INSIGHTS FROM SIMULATIONS OF FAULT SLIP WITH ENHANCED CO-SEISMIC WEAKENING: RUPTURE OF CREEPING REGIONS, SCALE-DEPENDENT RUPTURE PRESTRESS, AND EFFECT ON B-VALUES (Invited Presentation)

A number of observations suggest that well-developed, mature strike-slip faults such as the San Andreas Fault (SAF) are generally “weak,” i.e. operate at low overall levels of shear stress (10-20 MPa) in comparison with 100-200 MPa that would be expected from Byerlee’s law. Observations also show that many major strike-slip faults known to have had large earthquakes are largely silent in the interseismic period. Our numerical simulations of earthquake sequences on rate-and-state faults show that both observations can be explained by enhanced dynamic weakening (EDW). We use thermal pressurization of pore fluids as the EDW mechanism in our simulations, but other mechanisms which involve strong slip-rate dependence should produce similar results. On such simulated faults, average shear stress before dynamic rupture systematically varies with the rupture size: the smallest ruptures have prestress comparable to the (high) shear stress required for nucleation, while larger ruptures propagate over increasingly under-stressed areas due to dynamic stress concentration and result in progressively lower average prestress (Lambert, Lapusta, and Faulkner, JGR, 2021). The findings illustrate that large earthquakes can occur on faults that, on average, are far from being critically stressed, as long as there are locations of high stress on such faults suitable for nucleation. They further highlight the importance of finite-fault modeling in relating the local friction behavior determined in the lab to the field scale, as laboratory samples tend to be small and stressed almost uniformly to failure, conditions representative of nucleation zones but not of lower-stressed fault regions into which rupture propagates. Increasingly efficient EDW also allows more simulated events to grow and even penetrate into fault areas with slow slip, such as deeper fault extensions (Lambert, Lapusta, and Faulkner, JGR, 2021; Noda and Lapusta, Nature, 2013; Jiang and Lapusta, Science, 2016), decreasing the b-value and erasing the band of seismicity that otherwise would be expected at the bottom of the seismogenic zone. Hence our modeling suggests that paucity of small events or seismic quiescence on mature plate-boundary faults indicates faults that operate under low shear stress and have large events penetrating deeper into the stable fault extensions due to EDW.