HR: 11:50h
AN: T22A-07    [Abstracts]
TI: Transition from Frictional Weakening to Shear Heating Induced Thermal Pressurization
AU: * Segall, P
EM: segall@stanford.edu
AF: Stanford University, Department of Geophysics, Stanford, CA 94305 United States
AU: Rice, J R
EM: rice@esag.harvard.edu
AF: Harvard University, Division of Engineering and Applied Sciences and Department of Earth and Planetary Sciences 224 Pierce Hall 29 Oxford Street, Cambridge, MA 02138 United States
AB: Earthquake nucleation requires loss of frictional strength $\tau = \mu (\sigma - p)$ with slip or slip rate. For rate and state dependent $\mu$ at fixed $(\sigma - p)$ instabilities can occur when $d \tau_{ss} / d {\rm log} v = (\sigma - p)(a - b)$ is negative. Shear heating increases $p$ and, if dilatancy and pore pressure diffusion are limited, will cause $\tau$ to decrease. We examine how frictional weakening, shear heating, dilatancy and pore-pressure diffusion determine stability in a simplified fault model with a narrow core (thickness $h$) bordered by an impermeable wall, and an outer damage zone. We develop a fluid diffusion model accurate when along strike variations are much greater than $h$, and times are long compared to the pore pressure diffusion time through the fault core. Dilatancy and thermal diffusion are included in an approximate fashion. If the drained behavior is stable ($ a>b$), and wall zone permeability exceeds a critical value (estimated at $\sim 10^{-21}m^2$) then fault slip is linearly stable at all wavelengths. The critical permeability is less than that measured in active fault cores, even at effective stresses acting at 10 km depth. We conclude that shear heating can not generally nucleate slip instability and that frictional weakening is required. However, shear heating may nucleate instability on velocity strengthening faults following strong stress perturbations. On frictionally weakening faults shear heating becomes dominant at slip speeds of order %$v \sim L_p/t_p $\sim$ 1 mm/s, and displacements of $(\sim 0.001-0.01 m)$. Thus, dynamic rupture may be insensitive to frictional variations and dominated by shear heating effects. Time to failure calculations based on rate-state friction alone, however, should be approximately valid
DE: 8159 Rheology--crust and lithosphere
DE: 7209 Earthquake dynamics and mechanics
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting