VELOCITY STRUCTURE OF THE LONG VALLEY CALDERA FROM THE INVERSION OF LOCAL EARTHQUAKE P-TRAVEL AND S-TRAVEL TIMES

A high-resolution tomographic study of the Long Valley region began in 1989 with the installation of a special fan array of eight three-comp onent borehole (80-160 m depths) receivers on the northwestern rim of the caldera to provide the data necessary for a joint inversion for hy pocenters and the three-dimensional P and S wave velocity structure in and below the caldera. The experiment specifically targeted the subsu rface location of the previously inferred magma chamber beneath the Re surgent Dome. Additional coverage was provided by existing U.S. Geolog ical Survey and University of Nevada, Reno seismographic networks. We employed the progressive inversion scheme of Thurber (1983), with cubi c spline interpolation after Michelini and McEvilly (1991). The final model is based on data from 280 well-distributed local earthquakes. So me 6900 arrival times were used, including about 700 S wave times from the three-component stations. The resulting velocity structure reflec ts the known geology, defining the low-velocity caldera fill to 2 km d epth, contrasting markedly with the surrounding higher-velocity Sierra n block and highland terrane. No isolated distinct low-velocity anomal ies are revealed beneath the caldera floor, although a diffuse zone of reduced velocity persists to a depth of about 8 km. These lower veloc ities may be related to hydrothermal alteration and/or extensive fract ures. The Vp/Vs structure contains significant lateral variation withi n and beneath the caldera to about 8 km depth, and these variations ca n be related to accepted models of the active geothermal system. The l ack of a significant S wave velocity anomaly, along with the normal or low values of the Vp/Vs ratio, argues against the presence of a sizab le and distinct magma body at shallow to midcrustal depth beneath Long Valley caldera, although a very low percentage of partial melt cannot be precluded.