Contained Laboratory Earthquakes Ranging From Slow to Fast

Loading a 3-m granite slab containing a saw-cut simulated fault, we generated slip events that spontaneously nucleate, propagate, and arrest before reaching the ends of the sample. This work shows that slow (0.07 mm/s slip speeds) and fast (100 mm/s) contained slip events can occur on the same fault patch. We also present the systematic changes in radiated seismic waves both in time and frequency domain. The slow earthquakes are 100 ms in duration and radiate tremor-like signals superposed onto a low-frequency component of their ground motion. They are often preceded by slow slip (creep) and their seismic radiation has an omega(-1) spectral shape, similar to slow earthquakes observed in nature. The fastest events have slip velocity, stress drop, and apparent stress (0.2 m/s, 0.4 MPa, and 1.2 kPa, respectively) similar to those of typical M -2.5 earthquakes, with a single distinct corner frequency and omega(-2) spectral falloff at high frequencies, well fit by the Brune earthquake source model. The gap between slow and fast is filled with intermediate events with source spectra depleted near the corner frequency. This work shows that a fault patch of length p with conditions favorable to rupture can radiate in vastly different ways, based on small changes in ph*, where h* is a critical nucleation length scale. Such a mechanism can help explain atypical scaling observed for low-frequency earthquakes that compose tectonic tremor.