HR: 14:40h
AN: T13E-05 INVITED     [Abstracts]
TI: Shear Localization in Fluid-Saturated Fault Gouge by Instability of Spatially Uniform, Adiabatic, Undrained Shear
AU: Rice, J R
EM: rice@esag.harvard.edu
AF: Department of Earth and Planetary Sciences and Division of Engineering and Applied Sciences, Harvard University, 224 Pierce Hall, 29 Oxford St., Cambridge, MA 02138 United States
AU: Rudnicki, J W
EM: jwrudn@northwestern.edu
AF: Department of Mechanical Engineering and Department of Civil and Environmental Engineering, Northwestern University, A333 Technological Institute, 2145 Sheridan Road, Evanston, IL 60208 United States
AU: * Tsai, V C
EM: vtsai@fas.harvard.edu
AF: Department of Earth and Planetary Sciences, Harvard University, G-8 Hoffman Lab, 26 Oxford St., Cambridge, MA 02138 United States
AB: A number of authors have shown that the thermal expansion of pore fluids may play an important role in the earthquake process. Here, we examine the dynamics of a shear zone of fluid-saturated gouge under homogenous, undrained, adiabatic shear, and show that such deformation is unstable, in a manner that suggests strong shear localization. The material is assumed to follow the Coulomb friction law but to be rate-strengthening, such that its friction coefficient increases with shear strain rate γ̇ (if rate-weakening, deformation would localize at the onset of shear). This rate-strengthening model is of interest because it applies to stable regions in which rupture cannot nucleate and to initially unstable regions that have been driven into a stable temperature regime by shear heating. We describe an analysis by Rice and Rudnicki (2005) which shows that this shearing is linearly unstable in the sense that small perturbations from uniform shearing result in exponential growth for all wavelengths greater than a critical wavelength. Setting the shear zone width equal to this critical wavelength, they obtain a rough estimate of the largest width h over which uniform shearing is stable, given by h = 4π² (α_th + α_hy) / [(z+2)HV]. Here α_th and α_hy are the thermal and hydraulic diffusivities, 1/H is a characteristic weakening strain of the homogenous solution, V is the net slip rate across the shear zone, and 1/z = (γ̇ df/ d γ̇)/f is a measure of the strengthening of friction coefficient f with γ̇. Choosing z = 40 (z ~ 20-60 based on known lab experiments showing rate strengthening, unfortunately all done at low γ̇), average earthquake slip rate V = 1 m/s, and values [Rice, 2005] α_th = 0.7 mm²/s, α_hy = 4 mm²/s, and H = 0.1 thought to be relevant to deforming ultracataclastic gouge at typical centroidal depths of the crustal seismogenic zone, we estimate h ≈ 0.04 mm. This mechanism, therefore, may help explain the field observation of sub-millimeter-sized high shear zones [Chester et al., 2003] within a much thicker gouge layer. We also perform a more detailed, numerical analysis that takes into account nonlinearities and pressure and temperature dependencies of the shear zone poromechanical properties. This numerical analysis more accurately describes the dynamics within the shear zone, and demonstrates the approximate validity of some aspects of the linear stability analysis, e.g., in predicting the localized zone thickness, which lie beyond the range of its applicability. A more comprehensive description of this nonlinear work is in preparation [Tsai and Rice, 2005].
DE: 7209 Earthquake dynamics (1242)
DE: 8020 Mechanics, theory, and modeling
DE: 8045 Role of fluids
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8163 Rheology and friction of fault zones (8034)
SC: Tectonophysics [T]
MN: Fall Meeting 2005