HR: 14:25h
AN: T13E-04    [Abstracts]
TI: Effective Normal Stress Alteration During Earthquakes Due to Pore Pressure Induced by Contrasting Properties Bounding the Rupture Plane
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 60208-3109 United States
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
AB: Recent, detailed examinations of fault zones have shown that the fault walls are often bordered by materials that are different from each other and from the more uniform material further away and that the ultracataclastic core of mature fault zones, where slip is concentrated, is less permeable to flow across it than the adjoining material of the damage zone. Inhomogeneous slip at the interface between materials with different poroelastic properties and permeabilities causes a change in pore pressure there. Because slip causes compression on one side of the fault wall and extension on the other, the pore pressure on the fault increases substantially when the compressed side is significantly more permeable, and a decreases when, instead, the extended side is more permeable. This change in pore pressure alters the effective normal stress on the slip plane in a way that is analogous to the normal stress alteration in sliding between elastically dissimilar solids. The magnitude of the effect due to induced pore pressure can be comparable to or larger than that induced by sliding between elastic solids with a dissimilarity of properties consistent with seismic observations. The induced pore pressure effect is increased by increasing contrast in permeability but the normal stress alteration due to elastic contrast increases rapidly as the rupture velocity approaches the generalized Rayleigh velocity. Because the alteration in effective normal stress due to either effect can be positive or negative, depending on the contrast in properties, the two effects can augment or offset each other. We calculate details of the pore pressure change using the steady state, dynamic slip weakening, slip pulse model of Rice, Sammis and Parsons (BSSA, 2005). The calculations show that an induced pore pressure increase causes the slip resistance to drop much more rapidly than the intrinsic slip weakening (in the absence of pore pressure changes). Although the largest pore pressure changes occur near the onset of slip, they persist beyond the region of slip weakening to reduce the effective value of the residual shear stress.
DE: 5114 Permeability and porosity
DE: 7209 Earthquake dynamics (1242)
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