Intersonic shear rupture mechanism

Abstract

According to the theory the rupture speed in solids for mode-I cracks is limited by the Rayleigh speed c R, while mode-II cracks can propagate intersonically. These theoretical predictions and sustaining experiments were made for the idealized condition of a crack propagating along a predetermined weak and straight-line path in a homogeneous linear elastic material. In real materials, however, the mode-I crack speed has never been observed to exceed 0.65 c R. The reason for this is the natural tendency for physical cracks to follow a wavy path and for microbranching, which results in a significant increase in microcrack population and, consequently, in the fracture energy. At the same time, intersonic shear ruptures (mode-II cracks) have been reported for crustal earthquakes. It seems paradoxical because earthquake ruptures are normally associated with high complexity and extreme damage in the rupture zone. The present paper shows that nature has provided special shear rupture mechanisms acting in hard rocks at high confining pressure that minimize the rupture energy, causing the increase in rupture speed. These mechanisms are different for primary ruptures (continuous thin ruptures with uniform structure) and general faults (complex discontinuous systems). The general faults propagate in a jump-like manner, forming a cascade of segments due to an advanced triggering mechanism. The advanced triggering mechanism triples the propagation speed of a general fault compared with primary fractures involved in the fault. The propagation of primary ruptures is facilitated by another mechanism, which is a fan-shaped self-equilibrated mechanism created on the basis of an echelon of rotating blocks representing the intrinsic nature of the shear crack structure. These two mechanisms acting in combination can provide intersonic propagation of general faults.