Shear loading stiffness plays a critical role in conditioning the stability of slip on reactivated faults. However, a relationship linking peak slip velocities and shear loading stiffness is lacking. To explore this, we shear granite faults in double direct shear with shear loading stiffnesses spanning two orders to define the effects of shear loading stiffness in conditioning the transition from stable to unstable slip. Our results show that peak slip velocity and acceleration decrease as power law relationships with respect to the shear loading stiffness ratio, with identical exponent, and with a linear relationship between the peak slip velocity and acceleration. The maximum acceleration occurs during the velocity weakening process that limits the peak slip velocity. The power law exponent increases linearly with increasing normal stress. Experimental results also indicate that slip magnitude, stress drop and recurrence time of unstable slip events increase, and slip durations decrease with decreasing stiffness ratios. The spans of limit cycles of velocity versus shear stress decrease, and their shapes evolve from triangular to semicircular with increasing loading stiffness ratio. Stress drops mostly occur during deceleration. The deceleration phase dominates the unstable slip duration that decreases as peak slip velocity increases. Our results indicate that the average stress drop rates over a slip duration increase as a power law relationship with reducing shear loading stiffness, which also contributes to a lower shear loading stiffness producing increased slip velocities and accelerations. Our findings highlight that loading stiffness ratio is an underlying mechanism defining unstable slip behaviors with the normal stress merely conditioning the exponent. The present relationship of unstable slip velocity with shear loading stiffness suggests a way to evaluate hazard of an impending instability event based on the initial shear loading stiffness ratio that can be calculated through the linear-elastic behaviors at the early quasi-static phase.

中文翻译：

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不稳定滑移速度对剪切载荷刚度降低的幂律依赖性


剪切载荷刚度在调节重新激活断层上滑移的稳定性方面起着至关重要的作用。然而，缺乏峰值滑移速度和剪切载荷刚度之间的关系。为了探索这一点，我们在双直接剪切中剪切花岗岩断层，剪切载荷刚度跨越两个阶，以定义剪切载荷刚度在调节从稳定滑移到不稳定滑移的过程中的影响。我们的结果表明，峰值滑移速度和加速度与剪切载荷刚度比呈幂律关系，具有相同的指数，并且峰值滑移速度和加速度之间呈线性关系。最大加速度发生在限制峰值滑移速度的速度弱化过程中。幂律指数随着正应力的增加而线性增加。实验结果还表明，随着刚度比的减小，滑移幅度、应力降和不稳定滑移事件的重现时间都会增加，滑移持续时间会减少。随着载荷刚度比的增加，速度与剪应力的极限环跨度减小，形状从三角形演变为半圆形。应力下降主要发生在减速过程中。减速阶段主导着不稳定滑移持续时间，该持续时间随着峰值滑移速度的增加而减少。我们的结果表明，滑动持续时间内的平均应力下降率随着剪切载荷刚度的降低呈幂律关系而增加，这也有助于降低剪切载荷刚度，从而增加滑动速度和加速度。我们的研究结果强调，负载刚度比是定义不稳定滑动行为的基本机制，法向应力仅调节指数。 目前不稳定滑移速度与剪切载荷刚度的关系提出了一种基于初始剪切载荷刚度比来评估即将发生的不稳定事件的危险的方法，该初始剪切载荷刚度比可以通过早期准静态阶段的线弹性行为计算出来。