Supershear Ruptures Along a Circular, Bimaterial, Experimental Fault

Abstract:

Analyses of dynamic shear rupture along experimental faults have provided significant insight into the processes of earthquake dynamics. Most experimental faults were set in direct shear configurations with free edges in which the propagating ruptures encountered complicated boundary conditions that could affect the rupture characteristics. We eliminated the free edge effects by investigating spontaneous dynamic ruptures along a circular fault that may be considered as a long fault proxy. The experimental, ring-shape fault is composed of two PMMA cylinders (ID and OD of 78.5 mm and 102.2 mm, respectively), of different heights (upper and lower block heights of 22.4 mm and 41.2 mm, respectively). The stiffness difference created a geometrical biomaterial fault. We monitor dynamic rupture propagation through 10 sets of 3-component rosette strain gauges mounted at ~2.25 mm from the fault surface with 1 MHz sampling rate, along with macroscopic stresses and velocities monitoring at up to 5 KHz rate. The fault was loaded by rotation at circumferential velocity of 16-152 micron/s and normal stresses of 2.3-6.7 MPa.

During a stick-slip event, the ruptures originated spontaneously at a nucleation site, and propagated bilaterally along the circular interface. Ruptures propagating parallel and opposite to the fault slip direction are defined as positive and negative ruptures, respectively. Most ruptures propagated at supershear velocity (> 1350 m/s), with 79% and 60% supershear propagation in positive and negative ruptures, respectively. Negative ruptures typically started with slow speed of hundreds of m/s from the nucleation site, followed by sustained acceleration with propagation, whereas positive ruptures initiated with supershear ruptures, and maintained supershear speed with propagation. The supershear ruptures have a distinct shear-strain pattern characterized with initial rise followed by intense strain drop which occurred during a period of 2-4 ms. The pulse-like supershear rupture fronts span less than 2 cm, with particle velocity up to ~ 2 m/s. Rupture velocity increases with increasing normal stress. The rupture slip-pulses induce substantial fault strength reduction during slip-displacements up to tens of microns, with fault weakening intensity proportional to rupture front velocity.