A forty-station seismographic network operating in Long Valley caldera in California during the summer of 1997 recorded a number of earthquakes with unusual secondary phases. The phases can be seen on several stations throughout the network, allowing the use of particle motions and differential travel times to examine their nature and points of origin. Based on amplitude, frequency, and travel-time considerations, it appears that some of the phases are mode-converted S-to-P reflections from a zone below the Casa Diablo area near Mammoth Lakes, CA. The locus of reflection points was imaged using time-domain signal migration and stacking under the assumptions of first-order scattering theory. The results indicate that the reflecting zone is a curved, 1.5 km by 2 km surface 7.6 km below and 1.4 km west of the Casa Diablo geothermal plant.

Previous studies show low P-wave velocities and high S-wave attenuations in the same location as the reflecting zone. An amplitude analysis of the suspected reflections suggests they are produced by large contrasts in material properties, with the possible range of transitions including solid to partially molten or dry to fluid-saturated rock. Given these observations plus the volcanic history of the caldera and its recent tectonic activity, the most straightforward interpretation of the reflections is that they come from either the top of a sill or intrusion, or the overlying hydrothermal system of such structures.

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