Scale Dependence of Earthquake Rupture Prestress in Models With Enhanced Weakening: Implications for Event Statistics and Inferences of Fault Stress.

Determining conditions for earthquake slip on faults is a key goal of fault mechanics highly relevant to seismic hazard. Previous studies have demonstrated that enhanced dynamic weakening (EDW) can lead to dynamic rupture of faults with much lower shear stress than required for rupture nucleation. We study the stress conditions before earthquake ruptures of different sizes that spontaneously evolve in numerical simulations of earthquake sequences on rate‐and‐state faults with EDW due to thermal pressurization of pore fluids. We find that average shear stress right before dynamic rupture (aka shear prestress) systematically varies with the rupture size. The smallest ruptures have prestress comparable to the local shear stress required for nucleation. Larger ruptures weaken the fault more, propagate over increasingly under‐stressed areas due to dynamic stress concentration, and result in progressively lower average prestress over the entire rupture. The effect is more significant in fault models with more efficient EDW. We find that, as a result, fault models with more efficient weakening produce fewer small events and result in systematically lower b‐values of the frequency‐magnitude event distributions. The findings (a) illustrate that large earthquakes can occur on faults that appear not to be critically stressed compared to stresses required for slip nucleation; (b) highlight the importance of finite‐fault modeling in relating the local friction behavior determined in the lab to the field scale; and (c) suggest that paucity of small events or seismic quiescence may be the observational indication of mature faults that operate under low shear stress due to EDW. Plain Language Summary: Understanding the evolution of stress on faults over periods of slow and fast motion is crucial for assessing how earthquakes start, grow, and ultimately stop. Here we use computer models to explore the stress conditions required for simulated earthquake ruptures to occur. We find that the critical stress conditions for rupture propagation depend on the size of the rupture and how efficiently the fault shear resistance weakens during fast slip. In particular, larger earthquakes on faults that experience increasingly more efficient weakening during rupture can propagate under systematically lower stress conditions than those required for rupture nucleation. As a result, we find that faults that exhibit efficient weakening can host predominantly large earthquakes at the expense of smaller earthquakes. Our findings illustrate how large earthquakes can occur on faults that appear to be understressed compared to expected conditions for rupture nucleation. Moreover, our findings support a body of work suggesting that the scarcity of small earthquakes on some major mature fault segments, like the central section of the San Andreas Fault, may indicate that they experience substantial weakening during large earthquakes, a consideration that may be particularly useful for earthquake early warning systems. Key Points: Local shear prestress varies significantly within and among ruptures, being close to the quasi‐static fault strength in nucleation regionsEfficient weakening allows rupture propagation over areas of lower prestress, leading to lower average prestress over larger rupture areasFault models with more efficient dynamic weakening produce fewer smaller events and result in systematically lower b‐values [ABSTRACT FROM AUTHOR]