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Published July 7, 2009 | Published
Journal Article Open

Earthquake ruptures with thermal weakening and the operation of major faults at low overall stress levels

Abstract

We model ruptures on faults that weaken in response to flash heating of microscopic asperity contacts (within a rate-and-state framework) and thermal pressurization of pore fluid. These are arguably the primary weakening mechanisms on mature faults at coseismic slip rates, at least prior to large slip accumulation. Ruptures on strongly rate-weakening faults take the form of slip pulses or cracks, depending on the background stress. Self-sustaining slip pulses exist within a narrow range of stresses: below this range, artificially nucleated ruptures arrest; above this range, ruptures are crack-like. Natural earthquakes will occur as slip pulses if faults operate at the minimum stress required for propagation. Using laboratory-based flash heating parameters, propagation is permitted when the ratio of shear to effective normal stress on the fault is 0.2–0.3; this is mildly influenced by reasonable choices of hydrothermal properties. The San Andreas and other major faults are thought to operate at such stress levels. While the overall stress level is quite small, the peak stress at the rupture front is consistent with static friction coefficients of 0.6–0.9. Growing slip pulses have stress drops of ∼3 MPa; slip and the length of the slip pulse increase linearly with propagation distance at ∼0.14 and ∼30 m/km, respectively. These values are consistent with seismic and geologic observations. In contrast, cracks on faults of the same rheology have stress drops exceeding 20 MPa, and slip at the hypocenter increases with distance at ∼1 m/km.

Additional Information

©2009. American Geophysical Union. Received 7 October 2008; accepted 31 March 2009; published 7 July 2009. The largest simulations were performed on Harvard's BlueGene/L; we thank Aaron Culich, Robert Parrot, and Joyanta Sircar of the Harvard SEAS/IT group and Kirk Jordan of IBM for technical support. We also thank associate editor Dan Faulkner and two anonymous reviewers for their insightful suggestions. The study was supported at Harvard by NSF-EAR award 0510193 and by the Southern California Earthquake Center as funded by Cooperative Agreements NSF EAR-0106924 and USGS 02HQAG0008 (SCEC contribution number 1228).

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