HR: 11:35h
AN: S12B-06    [Abstracts]
TI: Numerical Studies of Small Repeating Earthquakes and Their Source Parameters Using Laboratory-derived Friction Laws
AU: * Chen, T
EM: tchen@gps.caltech.edu
AF: Caltech, MC 252-21, 1200 E California Blvd, Pasadena, CA 91125, United States
AU: Lapusta, N
EM: lapusta@its.caltech.edu
AF: Caltech, MC 252-21, 1200 E California Blvd, Pasadena, CA 91125, United States
AB: Small repeating earthquakes have short recurrence times and known locations, and hence they present a rare predictable opportunity for detailed observation and insights into earthquake physics. That has been exploited in the San Andreas Fault Observatory at Depth (SAFOD) drilling project. It is important to establish realistic models for their occurrence, to provide a framework for proper interpretation of SAFOD data and other studies. One of the intriguing observation about repeating earthquakes is the scaling of their repeat time T and seismic moment M₀ as T\propto M₀^0.17 (Nadeau and Johnson, 1998). The scaling is abnormal compared to T\propto M₀^1/3, the typical scaling that results from a simple conceptual model of circular ruptures with stress drop independent of the seismic moment and slip proportional to the repeat time. Several explanations for the discrepancy in scaling have been proposed, including high stress drop (Nadeau and Johnson, 1998), shading asperity (Sammis and Rice, 2001), and aseismic slip (Beeler et al., 2001). Our studies show that a model based on Dieterich-Ruina rate and state friction laws reproduces the observed abnormal scaling. In our 3D model, a small patch with rate-weakening friction is surrounded by a much larger region with rate-strengthening friction. Our simulations use the 3D methodology of Liu and Lapusta (AGU, 2006) that fully resolves all aspects of seismic and aseismic behavior of the fault. For a set of laboratory-based friction parameters, we can reproduce the observed scaling simply by varying the size of the rate-weakening patch. When the patch size is smaller than the nucleation size implied by the underlying rate and state formulation, the patch has fully aseismic slip. For larger patch sizes, small repeating events start to occur, with slip rates of order of 1 m/s and sharp stress drops. The events are accompanied by significant aseismic slip on the patch, providing a physical explanation for the idea of Beeler et al. (2001). Our recent and current work pursues several directions. We have studied properties of individual small events and found that their static stress drop, averaged over the zone of seismic slip, is in the typical range, although the stress drop is quite heterogeneous in space. We will use our simulations to investigate how well the average value of stress drop on the interface corresponds to the one determined from near-field seismograms. We have found a trade-off between values of frictional parameters and plate loading rate in determining the repeat time and moment of seismic events, and we will report on our current efforts to quantify it. Simulations in 2D models, needed to study a wider range of formulations, produce the same scaling, although individual small earthquakes have much larger moment and repeat time in 2D than in 3D. Our preliminary simulations with Ruina-Dieterich version of rate and state friction have results qualitatively similar to those with Dieterich-Ruina formulation. We are also exploring models that combine rate and state friction with dynamic weakening in the form of flash heating.
DE: 0545 Modeling (4255)
DE: 7209 Earthquake dynamics (1242)
DE: 7215 Earthquake source observations (1240)
DE: 8020 Mechanics, theory, and modeling
DE: 8118 Dynamics and mechanics of faulting (8004)
SC: Seismology [S]
MN: 2007 Fall Meeting