Sheared granular layers: Competition between flash heating and particle comminution

Quantitatively predicting the dynamic response and energy partition of granular fault gouges under shear remains a major challenge in seismology. Earthquake laboratory experiments are limited by capacity of equipments and thus cannot provide similar conditions in real earthquake ruptures. This motivates the development of computational models of granular fault gouge that accurately predict their behavior on both laboratory scale and seismic scale. Our model is based on Shear Transformation Zone theory, which contributes the shear movement of granular materials to localized confi gurational changes of small zones of particles. The model describes layers of sheared granular materials with multiple state variables, and the major energy partition is between thermal energy (temperature), confi gurational energy (effective temperature), and surface energy (for breakable particles). The evolution of the system is governed by equations of state variables coupled with thermally varying material properties. The local temperature at the grain contacts is estimated using a 1D heat conduction model. The model integrates important features including strain localization due to spatially heterogeneous disorder, fl ash heating mechanism, and comminution. Rapid weakening in shear strength and localization of heat and disorder are observed in our simulations due to strain localization. Those particles under higher shear strain experience great increase in contact temperature and thus signifi cantly decrease the minimum fl ow stress of the system. However, this effect is countered by diffusion across the layer and comminution of particles because contact time between particles decreases as their sizes reduce. Whether the system response is primarily controlled by fl ash heating or comminution depends on value of free parameters and initial value of state variables. Our preliminary results suggest that at low slip rates the effect of breakage is dominant, whereas at high slip rates fl ash heating becomes more effective. Our model provides a framework for integrating different weakening mechanisms in fault gouge within dynamic rupture simulations. In future, we are looking to determine the value of free parameters and material properties using laboratory and recorded seismic data, and optimize the algorithm to increase simulation speed. We believe this model will be very useful in studying earthquake rupture dynamics.