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.

中文翻译：

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全动态地震周期模拟中特征弱化距离对地震成核型式的影响


地震成核是后续同震破裂扩展的关键准备过程。在速率和状态摩擦 （RSF） 的框架下，发现 与 $b$ 参数的 $a$ 比值控制地震在动态不稳定之前是以扩大的裂纹形式成核还是以固定长度的形式发生。然而，特征弱化距离 $D_{RS}$ 控制了 RSF 中状态变量的弱化效率，并且也会影响成核类型。在这里，我们通过定量评估成核带从构造载荷速率加速到地震滑移速度时的演变，研究了在完全动态地震周期背景下 $D_{RS}$ 对成核类型的影响。需要更大的 $a/b$ （>0.75） 来产生相对较小 $D_{RS}$ 的 膨胀裂纹成核型，这表明固定长度的成核型型可能在自然和实验室断层中占主导地位。此外，当成核位点不在凹凸面的中心时，我们发现了一种更复杂的成核方式，并确定了一种类似孪晶的成核方式，其中包括两个初始加速阶段。我们得出结论，地震成核样式受 的值强烈控制 $D_{RS}$ 。固定长度成核样式可能占主导地位表明，地震破裂的最小规模可以在成核阶段的早期阶段估计。