A Systematic Analysis of Seismic Moment Tensor at The Geysers Geothermal Field, California

Abstract

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.

The Variability and Interpretation of Earthquake Source Mechanisms in The Geysers Geothermal Field From a Bayesian Standpoint Based on the Choice of a Noise Model

“…Retrieved mechanisms with positive volumetric components agree well with observations of Boyd et al (2015). However, these results are unexpected in a region that is undergoing volumetric contraction, as determined in geodetic studies.…”

“…Kirkpatrick et al (1996) used P and S wave amplitudes and polarities to obtain MT solutions for 2,552 events. Most recent studies using waveform data have found even larger ISO components (up to 47%) for certain earthquakes (Boyd et al, 2015;Johnson, 2014b). The ISO parts were up to 30% of the mechanism and had both positive and negative signs.…”

“…Normal mechanism earthquakes are common for larger events (e.g., Boyd et al, 2015), probably as a result of local stress variations. Solutions obtained with a two-attenuatedcosine C D have strike angles rotated by approximately 90 ∘ from strike angles obtained with a diagonal C D .…”

“…Larger earthquakes have recently been studied by Boyd et al (2015), who performed a regional full-waveform MT inversion of 53 events with moment magnitudes larger than 3. They found that, in comparison with the (black stars), in a previous study (gray star), or not analyzed (white stars), as explained in the text.…”

Moment Tensors of Induced Microearthquakes in The Geysers Geothermal Reservoir From Broadband Seismic Recordings: Implications for Faulting Regime, Stress Tensor, and Fluid Pressure

“…Several studies focusing on the source mechanisms of small groups of events (15-53 events) of larger magnitudes (M > 3.0) and/or with limited spatial distributions were reported using either local short-period or regional broadband networks (Boyd et al, 2015;Guilhem et al, 2014;L. R. Johnson, 2014aL.…”