Effect of thermal refraction on heat flow near the San Andreas Fault, Parkfield, California

Fulton, P. M.; Saffer, D. M.

doi:10.1029/2008jb005796

Cited by 17 publications

(27 citation statements)

References 54 publications

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“…Simulated heat flow from these model scenarios is marked by an increase of <10 mW m −2 near the fault (Figures 6e–8e). This spatial variability in heat flow is consistent with the range of observed heat flow scatter around the fault and is considerably less than the ∼40 mW m −2 near‐fault anomaly expected from frictional heating [e.g., Lachenbruch and Sass , 1980; Fulton and Saffer , 2009]. …”

Section: Resultssupporting

confidence: 78%

Potential role of mantle‐derived fluids in weakening the San Andreas Fault

Fulton¹,

Saffer²

2009

J. Geophys. Res.

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[1] On the basis of both geomechanical and thermal data, the San Andreas Fault (SAF) has been interpreted to act as a weak plane within much stronger crust, allowing it to slip at very low shear stresses. One explanation for this weakness is that large fluid overpressures exist locally within the fault zone. However, mechanisms for generating, maintaining, and localizing pressures within the fault are poorly quantified. Here we evaluate whether realistic sources of mantle-derived fluids, proposed on the basis of high mantle helium signatures near the SAF, can generate localized fluid pressures within the fault zone in a manner consistent with a wide range of observations along the fault and in the San Andreas Fault Observatory at Depth borehole. We first calculate a reasonable estimate of the magnitude and location of a mantle-derived flux of water into the crust. This fluid flux results from dehydration of a serpentinized mantle wedge following the northward migration of the Mendocino Triple Junction and the transition from subduction to strike-slip tectonics. We then evaluate the potential effect of this water on fluid pressures within the crust using 2-D cross-sectional models of coupled fluid flow and heat transport. We show that in models with realistic permeability anisotropy, controlled by NE dipping faults and fractures within the country rock, large localized fluid pressures can be focused within a SAF acting as a hydrologic barrier. Our results illustrate a simple and potentially plausible means of weakening the SAF in a manner generally consistent with available hydrologic, thermal, and mechanical constraints.

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Section: Resultssupporting

confidence: 78%

Potential role of mantle‐derived fluids in weakening the San Andreas Fault

Fulton¹,

Saffer²

2009

J. Geophys. Res.

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“…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 heterogeneous 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].…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2010

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[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?,

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“…The heat source increases linearly by 8.85 mW m −2 per km depth, corresponding to an average slip rate on the SAF of 3.1 cm yr −1 and an average resisting shear stress of 9 MPa km −1 over the seismogenic zone [ Lachenbruch and Sass , 1980]. We do not include radiogenic heat production, which could produce variability in surface heat flow of a much broader wavelength than that exhibited by the heat flow data [e.g., Fulton and Saffer , 2009]. …”

Section: Modeling Methodsmentioning

confidence: 99%

“…We prescribe a constant atmospheric pressure of 0.1 MPa and an atmospheric lapse rate of 6.9°C per km at the top boundary, with a temperature of 10°C at sea level. We assign a grain thermal conductivity of 2.9 W (m K) −1 to all units to exclude the effects of thermal refraction from simulated surface heat flow, which have already been explored in detail by others [e.g., Fulton and Saffer , 2009]. The bulk thermal conductivity is computed from porosity, using a weighted average for the solid grains and the fluid [e.g., Voss , 1984]; the resulting values of bulk thermal conductivity range from 2.1 to 2.8 W (m K) −1 .…”

Section: Modeling Methodsmentioning

confidence: 99%

“… Fulton et al [2004] quantified the effects of 3‐D topography and variable solar insolation on the Parkfield heat flow data set [e.g., Blackwell et al , 1980] and demonstrated that by accounting for these processes, the standard deviation of the surface heat flow data was reduced by 26%. Fulton and Saffer [2009] investigated the effects of subsurface thermal refraction and showed that this process could account for the scatter in surface heat flow near Parkfield in a purely conductive regime if the thermal conductivity of the Tertiary sediments is ∼60% of that for the underlying basement. However, contrasts in thermal conductivity in the Coast Ranges are poorly constrained because rocks from the heat flow boreholes are undersampled for thermal conductivity [ Mase et al , 1982; Williams et al , 1994; Sass et al , 1997; Fulton et al , 2004].…”

Section: Introductionmentioning

confidence: 99%

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Heat advection by groundwater flow through a heterogeneous permeability crust: A potential cause of scatter in surface heat flow near Parkfield, California

Popek¹,

Saffer²

2011

J. Geophys. Res.

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[1] Surface heat flow in the California Coast Ranges near Parkfield, California, exhibits substantial scatter, with differences as large as 20 mW m −2 over lateral distances of 5-70 km. This scatter has been an important limitation in using the heat flow data set to constrain geodynamic processes, but to date it has not been explained. Here we use a numerical model of coupled fluid and heat transport to test the hypothesis that heat advection by groundwater flow can generate the scatter. Our study significantly extends previous investigations in that we consider realistic and heterogeneous permeability architecture and topographic driving forces. We find that the magnitude and spatial characteristics of the scatter, including the standard deviation, variation in heat flow as a function of separation distance, and patterns of heat flow and elevation can be generated if the Tertiary sediments in the upper 2-3 km of the crust have a permeability ≥3 × 10 −16 m 2 , allowing recharge of ∼0.5 cm yr −1 or higher. These permeabilities and recharge rates are consistent with existing constraints on both quantities, suggesting that groundwater flow offers a plausible explanation for the observed scatter in the heat flow data set. Last, although not the primary focus of this study, we demonstrate that for a range of reasonable permeability architectures, topographically driven groundwater flow would not mask a thermal anomaly associated with frictional heating on the San Andreas Fault.

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Effect of thermal refraction on heat flow near the San Andreas Fault, Parkfield, California

Cited by 17 publications

References 54 publications

Potential role of mantle‐derived fluids in weakening the San Andreas Fault

Potential role of mantle‐derived fluids in weakening the San Andreas Fault

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

Heat advection by groundwater flow through a heterogeneous permeability crust: A potential cause of scatter in surface heat flow near Parkfield, California

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