Rheological Controls on Asperity Weakening During Earthquake Slip

Abstract

Evolution of fault strength during the initial stages of seismic slip plays an important role in the onset of velocity‐induced weakening, which in turn, leads to larger earthquake events. A key dynamic weakening mechanism during the early stages of slip is flash heating, where stress concentrations at contacts on the interface lead to the rapid generation of heat. Although potential weakening from flash heating has been extensively modeled, there is little recorded microstructural evidence of its physical manifestations. We present results of a series of triaxial experiments on synthetic faults in quartz sandstone. Samples were subjected to a variety of normal stresses and ambient temperatures, to induce a range of slip event sizes and sliding velocities. We show the microstructural evolution of asperity interactions from the onset of flash heating through to the formation of grain‐scale areas of sheared melt. Using microstructural observations and mechanical data from the experiments, we model temperature and the viscoelastic behavior of the glass. Results suggest that, in the earliest stages of slip asperity contacts melt, but temperatures remain too low for viscous shear to occur within the melt layer. Instead melted asperities behave as glassy solids, facilitating continued frictional heating. With further slip, increased asperity temperatures allow the transition to viscous shear within the melt layer, facilitating weakening. These results highlight the dynamic evolution of the viscoelastic properties of the melt and resulting effects on asperity strength. Such complexity has, to‐date, not been fully addressed in modeling of flash heating.

Shear Response of Rough Rock Discontinuities Subjected to Impact Loading: Experimental Study and Theoretical Modelling

“…Areas with melt-flow textures behaved as a liquid during the experiment. The observation of the viscously sheared films is consistent with those made by Hayward et al [40] for the fault surfaces of sandstone. Thus, a higher cohesion may be related to the viscosity of the sheared surface induced by the loading rate effect.…”

Scaling Seismic Fault Thickness From the Laboratory to the Field

“…Whether a melt behaves as a solid or a liquid depends on its relaxation time (Dingwell & Webb, 1989; Hayward et al. 2019). Most geological materials have non‐Newtonian viscosities, especially strain‐rate dependent.…”

“…At high slip rates, displacement is accommodated over finite distances with extremely high strain rates, i.e., far above the conditions for viscoelasticity. Whether a melt behaves as a solid or a liquid depends on its relaxation time (Dingwell & Webb, 1989;Hayward et al 2019). Most geological materials have non-Newtonian viscosities, especially strain-rate dependent.…”

Scaling seismic fault thickness from the laboratory to the field

“…far above the conditions for viscoelasticity. Whether a melt behaves as a solid or a liquid depends on its relaxation time (Dingwell & Webb, 1989;Hayward et al 2019). Most geological materials have non-Newtonian viscosities, especially strain-rate dependent.…”