Heat Flow Studies in the SAFOD Main Hole

Thermal investigations conducted in association with the San Andreas Fault Observatory at Depth (SAFOD) are focused on characterizing the in situ thermal state of the San Andreas fault (SAF) at seismogenic depths and detecting any thermal anomalies associated with frictional slip along the fault. Heat flow in the near-vertical, 2.2-km-deep SAFOD pilot hole, located 1.8 km west of the SAF near Parkfield, California averages 91 ± 3 mW m-2, a value consistent with other measurements in the Coast Ranges northwest of Parkfield (Williams et al., GRL, 2004, DOI 10.1029/2003GL019352). We are acquiring new temperature and thermal conductivity measurements from the SAFOD main hole, which starts at the same location as the pilot hole and then deviates to cross the active trace of the SAF at a depth of 3.2 km. Temperature profiles in the two holes are remarkably consistent, with the temperature at 2.2 km vertical depth in the pilot hole equaling 92.5 oC, and temperature at the same depth in the main hole approximately 1 km to the northeast equaling 93.0 oC. This difference is well within the temperature measurement uncertainty of ±0.01 oC and most likely reflects either variations in thermal properties between the two holes or uncertainties in the directional surveys. Preliminary heat flow measurements in the main hole from the depth intervals 2.24-2.33 km (0.8 to 0.7 km from the SAF) and 2.35-2.53 km (0.7 to 0.5 km from the SAF) equal 86 ± 9 mW m-2 and 100 ± 7 mW m-2, respectively. This variation in heat flow relative to the average measured in the pilot hole most likely reflects thermal refraction through the sedimentary rocks that occupy the interval between the SAF and the granitic basement penetrated by the pilot hole and the upper 1.8 km of the main hole. Alternatively, the observed variations could reflect a modest increase in heat flow toward the SAF. Such large-scale spatial trends, and their implications for frictional heating along the fault, will be resolved when we have completed heat flow measurements over the entire length of the main hole and across the SAF in late 2005.