Peng Zhai , Yihe Huang

Earthquake nucleation is a crucial preparation process of the following coseismic rupture propagation. Under the framework of rate-and-state friction (RSF), it was found that the ratios of a to b parameters control whether earthquakes nucleate as an expanding crack or a fixed length. However, the characteristic slip distance DRS controls the weakening efficiency of fault strength and can influence the nucleation styles as well. Here we investigate the effects of DRS on nucleation styles in the context of fully dynamic seismic cycles by evaluating the evolution of the nucleation zone quantitatively when it accelerates from the tectonic loading rate to seismic slip velocity. A larger a/b (>0.75) is needed to produce expanding crack nucleation styles for relatively small DRS, which suggests that fixed length nucleation styles may dominate on natural and laboratory faults. Furthermore, we find that when the nucleation site is not in the center of the asperity, the constant weakening rate near 1 induces a more complex nucleation style. We also identify two special nucleation styles: one containing a temporary acceleration phase (foreshock-like) and the other including a failed acceleration phase (twin-like). We conclude that the earthquake nucleation style is strongly controlled by the value of DRS. Future research needs to be cautious when selecting a few representative DRS to study earthquake nucleation as well as foreshock activities. The possible dominance of fixed length nucleation styles also suggests that the minimum size of earthquake rupture may be estimated at the early stage of the nucleation phase.