The PIs present new experimental data and theory that describe the thermal weakening of fine-grained gouges during earthquake slip. They postulate that the particles in fine-grained gouges thermally soften due to an intrinsic decrease in the elastic shear modulus and increases in plasticity in response to rapid heating of the gouge layer. High-speed weaken processes directly bear on earthquake propagation physics and the origin of the anomalously low state of resolved shear stress on many major plate boundary faults systems, hazardous long run out landslides, and the motion on misaligned faults and low angle detachments. They suppose a simple non-linear weakening model and supporting experimental evidence that explains the principal properties of initial high velocity weakening phase in terms of a thermally controlled increase in the plasticity and decrease in yield strength of the contacting asperities in fault in response to frictional heating. It relies on the well understood concepts of rock plasticity at elevated temperatures and stresses and accounts for the factor observed of 3 to 4 weakening seen in high speed friction tests by considering both the Wachtman-Anderson relationship for the temperature dependent decrease of the elastic modulus and the hyperbolic sine creep law to account for asperity creep at elevated stresses and temperatures.
They plan to test the assumption that flash weakening is a truly adiabatic process where the heat caused by the friction is confined to a very thin thermal boundary layer along the asperity contact leaving the bulk of the asperity and attached grains uninvolved in the heating and deformation. The initial data experimental data confirms there is a systematic evolution of the friction coefficient from ~0.5 to as low as 0.15 as velocities increase from 0.14 m/s to 2.5 M/s. However, it does not support the simplified current simple form of the flash-weakening hypothesis. To constrain the theoretical development They will do the following: (1) Obtain thermal record of the average temperatures in the shear zone so we can properly constrain the temperature dependence of the gouge rheology. (2) Examine the shear zone fabrics with a SEM at different velocity and normal stresses to look at the nature of the grain size distribution and evidence for the onset of plastic deformation and melting of gouge particles. (3) Check for the effects of composition (clay vs. crystalline rock) and presence of water on the friction properties. (4) Modify the rotary shear apparatus so that it will run 2 to 4 times slower to extend measurements over a wider velocity range.
