Teleseismic evidence for a low‐velocity body under the Coso Geothermal Area

Teleseismic P wave arrivals were recorded by a dense array of seismograph stations located in the Coso geothermal area, California. The resulting pattern of relative residuals reveals an area showing approximately 0.2‐s excess travel time that migrates with changing source azimuth, suggesting that the area is the ‘delay shadow’ produced by a deep, low‐velocity body. Inversion of the relative residual data for three‐dimensional velocity structure determines the lateral variations in velocity to a depth of 22.5 km beneath the array. An intense low‐velocity body, which coincides with the surface expressions of late Pleistocene rhyolitic volcanism, high heat flow, and hydrothermal activity, is resolved between 5‐ and 20‐ km depth. It has maximum velocity contrast of over 8% between 10 and 17.5 km. The shallowest part of this body is centered below the region of highest heat flow; at depth it is elongate in approximately the N‐S direction. The hypothesis that this low‐velocity body is caused by the presence of partial melt in the middle crust is consistent with the local seismic, geologic, and thermal data.