Spatiotemporal Structure of a Coupled Continuum-Granular Earthquake Experiment

Earthquake faults are complicated and hard to access experimentally. To complement field studies, laboratory and numerical models that focus on the universal features of natural earthquakes are extremely valuable. We have developed a laboratory experiment that includes sheared elastic plates separated by a narrow gap filled with a quasi-two-dimensional granular material. Local measurement of strain displacements of the plates at over 400 spatial points located adjacent to the gap allows direct determination of the moments and their spatial and temporal distributions. We show that events consist of laterally coherent, larger motions that we label as "brittle" events and spatially distributed, smaller "non-brittle" events. The non-brittle events have a probability distribution of event moment consistent with an M^-3/2 power law scaling and a Poisson distributed recurrence time distribution. Brittle events have a broad, log-normal moment distribution and a mean repetition time. As the applied normal force increases, there are fractionally more (less) brittle (non-brittle) events, and the brittle moment distribution broadens. The magnitude of the slip motion of the plates is well correlated with the RMS displacements of the granular matter. Our results are consistent with mean field descriptions of statistical models of earthquakes and avalanches.