HR: 15:25h
AN: S23C-08    [Abstracts]
TI: Laboratory Study of Transient Stress Effects on Fault Stability
AU: * Savage, H M
EM: hsavage@geosc.psu.edu
AF: The Pennsylvania State University, Deike Building, State College, PA 16802 United States
AU: Marone, C J
EM: cjm@geosc.psu.edu
AF: The Pennsylvania State University, Deike Building, State College, PA 16802 United States
AB: Transient dynamic stresses, such as seismic waves and Earth tides, can destabilize fault slip and trigger earthquakes. Understanding the effects of transient stressing on fault zone frictional strength would enhance our understanding of the earthquake cycle. Earthquake triggering does not depend simply on the magnitude of the trigger but rather might be a function of the dynamic stress associated with the propagating elastic wave. The amplitude and frequency of the seismic wave, along with material properties of the fault zone, may determine whether triggering will occur. In natural settings, earthquake triggering is difficult to study due to static stress transfer near the fault. Therefore, we use laboratory experiments to study this process. We analyzed the shear stress response of a laboratory fault to transient, periodic loading rate oscillations. Experiments were conducted using a servo-controlled, biaxial apparatus with a double-direct shear configuration. Layers of glass beads, 3 mm thick and with nominal frictional contact dimensions of 100 cm2 were loaded via a shear displacement boundary condition consisting of a linear function with a sinusoid superimposed to simulate oscillating, transient stressing. Normal stress was held constant at 5 MPa, which is low enough to avoid comminution of the material. We studied amplitude and frequency of the velocity oscillation in the range 2-10 micron/sec and 0.01-4 Hz, respectively. Glass beads were used because they exhibit repeated, consistent stick-slip frictional sliding. We measured the timing of stick-slip events relative to velocity oscillations to investigate the phase correlation between oscillations and instabilities. If instabilities occurred consistently at a given phase, the timing of the stick-slips was considered correlated with the oscillations. Preliminary results indicate that both amplitude and frequency of the oscillation determine the correlation between the phase of the velocity oscillations and stick-slips. Generally, higher amplitudes (approaching the background loading rate) are needed to produce consistent correlation between oscillations and instabilities. However, each suite of experiments indicates a critical frequency at which correlation between stick-slip and oscillation occurs at lower amplitudes. For instance, 1 Hz oscillations produced strong correlation at amplitudes of 3 micron/sec for experiments with a background loading rate of 10 micron/s. Higher and lower frequencies required larger amplitudes to produce the same correlation. Forward models of the laboratory experiments using rate and state friction equations and assuming quasi-static motion mimic the laboratory results, showing a critical frequency at which small amplitude oscillations correlate with instability. Parameteric studies of the forward models predict that this frequency is related to stiffness. Our results indicate that transient stressing and the passage of seismic waves at a critical frequency and amplitude can destabilize fault slip and cause stick-slip instabilities.
DE: 8020 Mechanics
DE: 7209 Earthquake dynamics and mechanics
SC: Seismology [S]
MN: 2004 AGU Fall Meeting