Scale dependence of earthquake rupture prestress in models with enhanced
weakening: implications for event statistics and inferences of fault
stress
Abstract
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 1) illustrate that large earthquakes can
occur on faults that appear not to be critically stressed compared to
stresses required for slip nucleation; 2) highlight the importance of
finite-fault modeling in relating the local friction behavior determined
in the lab to the field scale; and 3) 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.