Effect of stress state on slow rupture propagation in subduction fault zones

Slow slip events (SSEs) in subduction zones are characterized by a low rupture velocity (~1-10 km/day), long duration (days to years), and no measurable radiating seismic energy. In southwest Japan, short-term SSEs have been observed to occur preferentially along the plate interface beneath the serpentinized mantle wedge. The existence of high Poisson's ratios (~0.4) in these areas suggests that aqueous fluids are present under conditions of near-lithostatic pore pressure. These geophysical observations suggest that fluids play an important role in facilitating slow slip under low stress conditions. However, the physical mechanisms underlying the generation of SSEs are not yet fully understood. To explore the influence of stress state on the process of slow rupture along the plate interface, unstable slip experiments on simulated fault zones of serpentines (lizardite/chrysotile and antigorite) and olivine were performed in a gas-medium triaxial apparatus at room temperature and confining pressures of 60-170 MPa. We show that antigorite and olivine have friction coefficients that agree with Byerlee's law (μ ~0.7), while liz/ctl has a lower friction coefficient of ~0.5. During a single unstable slip, we clearly observe an initial quasi-static slow rupture phase that is almost always followed by an unstable, high-speed rupture. We find that the velocity and duration of slow rupture propagation become lower and longer, respectively, with decreasing confining pressure (at least from 140 down to 60 MPa) and in the case of lower fault-zone strength (i.e., lizardite/chrysotile). Our experimental results 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., serpentines and talc) and/or the direct involvement of fluid itself, leading to a reduction in the effective normal stress.