Modeling absolute stress levels on mature faults: Implications for seismic radiation and earthquake statistics

Nadia Lapusta, & Valere R. Lambert

Published August 13, 2021, SCEC Contribution #11389, 2021 SCEC Annual Meeting Poster #143

Accumulating geophysical evidence suggests that the shear resistance acting on mature faults must be low (< 20 MPa). Two hypotheses used to explain such low-stress, low-heat operation of mature faults are that they are either (I) chronically weak, such as from low effective confinement due to pervasive fluid overpressure, or (II) quasi-statically strong but undergo considerable enhanced dynamic weakening at seismic slip rates. Here, we explore relations among geophysical inferences in the corresponding fault models of matures faults, using fully dynamic simulations of earthquake sequences on rate-and-state faults with varying degrees of efficiency in enhanced weakening due to thermal pressurization of pore fluids.

We find that the two end-member models for low-stress operation can in principle be distinguished based on the radiated energy per seismic moment of the resulting earthquake ruptures. Models of type (I) produce predominantly crack-like ruptures and result in the radiated energy per moment comparable to teleseismic estimates from large megathrust earthquakes. Models of type (II) typically produce self-healing pulses, resulting in larger radiated energy per moment than typically inferred teleseismically for large megathrust earthquakes, yet potentially comparable to limited regional estimates from large crustal earthquakes.

Models (I) and (II) also result in substantially different relations between the shear stress required for rupture nucleation and the average stress conditions over larger fault areas. Our simulations show that larger ruptures on faults that experience more efficient weakening during rupture can propagate under systematically lower stress conditions that those required for rupture nucleation. As a result, fault models with more efficient weakening produce predominantly larger ruptures at the expense of smaller events. Our findings suggest that (1) persistently weak fault models with moderate enhanced dynamic weakening may be plausible representations of natural subduction zones, unless teleseismic estimates underestimate radiated energy, and (2) crustal mature faults may differ, with less chronic but more dynamic weakening. If mature crustal faults are indeed susceptible to substantial coseismic weakening, then they may have reduced complexity of earthquake sequences, all the way to quasi-periodic large events with seismic quiescence in between, with important implications for early warning.

Key Words
Fault stress, dynamic weakening, earthquake statistics

Citation
Lapusta, N., & Lambert, V. R. (2021, 08). Modeling absolute stress levels on mature faults: Implications for seismic radiation and earthquake statistics . Poster Presentation at 2021 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)