Slip Rate-Dependent Friction as a Universal Mechanism for Slow Slip Events

Abstract:

A growing body of observations worldwide has documented fault slip transients that radiate little or no seismic energy. The mechanisms that govern these slow slip events and their wide range of depths, slip rates, durations, stress drops, and recurrence intervals remain poorly known. Here we show that slow slip can be explained by a transition from rate-weakening frictional sliding at low slip rates toward rate-neutral or rate-strengthening behavior at higher slip rates, as has been observed experimentally, and that tremor-like vibrations are associated with these slow earthquakes when a zone of low effective normal stress is introduced. We use numerical simulations to illustrate that this rate-dependent transition to slow slip quantitatively explains experimental data for natural fault rocks representative of materials in the source regions of slow earthquakes. Our simulation results produce transitions in fault slip modes between stable sliding, stick-slip, slow stick-slip, and quasi-harmonic vibration. However, with a standard constant-parameter rate-and-state friction law, slow slip events arise only in for a narrow range of conditions at or near the threshold for slip instability. The inclusion of velocity dependent friction parameters significantly broadens the range of conditions for slow slip, and produces a wide range of event characteristics, including stress drop, duration, and recurrence, as are observed in nature. Upscaled numerical simulations that incorporate parameters consistent with laboratory measurements can reproduce geodetic observations of repeating slow slip events on tectonic faults. Additionally, when we incorporate a zone of low effective normal stress (assumed to arise from high pore pressure) in our simulations, tremor-like vibrations arise concurrently with the slow earthquake events – also as is commonly observed for many slow slip events on tectonic faults and in other geologic settings. We conclude that slip rate-dependent friction explains the ubiquitous occurrence of slow slip events in a broad range of geological environments.