Switching from seismic faulting to silent slips in harzburgite induced by H2O fluid at upper mantle pressures

Slow-slip events frequently occur, but regular earthquakes are much less active on the H2O fluid-rich subduction interface at depths of ~ 40 km. The characteristic duration for silent earthquakes, which are categorized as the great slow-slip events, is more than five orders of magnitude longer than that for regular earthquakes. Such phenomena are often attributed to the slippage of the softer part of the subduction interface, but the impact of H2O fluid on aseismic slip is still unsolved. Here, we conduct deformation experiments on water-saturated harzburgite at pressures of 1.2 to 3.0 GPa and temperatures of 770 to 1250 K, corresponding to the conditions of the lower part of the overriding plate just above the subduction interface. We observe deformation of the harzburgite followed by silent faulting at a significantly low stress level down to 0.3 GPa under fluid-bearing conditions, even though many acoustic emissions are generated at the onset of faulting in fluid-free harzburgite. We find that the observed silent faulting is caused by the detachment of asperity contacts by high pore pressures and lubrication of the fault plane by a hydration reaction. We therefore propose that H2O fluid may prevent the occurrence of regular intraslab earthquakes, but trigger silent earthquakes.