HR: 0830h
AN: S41C-0085    [PDF]
TI: The Effect of Dynamic Stress on Shear Strength and Stability in Laboratory Faults
AU: * Savage, H M
EM: hsavage@geosc.psu.edu
AF: Pennsylvania State University, Department of Geosciences, University Park, PA 16802 United States
AU: Marone, C
EM: cjm@geosc.psu.edu
AF: Pennsylvania State University, Department of Geosciences, University Park, PA 16802 United States
AB: Recent studies show that earthquake-induced stress changes can trigger fault slip, change seismicity rates, and possibly cause additional damaging earthquakes. Investigations of stress transfer often focus on changes in the static stress state. However, dynamic stressing can lead to destabilization of creep motion and resonant behavior including stick-slip instability, both locally and at greater distances from the earthquake event. Studies of stress transfer commonly find that earthquake triggering occurs from small changes in stress and is delayed relative to the arrival of maximum stress. This phenomena cannot be explained by the Coulomb failure model, which predicts a constant failure threshold independent of stressing rate or time. We test the effects of dynamic shearing, using oscillations of shear loading rate and we compare results to theory. Using a double direct shear configuration, we sheared 3mm thick layers of glass beads (size distribution 105-149 microns) at room temperature. Glass beads are an ideal substance for these tests due to the repeatability of stick-slip events in terms of their magnitude and recurrence interval at constant velocity. The samples were loaded to quasi-periodic failure under a normal stress of 5 MPa (eliminating any changes in stick-slip behavior with displacement) and a background shear loading rate with a sinusoidal oscillation superimposed. Amplitude and frequency of the sine wave were varied, along with background shear loading rate, to determine any systematic shear stress response. We studied amplitudes ranging from 1-30 microns/sec, frequencies ranging from 0.01-10 Hz and load rates of 0.1-20 microns/sec. Although loading rate changes sign, shear stress remains positive throughout the experiment. Shear stress response was determined by studying systematic variations in stick-slip properties, e.g. amplitude of stress drop, frequency, and phase of the stick-slip stress drops compared to the imposed loading rate oscillations. Laboratory results of frictional response are compared to forward models of the rate and state friction laws. Preliminary results indicate that velocity oscillations lengthen the stick-slip recurrence interval and create generally larger instabilities; lower frequency oscillations create longer recurrence intervals than higher frequency oscillations. The phase of the stress drop relative to the velocity oscillation varies with oscillation frequency. During 0.1Hz oscillations, stress drops occur before the maximum loading velocity is reached whereas during 1Hz oscillations stress drops are delayed relative to the maximum velocity.
DE: 7209 Earthquake dynamics and mechanics
DE: 8010 Fractures and faults
DE: 8020 Mechanics
SC: Seismology [S]
MN: 2003 Fall Meeting