Abstract
AbstractStress drops, inferred to be magnitude‐invariant, are a key characteristic used to describe natural earthquakes. Theoretical studies and laboratory experiments indicate that enhanced dynamic weakening, such as thermal pressurization of pore fluids, may be present on natural faults. At first glance, magnitude invariance of stress drops and enhanced dynamic weakening seem incompatible since larger events may experience greater weakening and should thus have lower final stresses and higher stress drops. We hypothesize that enhanced dynamic weakening can be reconciled with magnitude‐invariant stress drops due to larger events having lower average prestress when compared to smaller events. We conduct numerical simulations of long‐term earthquake sequences in fault models with rate‐and‐state friction and thermal pressurization, and in the parameter regime that results mostly in crack‐like ruptures, we find that such models can explain both the observationally inferred stress drop invariance and increasing breakdown energy with event magnitude. Smaller events indeed have larger average initial stresses than medium‐sized events, and we find nearly constant stress drops for events spanning up to two orders of magnitude in average slip, comparable to approximately six orders of magnitude in seismic moment. Segment‐spanning events have more complex behavior, which depends on the properties of the arresting velocity‐strengthening region at the edges of the faults.
Highlights
Stress drops and breakdown energy are important descriptors of natural earthquakes
We conduct numerical simulations of long-term earthquake sequences in fault models with rate-and-state friction and thermal pressurization, and in the parameter regime that results mostly in crack-like ruptures, we find that such models can explain both the observationally inferred stress drop invariance and increasing breakdown energy with event magnitude
Different dynamic weakening mechanisms produce different weakening behaviors, but here we focus on thermal pressurization as a representative dynamic weakening mechanism that can lead to continuous fault weakening with earthquake-source slip
Summary
Stress drops and breakdown energy are important descriptors of natural earthquakes. Stress drops characterize the average change in stress state from before to after the dynamic event (Kanamori & Anderson, 1975; Knopoff, 1958; Kostrov, 1974). We use fully dynamic simulations of earthquake sequences on rate-and-state faults to investigate this hypothesis and study if enhanced dynamic weakening can be compatible with magnitude-invariant stress drops while maintaining increasing breakdown energy with increasing event size. The argument should still hold, since larger ruptures with larger slip and more pronounced weakening should be able to propagate over larger areas of locally unfavorable prestress, as compared to smaller ruptures, potentially still resulting in nearly magnitude-invariant stress drops, but with some scatter due to heterogeneity. Such scenarios will be investigated in future work. The relative importance of the off-fault and on-fault dissipation during dynamic rupture is an important topic of ongoing studies which is beyond the scope of this work
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