Photo-acoustic study of subshear and supershear ruptures in the laboratory

We report supershear ruptures observed in spontaneously nucleating laboratory microearthquakes and describe the signature of the associated Mach wavefront radiation. Transducers detect the wavefield both close and at a distance from the fault. The rupture velocities are inferred from either photoelastic high-speed imaging or using the acoustic recordings; both methods yield compatible estimates of sufficient accuracy to discriminate between sub-shear and supershear ruptures. The acoustic records allow to characterize the Mach wavefront radiated from the supershear rupture front, in particular its amplitude and decay. Velocity functions recovered by integrating high frequency accelerometer signals in the case of supershear ruptures, consist in a double-pulse function: a first pulse traveling at supershear velocity followed by a second pulse traveling at the Rayleigh wave velocity. Conversely, the sub-shear event is characterized by a single pulse. Finally, we perform numerical simulations of our experiment using a prescribed supershear rupture velocity. The synthetic waveforms obtained from these simulations, including the Mach wave amplitude and phase, yield a satisfactory fit to the experimental results.