Lawrence Livermore National Laboratory deployed a small network of 14 three-component digital seismographs during August 1982 in the Mono Craters area, which is located just north of Long Valley caldera. The network recorded small earthquakes located to the south of Long Valley. One event shows a clear arrival between the P and S phases, which is interpreted as a reflected phase similar to that observed by other researchers in the Long Valley area. The reflected phase follows the P arrival by approximately 4.3 sec and has an apparent velocity of approximately 7.0 km/sec. Particle motions indicate that the reflection arrives at the surface as a P wave. Travel-time, amplitude, and particle motion modeling of the P, S, and reflected waves suggest that the reflected phase is best modeled as a P to P reflection from the base of a low-velocity layer. The layer is approximately 19 km thick and has a velocity contrast of 17 per cent with the basement rocks. The low-velocity layer extends to the lower crust and is floored with high-velocity (greater than 7.0 km/sec) material. A zone of partial granitic melt underlain by higher velocity basaltic rocks could account for the computed velocity structure.

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