Acoustic properties of the full spectrum of stick-slip events from stable sliding to dynamic rupture: insights on the mechanics of slow earthquakes and transient fault slip.

Abstract:

Seismic and geodetic observations show that fault slip occurs via a spectrum of behaviors that include slow earthquakes and tectonic tremor. These phenomena have been observed in a variety of tectonic environments worldwide, however the underlying processes are poorly understood. Here we report on lab experiments on simulated fault gouge. We used the double direct shear configuration and varied the loading system stiffness (k) to produce the full spectrum of stick-slip behaviors, with durations ranging from 10^-3 to 1 second. We measured frictional rheology and elastic wave properties throughout the stick-slip cycle for slow and fast events. When the loading stiffness is greater than the fault zone critical rheologic stiffness (k_c) we observe stable frictional sliding. For k≈k_c we document emergent slow-slip events from steady shear. When k_c>k we observe audible stick-slip. Stick slip stress drop and event duration vary systematically as a function of the ratio k/k_c. For slow-slip events, p-wave velocity (V_p) begins to decrease prior to the stress drop and the maximum slip velocity during failure coincides with the largest drop in V_p. Dynamic stick-slip events do not show precursory changes in V_p prior to failure. We find that fault creep and precursory changes in wave properties vary systematically with stick-slip event duration, with slower events showing larger precursory changes. In general, V_p begins to decrease prior to failure and drops abruptly as slip velocity accelerates to a peak value. The drop in V_p appears to be larger during dynamic stick-slip than for slow-slip events. Our results suggest that slow earthquakes and transient fault slip are governed by the same frictional processes as dynamic stick-slip and represent a continuum in the spectrum of fault slip. We show that fault gouge elastic properties evolve during the pre-seismic stage of slow-slip, which could provide an important means of assessing short term seismic hazard.