HR: 0830h
AN: T31E-0877    [PDF]
TI: 3-D Terrain Corrections to Heat Flow Data, Topographically-Driven Groundwater Flow, and the Strength of the San Andreas Fault at Parkfield, CA
AU: * Fulton, P M
EM: pfulton@uwyo.edu
AF: Department of Geology and Geophysics, University of Wyoming, P.O. Box 3006, Laramie, WY 82071 United States
AU: Saffer, D M
EM: dsaffer@uwyo.edu
AF: Department of Geology and Geophysics, University of Wyoming, P.O. Box 3006, Laramie, WY 82071 United States
AU: Bekins, B A
EM: babekins@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025 United States
AU: Harris, R N
EM: rnharris@mines.utah.edu
AF: Department of Geology and Geophysics, University of Utah, 135 S, 1460 E, Salt Lake City, UT 84112 United States
AB: The lack of a detectable heat flow anomaly along the San Andreas Fault (SAF) constitutes one important piece of evidence used to argue that the fault supports low shear stresses ($<$20 MPa averaged over the upper 10 km). However, key uncertainties in existing heat flow data, such as the effects of heat advection by topographically-driven groundwater flow, topographic refraction (terrain effects), subsurface heterogeneity (refraction caused by variable thermal conductivity), and uncertainty in thermal conductivity limit the utility of such analyses. Previous studies using heat flow data to investigate the strength of the SAF have taken into account effects of topographically-driven groundwater flow along a transect NW of the SAFOD site and included limited two-dimensional heat flow terrain effects, but remain inconclusive for interpreting possible frictional heating along the SAF near Parkfield, CA due to significant scatter remaining in the data and uncertainty in the extent of three-dimensional terrain effects. Here, we re-evaluate the effects of topographically-driven groundwater flow at Parkfield using full 3-D corrections to the heat flow data and including additional transects. In this study, we apply three-dimensional terrain corrections to temperature data for 22 boreholes near the SAFOD site. The corrected thermal gradients and available thermal conductivity data allow us to determine heat flow values free of terrain effects. The difference in heat flow for each borehole between published 2-D corrected values and the values corrected for 3-D terrain effects range from 0.2 to 21.0 mW/m$^{2}$, 6.9% on average. The standard deviation of the heat flow data is reduced by 25.8% by including the 3-D correction. Error bars based on the standard deviation of the thermal conductivity measurements for each borehole range from $\pm$3.2 to $\pm$25.7 mW/m$^{2}$ (10.3% on average for all data and 6.8% for high-quality data alone). We use the finite-element modeling code, SUTRA, to simulate steady-state coupled heat and groundwater flow within three cross-sections perpendicular to the fault. We consider a suite of hydrologic (groundwater flow) conditions to evaluate effects of topographically-driven groundwater flow and compute simulated heat flow values for both strong and weak fault frictional heat sources. Simulated heat flow values are corrected for all terrain effects using a two-dimensional Birch method correction, and then compared with the 3-D corrected heat flow data to evaluate plausible hydrologic and fault strength scenarios. For high-permeability scenarios, we predict a large variability in heat flow, as well as a systematic decrease in heat flow with elevation. These patterns are not present in the data, allowing us to estimate an upper limit on advection caused by groundwater flow. In general, models that incorporate a weak fault fit the data better than those with a strong fault. Uncertainty from poorly constrained (undersampled) thermal conductivity in some boreholes, and scatter caused by subsurface thermal refraction due to heterogeneous thermal conductivity structure still remain, but a pronounced near fault heat flow anomaly as predicted for a strong fault is not evident.
DE: 1832 Groundwater transport
DE: 8123 Dynamics, seismotectonics
DE: 8130 Heat generation and transport
DE: 8150 Plate boundary--general (3040)
SC: Tectonophysics [T]
MN: 2003 Fall Meeting