Supershear and Sub-Rayleigh Rupture Behavior Quantified by Full-field Laboratory Measurements

Abstract:

Supershear earthquakes, such as the 1906 San Francisco and the 2018 Palu earthquakes, result in more destructive shaking than sub-Rayleigh ones. At the same time, sub-Rayleigh earthquakes are more common. Thus it is important to characterize the pattern of deformation associated with both supershear and sub-Rayleigh ruptures. In this presentation, we characterize the fields of dynamic ruptures using densely-instrumented laboratory earthquakes featuring ultrahigh-speed digital image correlation. Our measurements show the full-field structure of dynamic ruptures at a level of detail that, until recently, was only attainable by numerical simulations. The maps of particle velocity confirm the dominance of the fault-parallel motion over the fault-normal for supershear ruptures (opposite to what happens for sub-Rayleigh ruptures), as observed in prior sparse pointwise measurements, while they simultaneously reveal additional spatiotemporal features. These new observations include the change in sign of the fault-normal velocity with increasing distance from the fault for supershear ruptures, and the transition from one to two peaks in the fault-parallel velocity time history for sub-Rayleigh ruptures. The full-field maps also illustrate the attenuation pattern of dynamic ruptures. The rupture features associated with shock fronts travel long distance away from the fault, while other features of supershear ruptures, such as the fault-normal motion at the rupture tip, are local in nature and rapidly attenuate. Discontinuity characteristics vanish in supershear ruptures traveling at the Eshelby speed of times the shear wave speed, and therefore these ruptures are relatively more benign in terms of their ground motion away from the fault. Conversely, in sub-Rayleigh ruptures certain features, such as the characteristic fault-normal motion, can be persistent at some distances from the fault. By quantifying the full-field behavior of dynamic ruptures, this work improves our understating of the ground motion with important implications for earthquake hazard assessment.