Heat flow near major strike-slip faults in California

Abstract

Seventeen heat-flow measurements were made in crystalline rock near the San Andreas, San Jacinto, and Garlock faults, California, in regions representative of several levels of seismic activity. Data from these measurements, together with other heat-flow determinations in California and offshore along the continental borderland, do not clearly demonstrate the existence of a heat-flow anomaly in the vicinity of these major faults, although regularities in the data are present. The mean value of the seventeen determinations is 1.65 μcal/cm^2/sec, ±0.28 s.d. It is concluded that any or all of the following are the case: (a) the amount of energy converted to heat near a fault is no larger than that appearing as seismic waves; (b) the presently inferred rates of slip on the faults studied have been going on for only the last few million years or less; (c) the high density of fault systems in central and southern California contributes to a regionally high heat flow but prevents the resolution of energy from any single member; (d) the frictional heat generation varies from place to place along the faults. In the region between Lake Hughes and San Bernardino, now seismically inactive, but in the zone of rupture from the ∼8-magnitude Fort Tejon earthquake, six measurements show no correlation with distance from the San Andreas fault. Near the San Jacinto fault in the Peninsular Ranges, a region characterized by frequent intermediate- and low-magnitude earthquakes, determinations at 1 and 4 km from the fault are the same; they are 20% higher than a measurement 13 km to the west but are not appreciably different from a probable regional average 25 km to the east in the Imperial Valley. Near Hollister, where the San Andreas fault is creeping at a rate of several centimeters per year, a measurement 3 km west of the fault gives a value similar to those found elsewhere along the fault, yet significantly higher than values to the east on the western flank of the Sierra Nevada. Finally, measurements across the historically inactive Garlock fault exhibit high fluxes near the fault in comparison with a determination 8 km to the north, but these measurements are only slightly higher than values characteristic of the Mojave block to the south.