HR: 17:30h
AN: S34A-07    [Abstracts]
TI: Modes of Dynamic Rupture Propagation and Rupture Front Speeds in Earthquake Models That Account for Dynamic Weakening Mechanisms
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, Pasadena, CA 91125 United States
AB: Laboratory experiments and theories of how fault materials respond suggest that the constitutive response of faults is far from simple. For slow slip rates, laboratory-derived rate and state friction formulations incorporate small, less than 10%, variations in frictional strength about a representative value which is the product of a typical slow-rate friction coefficient (0.6-0.7 for most rock surfaces and fault-like gouge) times the effective normal stress (which is comparable to overburden minus hydrostatic pore pressure, about 150 MPa at the representative seismic depth of 8 km). One could refer to this slow-rate frictional strength as (high) static fault strength. For fast sliding velocities and large slips, additional weakening mechanisms are activated that result in much lower frictional resistance during dynamic sliding. Hence we need to build earthquake models that would account for both high static strength and low dynamic strength of faults. At first, it seems that the combination of high static strength and low, near-zero, dynamic strength should create static stress drops that are large compared to 1-10 MPa static stress drops typically observed. However, Rice (AGU, 1994) and Lapusta and Rice (AGU, 2003, 2004) proposed a model that avoids that pitfall by incorporating small defect regions that nucleate ruptures while the average stress on the fault is still low compared to its static strength. By simulating earthquake sequences in the framework of a 2D depth-averaged elastic model of a faulted crustal plate, they showed that the fault would then operate with reasonable static stress drops, low shear stress, and low heat generation as follows: Earthquakes nucleate under low shear stress in a defect (weak) and then propagate into strong regions due to significant dynamic weakening. The simulations incorporated truly slow, tectonic-type loading of 35 mm/year and resolved all stages of the simulated earthquakes, including the nucleation process and inertial effects during dynamic rupture. The constitutive behavior of the model fault was governed by the Dieterich-Ruina rate and state friction law modified to permit much stronger weakening at high slip rates and/or large slips. Two dynamic weakening mechanisms were considered: mostly rate-dependent flash heating and mostly slip-dependent pore pressure evolution appropriate to undrained adiabatic shear heating of pore fluids. Here we extend that study by concentrating on other features of the simulated earthquake sequences such as modes of dynamic rupture propagation and rupture front speeds. For simulations with mostly rate-dependent flash heating, the rupture propagates as a pulse of slip when the prestress before the event is low, as an echelon of pulses when the prestress is higher, and as a crack when the prestress is high (comparable to the static strength). This is consistent with the study of Zheng and Rice (BSSA, 1998). The rupture speed is always close to the shear wave speed of the material. For simulations with mostly slip-dependent pore pressure evolution, the rupture always propagates in a crack-like mode. The rupture speed, however, can be a small fraction of the shear wave speed for sufficiently low prestress. The likely reason is that the continuing decrease of strength with slip due to pore pressure evolution results in a crack problem with large fracture energy. We will report on our current work that aims to address and quantify these issues in simulations that combine flash heating with pore pressure evolution.
DE: 0560 Numerical solutions (4255)
DE: 7209 Earthquake dynamics (1242)
DE: 7215 Earthquake source observations (1240)
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8163 Rheology and friction of fault zones (8034)
SC: Seismology [S]
MN: Fall Meeting 2005