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

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AbstractPrevious 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.