A fault constitutive relation accounting for thermal pressurization of pore fluid

Abstract

[1] The heat generated in a slip zone during an earthquake can raise fluid pressure and thereby reduce frictional resistance to slip. The amount of fluid pressure rise depends on the associated fluid flow. The heat generated at a given time produces fluid pressure that decreases inversely with the square root of hydraulic diffusivity times the elapsed time. If the slip velocity function is crack-like, there is a prompt fluid pressure rise at the onset of slip, followed by a slower increase. The stress drop associated with the prompt fluid pressure rise increases with rupture propagation distance. The threshold propagation distance at which thermally induced stress drop starts to dominate over frictionally induced stress drop is proportional to hydraulic diffusivity. If hydraulic diffusivity is 0.02 m 2 /s, estimated from borehole samples of fault zone material, the threshold propagation distance is 300 m. The stress wave in an earthquake will induce an unknown amount of dilatancy and will increase hydraulic diffusivity, both of which will lessen the fluid pressure effect. Nevertheless, if hydraulic diffusivity is no more than two orders of magnitude larger than the laboratory value, then stress drop is complete in large earthquakes.

Does hydrologic circulation mask frictional heat on faults after large earthquakes?

“…In a final set of models, we modify this initial condition to evaluate the effects of lateral fluid flow driven away from a highly pressurized fault zone, as might be expected from transient pressurization during slip [e.g., Andrews, 2002;Hirose and Bystricky, 2007] or from interseismic localization of pressure within the fault [e.g., Rice, 1992;Sleep and Blanpied, 1992;Fulton and Saffer, 2009b]. In these simulations, pore pressures within the fault zone and country rock are lithostatic and hydrostatic, respectively.…”

“…The low rates of frictional heating interpreted from existing thermal data could result from a low friction coefficient on the fault, elevated pore pressure, or a combination of the two [e.g., Lachenbruch and Sass, 1980;Rice, 1992;Fulton and Saffer, 2009]. In addition, elevated pore pressure that weakens the fault could be sustained throughout the seismic cycle or transiently generated during rapid slip [e.g., Rice, 1992;Segall and Rice, 2006;Andrews, 2002]. For simplicity, we represent different fault strength (frictional resistance) scenarios in terms of the equivalent friction coefficient assuming hydrostatic pore pressure, defined as the product of the friction coefficient during slip and effective normal stress divided by effective normal stress assuming hydrostatic pore pressure.…”

Off‐fault plasticity and earthquake rupture dynamics: 2. Effects of fluid saturation

“…Therefore we neglect in this study the effects of pore pressure increases or decreases on the slip-weakening response of the fault. Such slip weakening, as conventionally assumed, may in fact be a proxy for much more significant but highly localized pore pressure changes along the fault due to thermal pressurization [Sibson, 1973;Lachenbruch, 1980;Mase and Smith, 1987;Andrews, 2002;Noda and Shimamoto, 2005;Rice, 2006;Rempel and Rice, 2006;Suzuki and Yamashita, 2006;Bizzarri and Cocco, 2006].…”

Constitutive parameters for earthquake rupture dynamics based on high-velocity friction tests with variable sliprate

“…Shimamoto (2003, 2005) and Spray (2005) observed slip weakening during high slip rate sliding when melting occurred. Thermal pressurization was proposed by Sibson (1973) and modeled by Lachenbruch (1980), Mase and Smith (1987), Andrews (2002), Wibberley and Shimamoto (2005), Bizzarri and Cocco (2006) and Rice (2006) and many others. It is difficult to confirm in the laboratory whether thermal pressurization occurs or not because no footprints of thermal pressurization remain after high-speed slip; all the above studies are based on theoretical considerations.…”