A Continuum of Laboratory Fault Slip Reveals Distinct Seismic Energy Partitioning from Slow to Fast Slip

Understanding the physical controls on the transition between slow and fast earthquakes remains a fundamental challenge in earthquake physics. Here we show, through laboratory experiments on granular quartz gouge simulating natural fault zones, that both slow and fast slip can emerge on the same fault under identical stress conditions. The transition between slip modes is governed by the elastodynamic interaction between the fault and its surroundings. By systematically varying system stiffness at constant normal stress, we observe a continuous spectrum of slip behavior from stable sliding to slow events and ultimately fast rupture. Continuous acoustic monitoring reveals distinct seismic signatures: slow slip produces swarms of small events, while fast slip generates high-amplitude energy bursts. Continuous scaling of breakdown work with seismic moment supports a unified physical mechanism. Moment-duration scaling highlights a key transition in energy partitioning: in slow events, acoustic energy accounts for a minor portion of slip duration, whereas in fast events it contributes a much larger portion, indicating a shift in how seismic energy is radiated across slip modes. These findings suggest that slow and fast earthquakes are not distinct phenomena but reflect end-members of a fault slip continuum.