Combining Seismological Inferences to Constrain Physical Conditions Surrounding the Low Stress, Low Heat Operation of Mature Faults

Session: Crustal Stress and Strain and Implications for Fault Interaction and Slip
Type: Oral
Date: 4/28/2020
Time: 09:15 AM
Room: 240
Description: 

Numerous lines of evidence, including observations of heat flow, steep angles between inferred principal stress directions and fault traces, and the geometry of thrust-belt wedges suggest that the shear resistance during earthquakes 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 weakening at seismic slip rates. Many seismological studies seek to shed light on earthquake rupture physics using averaged event quantities, such as the average slip and static stress drop, as well as energetic estimates such as the radiated energy, breakdown energy and radiation efficiency. We explore the relationship among 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 including rate-and-state friction and enhanced weakening due to thermal pressurization (TP).

We find that simulations with mild TP produce crack-like ruptures, in which the average static and dynamic stress changes are comparable. Moreover, such models are able to reproduce observationally inferred trends of increasing breakdown energy with event size and radiation efficiencies between 0.1 and 1. However, to maintain reasonable temperatures, such models require persistently weak fault conditions. In contrast, simulations with more efficient TP can result in self-healing pulse-like ruptures, which are characterized by greater discrepancies between the average dynamic and static stress changes and thus result in inferred radiation efficiencies much greater than 1 and breakdown energies that are negative, which are rarely reported. Our findings taken along with seismological inferences suggest that either large earthquakes propagate predominantly as crack-like ruptures, suggesting that mature faults are chronically weak or that estimates of radiated energy are substantially underestimated.

Presenting Author: Valere Lambert

Authors