HR: 0800h
AN: T21A-0374    [Abstracts]
TI: Episodic Slow-Slip Transients and Rate-and-State Friction
AU: * Rubin, A M
EM: arubin@princeton.edu
AF: Department of Geosciences, Princeton University, Princeton, NJ 08540, United States
AU: Segall, P
EM: segall@stanford.edu
AF: Department of Geophsyics, Stanford University, Stanford, CA 94305, United States
AB: Numerical simulations dating back to Horowitz and Ruina [JGR 1989] suggest that aseismic slip transients may result if the velocity-weakening portion of a fault is large enough to nucleate an event but too small for that event to reach instability. Models of rate-and-state friction can credibly explain both the large dimensions of slow slip events in subduction zones and their propagation velocity, provided the effective normal stress is very low (of order 1 MPa). However, both standard laws for the evolution of fault "state" fail to explain other first-order features of these events. For lab-like values of a/b, where a and b are the coefficients of the velocity- and state-dependence of the frictional strength, the fault length needed for instability depends upon the effective fracture energy of the expanding nucleation zone. For the "aging" evolution law this fracture energy increases as the square of the logarithm of the velocity excursion, and the range of fault lengths hosting aseismic transients grows in rough proportion to (1-a/b)^-1. Near neutral stability this range is quite large. However, existing lab data strongly favor the "slip" evolution law as a predictor of nucleation style. For this law the effective fracture energy increases only as the logarithm of the velocity excursion, and the range of fault lengths capable of hosting slow slip events appears to be too small to explain why these events are so common. Even for the aging law, the range of fault lengths generating slip speeds reaching those inferred geodetically (~10-100 times the plate rate) is very small. The reason is that over most of parameter space where slow slip events are produced, the bulk of the increasing mechanical energy needed to grow the nucleation zone is supplied by the steady plate motion down- dip. The resulting slip speed cannot greatly exceed the plate rate. For steady-state friction that decreases as the logarithm of the slip speed, stable slip speeds significantly exceeding the driving rate can be reached over a substantial range of fault lengths if the fracture energy increases more rapidly than the square of the logarithm of the velocity excursion. An appealing mechanism is inelastic dilation of the fault zone coupled with pore pressure reduction, which becomes more effective at low effective stress. This is demonstrated in the companion presentation using a simplified representation of pore-pressure diffusion.
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting