Low velocity zone under Long Valley as determined from teleseismic events

Abstract

A temporary seismograph station network was used to estimate teleseismic P wave residuals in the vicinity of Long Valley geothermal area, California. Relative P wave delays of 0.3 s persist at stations in the west central part of the Long Valley caldera after regional and near-surface effects have been removed. Ray tracing indicates that low-velocity material exists beneath the caldera at depths greater than 7 km and less than 40 km, probably less than 25 km. The velocity contrast with normal crust must be at least 5% to satisfy the data and is probably in the range 10–15%. We believe that the low velocity indicates anomalously hot rock at depth and that relative teleseismic P residuals may be useful for investigation of sources of geothermal energy.