrete Element Method (DEM) simulations of a granular fault gouge that do a surprisingly good job of reproducing the phenomenology of the laboratory experiments, despite lacking significant time-dependence at grain/grain contacts. The DEM simulations offer the possibility of doing in numerical experiments what it is difficult to do with real rock -- observing the underlying mechanisms of rate-state friction. At the same time, however, the DEM results are disturbingly sensitive to approximations made to the underlying continuum equations.

4:00 pm, Friday, March 31, 2023
PAIS 1100 and via Zoom. Please take the Satisfaction Survey,

Refreshments will not be available at this event.

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