Abstract

Seismic-refraction profiles recorded north of Mammoth Lakes, California, using earthquake sources from the January 1983 swarm complement earlier explosion refraction profiles and provide velocity information from deeper in the crust in the area of the Long Valley caldera. Eight earthquakes from a depth range of 4.9 to 8.0 km confirm the observation of basement rocks with seismic velocities ranging from 5.8 to 6.4 km/sec extending at least to depths of 20 km. The data provide further evidence for the existence of a partial melt zone beneath Long Valley caldera and constrain its geometry. This evidence is in the form of pronounced secondary arrivals, which we interpret as waves that have propagated through a volume with low Q (the magma chamber) and reflected from the lower boundary of the southern edge of this volume at depths of 18 to 20 km.

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