Delayed Triggering and Insights into Rate- State Friction Properties During Stick-Slip

Heather M. Savage, & Nicholas J. van der Elst

Published December 2013, SCEC Contribution #2052

Remote triggering observations often include earthquakes that are delayed with respect to the passage of seismic waves. These delayed triggered earthquakes may simply be aftershocks of initial triggered events, or they may point to a slower or prolonged triggering process. The nature of this delay mechanism remains a topic of considerable debate. Stick-slip experiments on a dry, granular, laboratory fault show that stress perturbations do not only trigger slip during the transient stress, but also advance the timing of slip well after the transient has passed. Delayed triggering therefore does not necessarily require any secondary triggering or non-frictional mechanisms. Our experiments were performed in a biaxial deformation apparatus, with a double-direct shear sample geometry. Our fault consists of 3-mm thick layers of soda lime glass beads. Glass beads show remarkably consistent stick-slip recurrence intervals, making them an ideal material to study triggering. The background loading rate is constant, and small velocity oscillations of varying amplitude and frequency are introduced at different intervals in the stick-slip cycle. We find that for a given stress amplitude, higher frequency oscillations (2-3 Hz) lead to statistically significant delayed triggering whereas 1 Hz frequencies do not (the higher frequency oscillations also have a faster transient loading velocity). To determine what makes higher frequency triggers more effective at delayed triggering, we measure the transient mechanical response of the fault to a load point oscillation at different points in the stick-slip cycle. We find that the response varies considerably throughout the cycle, with slip velocity and dilation increasing systematically, and transient stress amplitude decreasing. The results suggest a monotonic decrease in the rate-state friction direct effect, a, throughout the stick-slip cycle. The evolution of frictional parameters during the interseismic cycle would have important ramifications for the time-varying seismic potential of faults.

Savage, H. M., & van der Elst, N. J. (2013, 12). Delayed Triggering and Insights into Rate- State Friction Properties During Stick-Slip. Presentation at 2013 AGU Fall Meeting.