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


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.

Seismological Research Letters