THERMAL REGIME OF THE SAN-ANDREAS FAULT NEAR PARKFIELD, CALIFORNIA

Knowledge of the temperature Variation with depth near the San Andreas fault is Vital to understanding the physical processes that occur wit hin the fault zone during earthquakes and creep events. Parkfield is n ear the southern end of the Coast Ranges segment of the San Andreas fa ult. This segment has higher mean heat flow than the Cape Mendocino se gment to the northwest or the Mojave segment to the southeast. Borehol es were drilled specifically for the U.S. Geological Survey's Parkfiel d earthquake prediction experiment or converted from other uses at 25 sites within a few kilometers of the fault near Parkfield. These holes , which range in depth from 150 to over 1500 m, were intended mainly f or the deployment of volumetric strain meters, water-level recorders, and other downhole instruments. Temperature profiles were obtained fro m all the holes, and heat flow values were estimated from 17 of them, Far a number of reasons, including a paucity of thermal conductivity d ata and rugged local topography, the accuracy of individual determinat ions was not sufficiently high to document local variations in heat ho w. Values range from 54 to 92 mW m(-2), with mean and 95% confidence l imits of 74 +/- 4 mW m(-2) This mean is slightly lower than the mean ( 83 +/- 3) of 39 previously published values from the central Coast Ran ges, but it is consistent with the overall pattern of elevated heat fl ow in the Coast Ranges, and it is transitional to the mean of 68 +/- 2 mW m(-2) that characterizes the Mojave segment of the San Andreas fau lt immediately to the south. The lack of a heat flow peak near the fau lt underscores the absence of a frictional thermal anomaly and provide s additional support for a very small resolved shear stress parallel t o the San Andreas fault and the nearly fault-normal maximum compressiv e stress observed in this region. Estimates of subsurface thermal cond itions indicate that the seismicaseismic transition far the Parkfield segment corresponds to temperatures in the range of 350 degrees-400 de grees C. Increasing heat flow to the northwest of Parkfield correspond s to a transition from locked to creeping sections and to a shallowing of the base of seismicity and confirms the importance of temperature in controlling the thickness of the seismogenic crust. Lateral variati ons in heat flow do not appear to have any major role in determining t he regularity of M5.5-6 earthquakes at Parkfield.