Structure of the Crustal Magmatic System in the Geysers-Clear Lake Area (Northern California) Imaged by Adjoint-State Travel-Time Tomography

Abstract

Abstract The Clear Lake volcanic field (CLVF) in northern California powers the world’s largest geothermal power plant, the Geysers Geothermal Complex. Volcanic-type earthquakes, hot springs, and seepage of volcanic gas suggest possible volcanic eruptions in the future. In this study, we apply a recently developed adjoint-state travel-time tomography method to high-quality manually picked P- and S-wave travel-time data to create 3D crustal VP and VS models for the Geysers-Clear Lake area. The adjoint-state travel-time tomography method has advantages of computational efficiency, ease of parallel implementation, and high accuracy in dealing with complex media. Strong velocity heterogeneities are revealed in the Geysers-Clear Lake area and its surrounding regions. Within the overall high VP, high VS Coast Ranges, a low VP, low VS anomaly is imaged beneath Mount Hannah, which indicates the existence of a large magma chamber. Our tomographic results suggest a simplified three-layer magmatic model beneath the CLVF: the upper layer (<7 km) consists of some intrusions of silicic magma from beneath Mount Hannah to the Geysers Geothermal Field in the southwest and to the Wilbur Springs area east of Clear Lake; the median layer includes the main body of the magma chamber at 5–13 km in depth and in the shape of an oblate ellipsoid; and the lower layer includes some mafic intrusions and molten or partially molten volcanic rocks from the upwelling mantle. The detailed velocity heterogeneities revealed by the newly picked travel-time data with the adjoint-state travel-time tomography method provide necessary constraints on seismogenic, volcanic, and geothermal processes in the Geysers-Clear Lake area, which are also useful for the mitigation of geologic hazards in northern California.