The role of gouge and temperature on flash heating and its hysteresis
Abstract:Geophysical observations and high-velocity friction experiments suggest that mature faults weaken significantly during earthquakes. One proposed weakening mechanism is the breakdown of frictional contacts at a critical weakening temperature, a process known as flash heating. For bare surface sliding Rice  showed that heat generation at frictional contacts triggers flash heating above a critical weakening velocity Vw of ~0.1 m/s. However, all faults generate a gouge layer at least a few millimeters wide, and the efficiency of flash heating in gouge is still unknown.
Building on Beeler et al. , we model flash heating in gouge by assuming that the total slip rate applied across the deforming zone is shared between multiple frictional contacts. Solving for the contact temperature we show that flash heating occurs when the strain rate exceeds a critical weakening strain rate controlled by the gouge properties. For a deforming zone 100 microns wide the equivalent Vw is ~4 m/s, making flash heating much less efficient in gouge than for bare surfaces.
The lower contact slip rate associated with distributed shear leads to longer contact lifetimes, increasing the thickness of the thermal boundary layer at a slipping contact W. We show that W can become comparable to the expected spacing between slipping contacts D. Accounting for this in a new model for contact temperature we show that when W » D flash heating begins at much lower slip rates, and friction decreases slowly as the slip rate increases.
Finally we study the hysteresis commonly seen in bare surface experiments, with higher friction observed during acceleration than deceleration. Accounting for the sensitive dependence of Vw on sliding surface temperature Tf allows us to match some experimental data for both acceleration and deceleration over a wide range of slip rates. Building on this we discuss how flash heating may operate near the trailing edge of a rupture where temperatures are high and slip is decelerating.