Three-Dimensional Imaging of the Crust and Upper Mantle in the Long Valley-Mono Craters Region, California, Using Teleseismic P-Wave Residuals

Abstract

Teleseismic travel time residuals measured at 90 seismic stations centered on the Long Valley caldera in eastern California were inverted to obtain a three-dimensional image of the velocity structure beneath the array. Direct inversion of these data indicate that the 2- to 4-km thick low-velocity caldera fill contaminates the signal from mid-crustal velocity anomalies beneath the caldera. Thus, two methods were used to strip the effects of the upper crust from the travel time residuals: (1) ray tracing through upper crustal velocity models provided by seismic refraction experiments and gravity surveys, and (2) an iterative stripping scheme using the inversion process itself. These methods adequately remove the effects of the shallowest crustal structures, including the caldera fill. The resulting “stripped” models show two well resolved mid-crustal low-velocity bodies in the Long Valley region. The first body is centered between 7 and 20 km depth beneath the resurgent dome of the Long Valley caldera and has a volume of 150 to 600 km3. The second, with a similar volume, is centered between 10 and 20 km depth beneath the Mono Craters, about 10 km north of Long Valley. Velocity contrasts in both of these bodies are about 6 to 10%, and the features are interpreted as silicic magma chambers. Upper mantle velocities are lower than average beneath the Mono Craters, and higher than average beneath Long Valley. The high eruptive rate of the Mono Craters and these upper mantle structures suggest that the focus of volcanism is shifting north from Long Valley to the Mono Craters.