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Combining kinematic and energetic inferences to constrain physical conditions for the low-stress, low-heat operation of mature faults

Valere R. Lambert, & Nadia Lapusta

Published December 8, 2019, SCEC Contribution #10092

Numerous lines of evidence, including observations of heat flow, steep angles between largest principal stress and fault traces, and the geometry of thrust-belt wedges, suggest that the shear resistance on mature faults must be low ( < 20 MPa). Two potential explanations for such low-stress, low-heat operation of mature faults are that they are 1) chronically weak or 2) may be statically strong but undergo considerable enhanced dynamic weakening during earthquakes. Many seismological studies seek to shed light on earthquake rupture physics using averaged quantities, such as stress drop, radiated energy, available energy, breakdown energy, and radiation efficiency. We explore these seismologically inferable quantities within parameter regimes consistent with the low-stress, low-heat operation of mature faults, using fully dynamic simulations of sequences of seismic and aseismic slip on faults with rate-and-state friction and enhanced weakening due to thermal pressurization (TP).

We find that simulations with relatively mild TP produce crack-like ruptures, in which the energy balance is well captured by typical seismological energy analyses. Such models are able to reproduce observationally inferred trends of magnitude-invariant static stress drops, near-constant apparent stress, increasing breakdown energy with event size, and radiation efficiencies between 0.1 and 1. However, to maintain fault temperatures below melting, the models require near-lithostatic fluid pressures or much lower static friction coefficients than typical laboratory values of 0.6-0.8. In contrast, simulations with more efficient TP result in self-healing pulse-like ruptures, characterized by a substantial stress undershoot, larger apparent stress than observed, and larger available energy than would be estimated based on typical energy balance considerations. As a result, self-healing pulse-like ruptures have inferrable radiation efficiencies much greater than 1 and seismologically estimated average breakdown energies that are negative, values that are rarely reported. Our findings suggest that either large earthquakes propagate predominantly as crack-like ruptures, implying that mature faults are chronically weak, or that seismological estimates of radiated energy are substantially underestimated.

Lambert, V. R., & Lapusta, N. (2019, 12). Combining kinematic and energetic inferences to constrain physical conditions for the low-stress, low-heat operation of mature faults. Poster Presentation at American Geophysical Union Fall Meeting.