Thermal Weakening of Asperity Tips on Fault Planes at High Sliding Velocities

Norman H. Sleep

Published February 7, 2019, SCEC Contribution #8995

Real contacts on the scale of ~10 ┬Ám support gigapascal shear tractions and normal tractions on rapidly sliding faults. At sliding velocities above ~0.1 m s-1, the asperity tips of the contacts become hot and weak. The macroscopic friction depends on the average strength of the asperity tips during their lifetimes of contact. The strength of the asperity tips does not go essentially to zero once they become weak because frictional heating would then cease and the tips would cool and strengthen. Rather, the contact retains finite strength so that frictional heating balances the heat lost from the asperity tip by conduction. Crudely, the macroscopic coefficient of friction at high sliding velocities decreases with the inverse of the square root of velocity rather than the inverse of the velocity in the widely used model of Rice (2006). Numerical thermal-mechanical models support this inference. The finite strength of the asperity tips retards lateral extrusion of weakened material from the tips. Otherwise, extrusion would allow the sliding surfaces to converge establishing new contacts, which would renew the strength of the surface. The macroscopic coefficient of friction with somewhat weakened asperities tips remains high enough that the fault surface becomes hot on a millimeter scale in large crustal earthquakes. Fluid pressurization and eventually millimeter-scale melting then reduce the macroscopic shear traction to lower values than does asperity tip weakening acting alone.

Key Words
Dynamic weakening, fluid pressurization, flash weakeniing, near-fault damage

Citation
Sleep, N. H. (2019). Thermal Weakening of Asperity Tips on Fault Planes at High Sliding Velocities. Geochemistry, Geophysics, Geosystems, 20(2), 1164-1188. doi: 10.1029/2018GC008062.


Related Projects & Working Groups
FARM, Computational Science, Seismology