# Nucleation Mechanism and Rupture Dynamics of Laboratory Earthquakes at Different Loading Rates
###### Abstract  
The loading rate of tectonic stress is not constant during long-term geotectonic activity and significantly affects the earthquake nucleation and fault rupture process. However, the mechanism underlying the loading rate effect is still unclear. In this study, we conducted a series of experiments to explore the effect of the loading rate on earthquake nucleation and stick-slip characteristics. Through lab experiments, faults were biaxially loaded at varying rates to produce a series of earthquakes (stick-slip events). Both shear strain and fault displacement were monitored during these events. The findings indicate a substantial effect of the loading rate on the recurrence interval and the shear stress drop of these stick-slip events, with the recurrence interval inversely proportional to the loading rate. The peak friction of the fault also decreases with the increasing loading rate. Notably, prior to the dynamic rupture of earthquakes, there exists a stable nucleation phase where slip occurs in a quasi-static manner. The critical nucleation length, or the distance required before the dynamic rupture, diminishes with both the loading rate and normal stress. A theoretical model is introduced to rationalize these observations. However, the rupture velocity of these lab-simulated earthquakes showed no significant correlation with the loading rate. Overall, this study enhanced our comprehension of earthquake nucleation and rupture dynamics in diverse tectonic settings. rupture velocity; loading rate; nucleation; rupture dynamics +
Footnote †: journal: Medical  
the nucleation phase of certain earthquakes [8; 9; 10]. Recently, the accelerated slip phase accompanied by a swarm of earthquakes was recognized weeks to months before major earthquakes [11; 12]. Particularly, two slow slip transients propagating toward the epicenter of the 2011 Mw 9.0 Tohoku-Oki earthquake were believed to promote the occurrence of the mainshock [9]. Since the occurrence time and location of mainshocks are intimately related to the nucleation process [13], understanding the mechanism of earthquake nucleation is of great importance for earthquake prediction. Over the past few decades, many efforts have been devoted to quantitatively characterizing the earthquake nucleation process, especially the factors controlling the critical nucleation length. Laboratory experiments [14; 15; 16; 17] and numerical simulations [18] showed that the critical nucleation length scales inversely with the normal stress, which is consistent with the prediction from the theoretical model [19]. Another factor that may control the rupture behaviors of faults is the loading rate [20; 21; 22; 23; 24]. The loading rate of natural faults, typically determined through geodetic measurements, can be significantly influenced by the creep of adjacent segments along the fault system. This phenomenon can result in local loading rates that vary by order of magnitude compared to the overall tectonic loading rate, causing substantial differences in stress accumulation along the fault [25; 26; 27; 28]. Some laboratory experiments and theoretical investigations showed that a higher loading rate contributes to a transition from stick-slip to stable slip [20; 22; 29; 30; 31]. On the contrary, some experimental results suggested that a higher loading rate may promote unstable slip [3; 6; 32] or favor ruptures with higher speed [33]. It was also suggested that the impact of loading rates on fault slip behaviors hinges on the fault's proximity to a steady state. While typically higher rates drive faults away from steadiness, causing increased instability, in the context of stiff loading systems applying constant, high rates, faults are instead nudged toward a steady state, resulting in stable sliding [15]. This nuanced interaction underscores the complexity of fault responses in varying loading scenarios and surroundings [34]. Furthermore, the loading rate could also affect the nucleation process of earthquakes.