is instrumented with an EMI MT-1 system, consisting of three orthogonal induction coil magnetometers, two orthogonal dipoles to measure induced electric field variations, and Quanterra data loggers (digitizing at 1 Hz for this study). The array is being used to explore the possibility that electromagnetic (EM) precursors to earthquakes might be generated by tectonic activity along the SAF. A major rationale for having two stations was to use data from one site to estimate the EM signal of ionospheric/magnetospheric origin at the other site. By doing this one might be able to detect anomalous signals of smaller amplitude, and possibly extend the reported association between anomalous EM signals and earthquakes to more frequent smaller events.Underlying this plan was the usual MT assumption that external magnetic fields are spatially uniform, at least over distances of a few hundred kilometers. Under this assumption the horizontal magnetic fields at one site (e.g., SAO) can be related to the those at the other site (PKD) via a frequency Inter-station Transfer FunctionsWe analyzed the data using robust multiple station TF methods [Egbert, 1997]

The Geysers geothermal field is one of the most seismically active regions in northern California. Most of the events occur at shallow depths and are related to stress and hydrological perturbations due to energy production operations. To better understand the relationships between seismicity and operations, better source mechanism information is needed. Seismic moment tensors offer insight into the nature of equivalent forces causing the seismicity. Fifty-three M >3 events located at The Geysers geothermal field were selected from the University of California Berkeley Moment Tensor Catalog for analysis of seismic moment tensor solutions and associated uncertainties. Deviatoric and full moment tensor solutions were computed, and statistical tests were employed to assess solution stability, resolution, and significance. In this study, we examine several source models including double-couple (DC), pure isotropic (ISO; volumetric change), and volume-compensated linear vector dipole (CLVD) sources, as well as compound sources such as DC CLVD, DC ISO, and shear-tensile sources. In general, we find from a systematic approach toward characterizing uncertainties in moment tensor solutions that The Geysers earthquakes, as a population, deviate significantly from northern California seismicity in terms of apparent volumetric source terms and complexity.

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