Slip Instability and Rupture Propagation During Slow Slip Events

Understanding the mechanics of earthquakes requires quantitative knowledge of fault instability. While the rate-and-state theory provides a general framework for understanding earthquake nucleation processes, recent discoveries of slow slip events, from low frequency earthquakes to episodic tremor and slip events, create new challenges to our understanding of source processes. Seismic data indicate that the seismic moments of various slow slip events scale linearly with their characteristic durations, which is different from the scaling law of regular earthquakes [Ide et al., 2007]. These results suggest that slip instabilities associated with slow events are likely different from those linked to regular earthquakes. However, to date, experimental evidence of different slip behaviors responsible for these events is scarce. In this study, we conducted deformation experiments on porous sedimentary rocks. Effects of decreasing normal stress on slip instability and fracture propagation are investigated. Our data indicate that while the effect of variable normal stress on the brittle strength is negligible, rupture propagation under decreasing normal stress can be considerably slower. Slip instabilities under decreasing normal stress were modeled using an inclined-spring-slider model [Dieterich and Linker, 1992]. We demonstrate that under decreasing normal stress, it is possible for rupture to occur under either velocity strengthening or weakening conditions. The scaling relation between fracture energy release and slip distance in samples undergone slow rupture propagation is different from that during fast rupture propagation.