HR: 0800h
AN: T21B-0487    [Abstracts]
TI: Laboratory constraints on earthquake nucleation
AU: Kilgore, B
EM: bkilgore@usgs.gov
AF: USGS, 345 Middlefield Rd MS977, Menlo Park, CA 94025
AU: * Beeler, N M
EM: nbeeler@usgs.gov
AF: USGS, 345 Middlefield Rd MS977, Menlo Park, CA 94025
AB: A simple view of time dependent earthquake nucleation from laboratory experiments is there are minimum and dominant nucleation patch sizes that are determined by the asperity contact dimension and the dependence of the fault strength on slip rate. Direct observations of nucleation (Okubo and Dieterich, 1984; 1986), 1D slider block analysis with lab-based constitutive equations (Dieterich, 1986; Dieterich and Kilgore, 1996), and some plane strain simulations (Dieterich, 1992)support this simple view. Extrapolations to natural stressing rates using laboratory measured parameters suggest that it would be impossible to resolve nucleation using surface and space-based strain sensors, and would be extremely difficult to detect in the subsurface using the most sophisticated borehole strain meters. However, recent plane strain simulations using rate and state constitutive relations (Rubin and Ampuero, in press, JGR and unpublished) show that only for relatively large negative rate dependence is there a dominant nucleation patch size and that different empirical lab-based evolution relations predict dramatically different behaviors. Rubin and Ampuero show that a key fault property controlling nucleation patch growth is the effective shear fracture energy- a parameter that unfortunately was not explicitly considered during the development of the various constitutive relations. For fracture energy that increases with slip rate the nucleation patch size can grow slowly in time and produce seismically detectable precursory slip. We present experimental constraints on nucleation size, time-dependent growth and effective shear fracture energy from experiments conducted on a 2 m long fault (Okubo and Dieterich (1984; 1986). We have conducted new constant loading rate nucleation experiments, using a dense array of strain and displacement sensors on the fault, loading rates of 0.001 to 0.0001 MPa/s and a narrow range of normal stress (4- 6 MPa). These produce a more complete view of the spatial and temporal evolution of slip and energy dissipation during nucleation than previously available. We have also undertaken novel static fatigue and hold tests to characterize slip- and time-dependent effects during nucleation. Preliminary results are consistent with a characteristic nucleation patch and show no evidence of the slow growth seen in simulations. At the same time, the detailed behaviors, particularly fracture energy, and the instantaneous rate dependence are not completely consistent with 1D implementations of existing empirical constitutive relations.
DE: 7215 Earthquake source observations (1240)
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8163 Rheology and friction of fault zones (8034)
SC: Tectonophysics [T]
MN: Fall Meeting 2005