Effect of stress state on slow rupture propagation in synthetic fault gouges

Abstract

Slow slip events (SSEs) in subduction zones are known to proceed so sluggishly that the associated slow ruptures do not generate any detectable radiating seismic waves. Moreover, they propagate at speeds at least four orders of magnitude slower than regular earthquakes. However, the underlying physics of slow slip generation has yet to be understood. Here, we carry out laboratory studies of unstable slip along simulated fault zones of lizardite/chrysotile (liz/ctl) and antigorite (i.e., low-and high-temperature serpentine phases, respectively) and olivine, under varying conditions of normal stress, with the aim of better understanding the influence of stress state on the process of slow rupture along the plate interface. During a single unstable slip, we clearly observe a slow rupture phase that is often followed by an unstable, high-speed rupture. We find that lower fault-zone friction coefficients (μ values from 0.7 down to 0.5) lead to increasing degree of the slow rupture mode, and also that the slow rupture velocities (V r = 0.07 to 5.43 m/s) are largely consistent with those of short-term SSEs observed in nature. Our findings suggest that the generation of SSEs is facilitated by conditions of low normal stress and low fault-zone strength along the plate interface, which may be weakened by metamorphic reactions that result in the production of hydrous phases (e.g., serpentine) and/or the direct involvement of fluid itself, leading to a reduction in effective normal stress.