Velocity‐density properties of the lithosphere from three‐dimensional modeling at the Geysers‐Clear Lake Region, California

Abstract

The analysis of travel time delays from 94 teleseisms recorded at The Geysers‐Clear Lake, California, region reveals a broad area of delay centered at Mount Hannah and encompassing The Geysers geothermal field. A three‐dimensional velocity inversion of the delays indicates that the source of the delays is a low‐velocity body largely within the top 30 km of the lithosphere. Velocity decreases of as much as 20% are observed in the top 15 km near Mount Hannah and diminish with depth. No significant velocity fluctuations are observed below 60 km. The block model of velocity perturbations resulting from the teleseismic residual inversion is used to predict the observed Bouguer gravity field by solving for the relation between velocity and density perturbation. Comparison between the predicted gravity and observed gravity indicates that the same body responsible for the teleseismic delays accounts for the long‐wave‐length 30‐mGal Bouguer gravity minima at Mount Hannah. The results of the velocity‐density modeling suggest that the material beneath Mount Hannah and The Geysers may be partially molten and somewhat more silicic than the surrounding medium.Supplemental table is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document J81‐006; $1.00. Payment must accompany order.