Does hydrologic circulation mask frictional heat on faults after large earthquakes

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, B09402, doi:10.1029/2009JB007103, 2010 Does hydrologic circulation mask frictional heat on faults after large earthquakes? Patrick M. Fulton, 1 Robert N. Harris, 1 Demian M. Saffer, 2 and Emily E. Brodsky 3 Received 2 November 2009; revised 17 March 2010; accepted 30 April 2010; published 3 September 2010. [ 1 ] Knowledge of frictional resistance along faults is important for understanding the mechanics of earthquakes and faulting. The clearest in situ measure of fault friction potentially comes from temperature measurements in boreholes crossing fault zones within a few years of rupture. However, large temperature signals from frictional heating on faults have not been observed. Unambiguously interpreting the coseismic frictional resistance from small thermal perturbations observed in borehole temperature profiles requires assessing the impact of other potentially confounding thermal processes. We address several issues associated with quantifying the temperature signal of frictional heating including transient fluid flow associated with the earthquake, thermal disturbance caused by borehole drilling, and heterogeneous thermal physical rock properties. Transient fluid flow is investigated using a two‐dimensional coupled fluid flow and heat transport model to evaluate the temperature field following an earthquake. Simulations for a range of realistic permeability, frictional heating, and pore pressure scenarios show that high permeabilities (>10 −14 m 2 ) are necessary for significant advection within the several years after an earthquake and suggest that transient fluid flow is unlikely to mask frictional heat anomalies. We illustrate how disturbances from circulating fluids during drilling diffuse quickly leaving a robust signature of frictional heating. Finally, we discuss the utility of repeated borehole temperature profiles for discriminating between different interpretations of thermal perturbations. Our results suggest that temperature anomalies from even low friction should be detectable at depths >1 km 1 to 2 years after a large earthquake and that interpretations of low friction from existing data are likely robust. Citation: Fulton, P. M., R. N. Harris, D. M. Saffer, and E. E. Brodsky (2010), Does hydrologic circulation mask frictional heat on faults after large earthquakes?, J. Geophys. Res., 115, B09402, doi:10.1029/2009JB007103. 1. Introduction [ 2 ] Frictional resistance along faults is an important parameter controlling earthquake nucleation and propaga- tion. Because friction is central to earthquake mechanics, considerable effort has gone into characterizing fault zone friction both in the laboratory and in situ [e.g., Scholz, 2002]. Laboratory measurements suggest that the intrinsic low‐ speed friction coefficient for most rocks is approximately 0.60−0.85 [Byerlee, 1978]. This magnitude of friction is hypothesized to generate large thermal anomalies on natural faults with large slip rates and/or large total displacements, assuming hydrostatic pore pressure. Curiously, analysis of surface heat flow data [e.g., Brune et al., 1969; Lachenbruch and Sass, 1980; Wang et al., 1995] and subsurface temper- College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon, USA. Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania, USA. Department of Earth and Planetary Sciences, University of California, Santa Cruz, California, USA. Copyright 2010 by the American Geophysical Union. 0148‐0227/10/2009JB007103 ature profiles [Yamano and Goto, 2001; Kano et al., 2006; Tanaka et al., 2006, 2007] that cross fault zones do not show substantial, unequivocal anomalies from frictional heating. These observations prompt two questions: (1) could the fric- tional resistance be as large as expected from Byerlee’s law and hydrostatic pore pressure, but the heat signal is masked or dissipated by other processes? (2) If not, what is the in situ value of frictional resistance during fault slip? [ 3 ] Much effort has been spent recently on the second of these questions resulting in theoretical models supported by both laboratory and field observations that suggest that coseismic friction may be quite low [e.g., Brodsky and Kanamori, 2001; Di Toro et al., 2004; Rice, 2006; Ma et al., 2006], but considerably, less work has been con- ducted on the first question. Studies of processes that may mask or dissipate the frictional heat signal have focused on steady state topographically driven or buoyancy‐driven groundwater flow [Williams and Narisimhan, 1989; Saffer et al., 2003; Fulton et al., 2004] and the effects of heteroge- neous thermal properties [Tanaka et al., 2007; Fulton and Saffer, 2009a]. One candidate for obscuring a frictionally generated thermal signal that has not been fully explored is transient groundwater flow following an earthquake [e.g., Kano et al., 2006; Scholz, 2006]. B09402 1 of 13