The Effects of Characteristic Weakening Distance on Earthquake Nucleation Styles in Fully Dynamic Seismic Cycle Simulations

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 with a fixed length prior to the dynamic instability. However, the characteristic weakening distance $D_{RS}$ controls the weakening efficiency of state variables in RSF and can influence the nucleation styles as well. Here we investigate the effects of $D_{RS}$ 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 $D_{RS}$, which suggests that fixed length nucleation styles may dominate on natural and laboratory faults. Furthermore, we find a more complex nucleation style when the nucleation site is not in the center of the asperity and identify a twin-like nucleation style which includes two initial acceleration phases. We conclude that the earthquake nucleation style is strongly controlled by the value of $D_{RS}$. The possible dominance of fixed length nucleation styles suggests that the minimum size of earthquake rupture may be estimated at the early stage of the nucleation phase.