Characterizing the Final Stage of Simulated Earthquake Nucleation Governed by Rate-And-State Fault Friction

Previous experiments and modeling on earthquake nucleation reveal accelerating slip and development of a final patch of fixed or expanding length, but whether the nucleation phase is spatially large enough to be detected in real-Earth conditions with durations long enough to be helpful is unknown. This study performed a suit of simulations of the nucleation process based on the rate-and-state friction law on homogeneous faults started from a "locked" state. Our results reveal the development of a weakening-zone core (WZcore, defined as a region bounded by two outwardly moving shoulders of stress distribution during the early phase, and later bounded by two inwardly shifting edges between the stress-releasing region and stress-increasing exterior) where stress releases continuously, which expands first to a dimension less than the half fault length then shrinks to a final length before re-expanding. The slip rate within the WZcore is above several times of the driving rate, and the high-rate region expands constantly up to the instability. The WZcore is similar to a strain-releasing zone in our new experiment. Under the assumption that much higher seismicity rate can be observed within the WZcore due to higher slip rate and stress-releasing state as augmented driving force for small asperities, these nucleation features, as well as the slip rate levels in our model, are consistent with step-like increases in seismicity preceding large subduction earthquakes. On the time scale, duration of the WZcore is weeks before large subduction earthquakes under high driving rate, but years on low-rate continental faults.