Seismic Energy Partitioning Across the Continuum of Laboratory Fault Slip Modes

Understanding the physical controls on the transition between slow and fast earthquakes remains a fundamental challenge in earthquake physics. Here we study simulated earthquake fault zones and show that both slow and fast slip can emerge under identical stress conditions. The transition between slip modes is governed by the elastodynamic interaction between the fault and its surroundings. By systematically varying fault loading 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 acoustic emissions, which are the equivalent of microearthquakes, while fast slip generates higher-amplitude energy bursts. We evaluate the energy release in the full range of lab earthquakes from slow to fast and find a continuous scaling of the breakdown work with seismic moment, which 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. Our results suggest that slow and fast earthquakes are not distinct phenomena but reflect end-members of a fault slip continuum.