HR: 0800h
AN: S11A-0992    [Abstracts]
TI: Simulations of Earthquake Nucleation, Its Static Perturbation, and Aftershock Rates on Faults with Rate and State Friction
AU: * Kaneko, Y
EM: ykaneko@gps.caltech.edu
AF: Division of Geological and Planetary Sciences, California Institute of Technology, 1200 East California Boulevard, MC 252-21, Pasadena, CA 91125 United States
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 East California Boulevard, MC 104-44, Pasadena, CA 91125 United States
AB: Large earthquakes are usually followed by an increased seismic activity that decays to the background earthquake rate over time. The decay is well-described by the empirical Omori's law. Dieterich (JGR, 1994) proposed that Omori's law could result from statically perturbing a population of nucleation sites that are governed by laboratory-derived rate and state friction laws. Dieterich considered a spring-slider setting for each nucleation site with the assumption that the acceleration during nucleation is such that the state variable is significantly behind its steady-state value. This allowed him to derive analytical formulae for the nucleation process as well as for the resulting earthquake rate increase. This model has been used to analyze aftershock sequences and inspired further theoretical studies. In particular, Gomberg et al. (JGR, 2000) considered numerically aftershock rates in the spring-slider block model with the full rate and state friction relations. Our goal is to verify Dieterich's conclusions in a 2-D continuum setting, where the nucleation process can be more complicated than assumed in Dieterich's model (Lapusta, SCEC meeting, 2003). First, we simulate a sequence of earthquakes using the methodology of Lapusta et al. (JGR, 2000) and select the nucleation process of a typical event for further study. This ensures that the nucleation process proceeds in conditions naturally occurring in the model and not arbitrarily imposed. Next, we perturb the model by a static stress change at some time during the nucleation process and simulate the resulting behavior, which lets us to construct numerically the dependence F of the new time to instability on the timing of the static perturbation. Finally, following Dieterich (JGR, 1994), we imagine a population of nucleation sites each of which following the behavior observed in our model but being at a different point in the nucleation process such that the whole population would result in a uniform background rate. Then we use the dependence F to find how the earthquake rate generated by this population is changed when the static stress perturbation is imposed. We find that if the nucleation process proceeds in relatively uniform conditions then the results, in terms of the changed earthquake rate, are well-described by the Dieterich's model. This is despite the fact that the nucleation process is non-uniform spatially (the slowly sliding nucleation region is spreading) and hence is not well-represented by a single degree of freedom spring-slider model. If, on the other hand, the nucleation process occurs in more complicated conditions, such as stress concentration caused by the nearby creeping region, the resulting aftershock behavior can be somewhat different, with additional peaks in the aftershock activity. The important distinguishing feature seems to be the validity of the Dieterich's assumption about the behavior of the state variable; it is almost always valid in the uniform case but fails for some parts of the nucleation process in more complex scenarios. We will report on our current efforts to study the parameter space and understand the significance, if any, of the observed differences.
DE: 7230 Seismicity and seismotectonics
DE: 7260 Theory and modeling
DE: 7209 Earthquake dynamics and mechanics
DE: 7215 Earthquake parameters
SC: Seismology [S]
MN: 2004 AGU Fall Meeting