Slow and fast ruptures on a laboratory fault controlled by loading characteristics

Recent geodetic observations indicate that a single fault may slip slowly and silently or fast and seismically in different circumstances. We report laboratory experiments that demonstrate how the mode of faulting can alternate between fast “stick‐slip” events (70 to ~500 mm/s sliding rates) and slow silent events (~0.001 to 30 mm/s) as a result of loading conditions rather than friction properties or stiffness of the loading machine. The 760 mm long granite sample is close to the critical nucleation length scale for unstable slip, so small variations in nucleation properties result in measurable differences in slip events. Slow events occur when instability cannot fully nucleate before reaching the sample ends. Dynamic events occur after long healing times or abrupt increases in loading rate which suggests that these factors shrink the spatial and temporal extents of the nucleation zone. Arrays of slip, strain, and ground motion sensors installed on the sample allow us to quantify seismic coupling and study details of premonitory slip and afterslip. We find that seismic coupling decreases when slip rates fall below about 70 mm/s. The slow slip events we observe are primarily aseismic (less than 1% of the seismic coupling of faster events) and produce swarms of very small M  −6 to M  −8 events. These mechanical and seismic interactions suggest that faults with transitional behavior—where creep, small earthquakes, and tremor are often observed—could become seismically coupled if loaded rapidly, either by a slow slip front or dynamic rupture of an earthquake that nucleated elsewhere.