HR: 11:20h
AN: T22A-05    [Abstracts]
TI: Earthquake Sequences on Rate and State Faults With Strong Dynamic Weakening
AU: * Lapusta, N
EM: lapusta@caltech.edu
AF: Division of Geological and Planetary Sciences and Division of Engineering and Applied Science, California Institute of Technology, 1200 E. California Boulevard, Pasadena, CA 91125 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, 29 Oxford Street, Cambridge, MA 02138 United States
AB: The laboratory-derived rate and state friction laws provide a unique tool for simulating earthquake sequences in their entirety, from accelerating slip in slowly expanding nucleation zones to rapid dynamic propagation of earthquake rupture to post-seismic slip and interseismic creep to fault restrengthening between seismic events. We (Lapusta et al., JGR, 2000) developed an efficient methodology that allows simulating all these stages of the earthquake process within a single algorithm in a model based on the Dieterich-Ruina rate and state laws. However, the laws have been derived from experiments with slip velocities small compared to those in seismic range. Hence these logarithmic laws, although reflecting important physics at slow slip velocities, fail to capture potential much stronger variations of fault strength during dynamic slip. Such variations, in particular much stronger weakening, have been receiving more and more experimental and theoretical support. Hence we study the behavior of 2-D rate and state faults with the constitutive relation modified at high seismic slips and/or slip velocities. One option, motivated by theoretical and experimental results on flash heating and, potentially, thermal pressurization of partially drained fault gauge, is to modify the steady-state frictional strength by the factor 1/(1+V/Vw), where Vw ~ 0.1-1 m/s is the parameter regulating at what slip velocities V the additional weakening steps in. Earthquake sequences simulated with such a law, that also includes static (slow-velocity) friction coefficient of about 0.6 as determined in the lab and effective normal stresses comparable to the overburden minus hydrostatic pore pressure, produce earthquakes that nucleate at high static shear stresses and propagate (and arrest) at low dynamic shear stresses, resulting in huge static stress drops. To avoid large static stress drops, it is NOT necessary to assume uniformly low static friction coefficient and/or uniformly low effective normal stress. It is enough to incorporate isolated weak regions that would nucleate earthquake ruptures under low overall shear stresses. Some of these ruptures then continue into the statically strong fault regions due to dynamic weakening. Such model results in fault operation at low shear stresses and low heat production, with reasonable static stress drops and earthquake ruptures propagating as self-healing pulses and hence satisfies a number of basic observational constraints. Similar results have been obtained (Rice, AGU, 1996) when the rate and state friction was modified by adding slip-dependent pore pressure evolution appropriate to undrained adiabatic shear heating of pore fluids. The simulations were done using quasi-dynamic approximation. We will report on our current efforts to incorporate such pore pressure evolution into a fully dynamic simulation of earthquake sequences as well as to combine it with the flash heating weakening mechanism discussed above. Flash heating and pore pressurization are likely to coexist during rapid frictional sliding, as the former occurs at large slip velocities and small slips while the latter occurs at large slip velocities and large slips. Flash heating decreases frictional resistance very early in the sliding process and hence it can influence the effectiveness of the pore pressurization mechanism.
DE: 7260 Theory and modeling
DE: 8123 Dynamics, seismotectonics
DE: 7209 Earthquake dynamics and mechanics
DE: 7215 Earthquake parameters
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting