Heat Flow in the SAFOD Pilot Hole and Implications for the Strength of the San Andreas Fault
Abstract
As part of an investigation into the physical properties of the San Andreas fault (SAF) and adjacent crust, detailed thermal measurements have been acquired in the 2.2-km-deep pilot hole for the San Andreas Fault Observatory at Depth (SAFOD), located 1.8 km west of the SAF near Parkfield, California. Precision temperature logs have been combined with thermal conductivity measurements on drill cuttings in a detailed vertical profile of heat flow. The temperature at the bottom of the borehole is 92 ° C, and heat flow from the basement section of the borehole (770 to 2160 m) is 91+/-2 mW m-2. Within the resolution of the measurements, heat flow is constant across the identified faults that intersect the borehole, suggesting that any active fluid flow along these faults is at rates too low to alter the background conductive thermal regime. Heat flow in the SAFOD pilot hole is significantly higher than the 74 mW m-2 average for the Parkfield area reported by Sass et al. (JGR, v. 102, 1997) based on measurements in shallow holes but consistent with five measurements ranging from 84 to 100 mW m-2 near the SAF in Pancho Rico Canyon 20 km to the northwest. Reanalysis of the regional heat flow pattern indicates that high heat flow at the SAFOD site reflects an abrupt increase in heat flow along the SAF and within the Coast Ranges northwest of Parkfield. This transition corresponds to a shallowing of the base of seismicity on the SAF and may be related to a change in the mechanical behavior of the fault near the northern terminus of the M6 1966 Parkfield earthquake rupture. The persistence of elevated heat flow at sites more than 40 km west of the SAFOD pilot hole appears to rule out frictional heating on the SAF as a major source of the high SAFOD value. However, the correlation of along-strike variations in heat flow with changes in rupture patterns and fault characteristics may indicate a previously overlooked connection between laterally heterogeneous frictional properties and active thermal processes.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2003
- Bibcode:
- 2003AGUFM.S41B..06W
- Keywords:
-
- 5134 Thermal properties;
- 8123 Dynamics;
- seismotectonics;
- 8130 Heat generation and transport;
- 8159 Rheology: crust and lithosphere