Earthquake Breakdown Energy Scaling Despite Constant Fracture Energy

Abstract: In the quest to determine fault weakening processes that govern earthquake mechanics, it is common to infer the earthquake breakdown energy from seismological measurements. Breakdown energy is observed to scale with slip, which is often attributed to enhanced fault weakening with continued slip or at high slip rates, possibly caused by flash heating and thermal pressurization. However, breakdown energy varies by more than six orders of magnitude, which is physically irreconcilable with prevailing material properties. We present a dynamic model that demonstrates that breakdown energy scaling can occur despite constant fracture energy and does not require thermal pressurization or other enhanced weakening. Instead, earthquake breakdown energy scaling occurs simply due to scale-invariant stress drop overshoot, which is affected more directly by the overall rupture mode -- crack-like or pulse-like -- rather than from a specific slip-weakening relationship. Our findings suggest that breakdown energy may be used to discern crack-like earthquakes from self-healing pulses with negative breakdown energy.