No abstract

Temperatures were measured in Shady Rest drill hole RD08, Mammoth Lakes, Mono County, Calif., on five occasions in July and once in September 1986. The interpretation of these temperature logs was complicated by a complex drilling history, multiple strings of casing, lost circulation, and the introduction of a large quantity (-1,100 gal) of cold water into the annulus between the HQ and NQ casings in order to determine the top of the cement. Nevertheless, the general pattern from these logs indicates a high temperature (maximum 202°C at-1,110 ft) and significant formation permeability above this level, as evidenced by the lost circulation during drilling and the ability of the formation to absorb injected fluid after the well was completed. Changes in temperature logs run in July and September appear to result from fluid loss to the formation during and following drilling and possibly to ground shaking associated with present and past seismic activity in the Long Valley region. Natural gamma-ray logs obtained on four occasions in September 1986 vary in detail but generally follow the same trend from one log to another with pronounced breaks at about 800 and 1,076 ft, corresponding to changes in lithology. The fact that temperatures in RD08 are close to boiling at 1,110 ft and that RD08 is located near the western terminus of a WNW zone of strike-slip faulting some 10 km long, 2 km wide, and extending to a depth of about 8 km leads us to speculate as to the cause and consequence of this seismicity, particularly with regard to spasmodic tremor evident in earthquake swarms in this zone. The fragile hydrothermal regime of Long Valley caldera was severely disrupted by strong local earthquakes beginning between 1978 and 1980. Subsurface boiling may have occurred along dilational jogs (in the manner of Sibson, 1986) in the south moat. Consequent brecciation of rocks may have produced a significant positive change in volume, the process being fortified by the large heat release upon hydrothermal alteration of previously unaltered rocks. Whether this hypothetical volume change is large enough to have significantly influenced tectonic events remains obscure.

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards and stratigraphic nomenclature. Any use of trade names is for descriptive purposes only and does not imply endorsement by the USGS.Although these programs have been extensively tested, the U.S. Geological Survey cannot guarantee that they will give accurate results for all applications or that they will work on all computer systems.

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards and strati graphic nomenclature. Any use of trade names is for descriptive purposes only and does not imply endorsement by the USGS.

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards and stratigraphic nomenclature. Any use of trade names is for descriptive purposes only and does not imply endorsement by the USGS.Although the program has been extensively tested, the U.S. Geological Survey cannot guarantee that it will give accurate results for all applications or that it will work on all computer systems. No warranty, expressed or implied, is made by the USGS as to the accuracy and functioning of the program and related program material, nor shall the fact of distribution constitute any such warranty, and no responsibility is assumed by the USGS in connection therewith.