Concurrent slow and fast frictional ruptures in laboratory earthquakes

Frictional motion is initiated by interface failure that is mediated by ruptures—akin to earthquakes—that typically accelerate to near-sonic velocities. However, slow ruptures may occur in both laboratory and natural fault settings, but the mechanisms that drive them are not fully understood. Although fracture mechanics describes fast frictional ruptures well, its relevance to slow ruptures is uncertain. Here we experimentally show that both extremely slow and fast ruptures—on scales of cm s^–1 and km s^–1, respectively—can repeatably propagate within the same frictional interface. We demonstrate that a dynamic equilibrium between the loading rates and velocity dependencies of both interface resistance and fracture energy enables slow ruptures to nucleate and propagate at very low applied shear stresses. In the same interfaces, fast ruptures also occur, but only when their nucleation becomes possible under higher stress conditions. We find that the dynamics and structure of both rupture classes are well described by fracture mechanics. Their existence results from a close interplay between the interface properties and rupture velocity. These results provide key insights into fault dynamics and related frictional motion.