Magnitude-invariant stress drops and increasing breakdown energy in earthquake sequences on rate-and-state faults with thermal pressurization.

Stephen M. Perry, Nadia Lapusta, & Valere R. Lambert

In Preparation June 24, 2019, SCEC Contribution #8128

Stress drops, observed to be magnitude invariant, are a key characteristic used to describe natural earthquakes. Theoretical studies and laboratory experiments indicate that dynamic weakening, such as thermal pressurization of pore fluids, may be present on natural faults. At first glance, these two observations seem incompatible since larger events may experience greater weakening and should thus have lower final stresses. We hypothesize that dynamic weakening can be reconciled with magnitude-invariant stress drops due to larger events having lower average prestress when compared to smaller events. Through studies of long-term earthquake sequences including rate-and-state friction and thermal pressurization, we show that such models can explain both 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 five orders of magnitude in seismic moment. Segment-spanning events have more complex behaviour, which is dependent on the properties of the velocity-strengthening region at the edges of the faults.

Citation
Perry, S. M., Lapusta, N., & Lambert, V. R. (2019). Magnitude-invariant stress drops and increasing breakdown energy in earthquake sequences on rate-and-state faults with thermal pressurization.. Journal of Geophysical Research: Solid Earth, (in preparation).