The rupture fronts that mediate the onset of frictional sliding may propagate at speeds below the Rayleigh wave speed or may surpass the shear wave speed and approach the longitudinal wave speed. While the conditions for the transition from sub-Rayleigh to supershear propagation have been studied extensively, little is known about what dictates supershear rupture speeds and how the interplay between the stresses that drive propagation and interface properties that resist motion affects them. By combining laboratory experiments and numerical simulations that reflect natural earthquakes, we find that supershear rupture propagation speeds can be predicted and described by a fracture mechanics-based equation of motion. This equation of motion quantitatively predicts rupture speeds, with the velocity selection dictated by the interface properties and stress. Our results reveal a critical rupture length, analogous to Griffith's length for sub-Rayleigh cracks, below which supershear propagation is impossible. Above this critical length, supershear ruptures can exist, once excited, even for extremely low preexisting stress levels. These results significantly improve our fundamental understanding of what governs the speed of supershear earthquakes, with direct and important implications for interpreting their unique supershear seismic radiation patterns.

中文翻译：

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超剪切摩擦破裂前沿的运动方程。

介导摩擦滑动开始的破裂前沿可能以低于瑞利波速度的速度传播，或者可能超过剪切波速度并接近纵向波速度。虽然已经广泛研究了从亚瑞利传播到超剪切传播的过渡条件，但对于决定超剪切破裂速度的因素以及驱动传播的应力与抵抗运动的界面特性之间的相互作用如何影响这些知之甚少。通过结合反映自然地震的实验室实验和数值模拟，我们发现可以通过基于断裂力学的运动方程来预测和描述超剪切破裂传播速度。这个运动方程定量地预测破裂速度，速度选择取决于界面属性和应力。我们的结果表明，临界断裂长度类似于亚瑞利裂纹的格里菲斯长度，在该长度以下是不可能发生超剪切传播的。超过此临界长度，即使受到极低的预先应力水平，一旦受激，仍可能存在超剪切断裂。这些结果大大提高了我们对控制超剪切地震速度的基本理解，对解释其独特的超剪切地震辐射方向图具有直接而重要的意义。即使对于极低的既有压力水平也是如此。这些结果大大提高了我们对控制超剪切地震速度的基本理解，对解释其独特的超剪切地震辐射方向图具有直接而重要的意义。即使对于极低的既有压力水平也是如此。这些结果大大提高了我们对控制超剪切地震速度的基本理解，对解释其独特的超剪切地震辐射方向图具有直接而重要的意义。