Frictional Weakening During Earthquake Slip on Faults: A Laboratory Study of Sliding-Surface Temperature During High-Speed Slip in Granite Under Biaxial Loading Conditions

Abstract

Flash heating can result in dramatic reductions in the frictional strength of rock sliding surfaces as velocities approach seismic rates. Models for flash-weakening show good overall agreement with the total magnitude of weakening, however deviations during the acceleration and deceleration phases, and at low and intermediate sliding velocities, are unresolved. To explore the role of surface temperature on flash-weakening, high-velocity rock friction experiments on Westerly granite were conducted using a high speed biaxial apparatus equipped with a high-speed infrared camera. Two sliding surfaces geometries were used to control mm-scale life-times and rest-times, which permitted accurate modeling of the flash temperatures observed. Surface temperature is inhomogeneous during all phases of sliding with steep temperature gradients that increase with displacement. Thermal models constrained by measured surface temperatures determine local (contact) normal stress at the mm-scale and are coupled with a conventional flash-weakening model. The majority of the total load is carried by 5-10% of the apparent contact area. Hot spots approximately 1 mm in diameter support local normal stresses exceeding 20 times the macroscopic stress with steep gradients that decrease with displacement. The stress concentrations at these mm-scale hotspots relative to micrometer asperity contacts is consistent with a proposed scaling of rock strength over several orders of magnitude. Classic models of flash-weakening that do not accurately reproduce weakening and re-strengthening during acceleration and deceleration phases are improved by considering multi-scale contact geometry and contact processes. Neither surface mineralogy nor surface topography, pre- or post-deformation, plainly explain hotspot distributions in repeat experiments. Work investigating mineral distribution below the sliding surface, wear product characteristics, and bonding during flash-weakening may further advance models for flash-weakening.