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 constrained 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 (0.4CS) and then transits to a very slow velocity (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 triggering 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 theoretical framework for dynamic triggering at greater distances.

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