Effects of frictional heating on the thermal, hydrologic, and mechanical response of a fault

Abstract

The mechanical response of a fault zone during an earthquake may be controlled by the diffusion of excess heat and fluid pressures generated by frictional heating. In this study we formulate a fault model which incorporates the effects of frictional heating on the thermal, hydrologic, and mechanical response of a small patch of the failure surface. This model is used to examine the parameters that control the fault response and to determine their critical range of values where thermal pressurization is significant. The problem has two time scales: a characteristic slip duration and a characteristic time for thermal pressurization. The slip duration is set by the fault geometry. The characteristic time for thermal pressurization is set by the slip rate, the friction coefficient, and the thermal and hydraulic characteristics of the medium. The response of the fault depends on the relative magnitude of these two times. Results suggest that the fault width and hydraulic characteristics of the fault zone and adjacent medium are the primary parameters controlling the mechanical response. For earthquakes occurring across zones of low porous medium compressibility (< 10−9 Pa−1) and permeability (< 10−18 m2) the characteristic time for thermal pressurization is small. In this case, frictional heating can cause fluid pressures to approach lithostatic values, the shear strength to approach zero, and the temperature rise to stabilize at a maximum value dependent on the pore dilatational and transport properties of the porous medium. Whether the patch acts as a barrier to slip or exhibits substantial strain weakening is dependent on the shear strain across the fault. Moderate slip events where shear strains exceed two cause substantial strain weakening and, consequently, large stress drops, accelerations, and displacements. Thus it is possible for the patch to act as a barrier for small earthquakes but not for large ones. Both the dynamic stress drop and total displacement decrease for zones with larger compressibility, permeability, or width. If the compressibility or permeability exceeds 10−8 Pa−1 or 10−14 m2 or the shear strain is less than one, then the effects of frictional heating may be negligible and the fault will exhibit no strain‐weakening characteristics. Consequently, the patch acts as barrier that halts or resists further fault motion. Extrapolation of these results suggests that spatial variations in fault width and hydraulic characteristics will cause a heterogeneous stress drop and fault slip over the failure surface, explaining many of the features of active faulting (e.g., barriers, nonuniform slip, rupture stoppage, random ground accelerations, strong motions, and frequency‐magnitude relations).