Earthquake Delay and Rupture Velocity in Near-Field Dynamic Triggering Dictated by Stress-Controlled Nucleation

Dynamic triggering of earthquakes by seismic waves generated by another earthquake is widely observed, while the underlying nucleation mechanisms remain unclear. We report here dynamically triggered earthquakes on laboratory faults with tightly con-strained imaging of the triggering process. The arriving stress wave alters the contact state of the laboratory fault and initiates rupture nucleation in two distinct phases. The triggered rupture grows at a fraction of the shear-wave velocity ( similar to 0.4CS) and then transits to a very slow velocity (similar to 0.1CS) before culminating into runaway shear. This intervening very slow rupture phase is present only for seismic ratios conducive to sub -Rayleigh ruptures and is notably absent for supershear events. Thus, the delay in trig-gering decreases to a minimum for triggered supershear ruptures, whereas it scales with the stress state for triggered sub-Rayleigh ruptures. These results may help explain key characteristics of delayed near-field dynamic triggering and provide a simple theo-retical framework for dynamic triggering at greater distances.