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James R. Rice

Heating, weakening and melting of wet fault gouge during earthquake slip

Abstract: The Sibson-Lachenbruch mechanism of fault weakening, by thermal pressurization of pore fluid through shear heating, is further developed theoretically and tested against seismological constraints. Results of recent geological studies of mature fault zone structure, and of associated lab studies of fault material hydrologic and poroelastic properties, are combined to make reasonably well-constrained predictions of the weakening process. A testable prediction is of the shear fracture energies of crustal earthquakes that would be implied if dynamic ruptures were controlled by that mechanism. The predicted values for shallow crustal earthquakes cover a range from 0.4 to 5 MJ/m^2. Remarkably, that range overlaps with values of fracture energy we have inferred in an independent manner from seismology. Such seismological results, for a group of approximately 20 events with slip > 0.1 m, cover the range from about 0.1 to 10 MJ/m^2, and average around 2-4 MJ/m^2. The rough coincidence suggests that thermal pressurization of pore fluids may be a major contributor to fault weakening during the crustal earthquake process, at least along highly slipped mature faults with well-developed gouge layering. Further, the constraints on hydrologic and poroelastic properties are such that only limited escape of fluid is expected from the intensely shearing zone during earthquake slip. Thus strength drop should often be nearly complete at large slip, and the onset of melting should often be precluded over most of the seismogenic zone, except in the largest slip events. Predicted conditions for melt onset will be reported.