Analysis of Seismic Moment Tensors, In-Situ Stress, and Finite-Source Scaling of Earthquakes at The Geysers Geothermal Field, California
The objective of this research was to develop methodologies of interest to the geothermal industry to estimate critical parameters needed to characterize the fracture network generated during Enhanced Geothermal System (EGS) development. This work was supported by the U.S. Department of Energy Geothermal Technologies Office (DOE GTO). The research focused on seismicity at The Geysers geothermal field located 120 km north of San Francisco, California. The Geysers geothermal field is the world’s largest geothermal reservoir with approximately 1.6 GW installed electric capacity, 22 geothermal power plants with current average production of over 900 MW. Geothermal energy has been produced at The Geysers geothermal field since the early 1960s. Most of the earthquakes occur at shallow depths and are related to stress and hydrological perturbations due to geothermal energy operations including, on average, more than 57 million liters of treated wastewater injected daily. While the locations of earthquakes, as well as the timing and rates of their occurrence correlate with geothermal energy activities, greater understanding about the physical mechanisms is needed.
We developed and implemented a suite of methodologies to support the geothermal industry during EGS development. Our research focused on 1.0 < MW < 5.0 seismicity at The Geysers geothermal field, California to remotely estimate critical source parameters needed to characterize the fracture network generated during the EGS development. Initially 53 larger magnitude M > 3 earthquakes occurring throughout The Geysers geothermal field from 1992 to 2014 were investigated using an approach designed to assess resolution and uncertainty of seismic moment tensors. Deviatoric and full moment tensor solutions were computed, and statistical tests were implemented to assess solution stability, resolution and significance, particularly with respect to possible volumetric or tensile components. Several source models were examined including double-couple (DC), pure isotropic (ISO; volumetric change), and volume-compensated linear vector dipole (CLVD) sources, as well as mixtures of these such as DC + CLVD, DC + ISO and shear-tensile sources. In general, it was found that The Geysers earthquakes as a population deviate significantly from northern California seismicity in terms of apparent volumetric source terms and complexity. The mean volumetric component of earthquake mechanisms at The Geysers is approximately 30% compared to essentially zero for earthquake mechanisms outside The Geysers geothermal field. The volumetric moment tensor components of The Geysers earthquakes could arise from the flashing of injected water to steam, or more likely from the tensile stress induced from rapid cooling of the hot rock through the introduction of water.
In another study, seismicity in the vicinity of an EGS demonstration project in the Northwest Geysers at the Prati 32 (P-32) injection well and the Prati State 31 (PS-31) production well is investigated to determine earthquake focal mechanisms, moment magnitudes, and in-situ stress of events that occur before and during reservoir stimulation starting October 6, 2011. We compiled a catalog of 167 waveform-based seismic moment tensors ranging in moment magnitude from 0.6 to 3.9. Owing to the large number of events a semi-automated approach was developed to align the data with the Green’s functions used in the inversion. The automatic results were then reviewed to find the optimal shift for the inversion. The moment tensor catalog is subsequently used to invert for the stress tensor and to investigate possible temporal stress changes resulting from fluid injection. The relatively small uncertainties of the recovered stress tensors demonstrate the quality of the input mechanisms from the seismic moment tensor catalog. An approximate 15-degree counter-clockwise rotation of the least compressive stress σ3 was found to occur during injection. More remarkable was a change in the orientation of the maximum compressive stress σ1 from subhorizontal to vertical when injection operations temporarily halted. The orientation of σ1 returned to subhorizontal when injection operations resumed. It is found that there was a systematic reduction in the stress shape factor, R, as the injected water volume increases, indicating an evolution toward a more transtensional stress state. This work shows that effective tracking of the evolution of the state of stress in a system due to the introduction of water may be accomplished utilizing a semi-automatic full waveform moment tensor method. In addition, the full waveform approach was found to provide more robust estimates of the magnitudes of the micro-earthquakes at The Geysers.
Finally, a finite-source inverse approach was used to estimate the rupture area for ten earthquakes ranging in magnitude from MW 1.0 to 5.0. The slip models for these small earthquakes are complex with non-uniform distribution and in some cases with multiple asperities of slip release. They have complexity comparable to what is commonly found for larger earthquakes. We found the rupture area scaling with moment magnitude to be consistent with published relationships developed for globally distributed earthquakes with larger magnitudes (MW ≥ 5.5). This scaling relationship in conjunction with results from the stress inversion was subsequently applied to a set of earthquakes with high location accuracy to generate a statistical representation of the 3-D fracture network that was activated during the EGS demonstration project at Prati 32 injection well. We found the highest concentration of fractures located approximately 300 m to the north and 400 m below the bottom of the injection and production wells. Although this approach is approximate with respect to the actual orientation and dimension of the micro-earthquakes, it is a means to estimate a representation of the stimulated fracture network and to evaluate fracture density which could be used to help industry target where to place steam recovery wells. Additionally, the detailed source analysis and scaling information together with characterizations of rates of earthquakes can help to inform on possible seismic hazard related to such operations. Finally, an objective of the research was the development of portable tools that may be applied to study seismicity in other geothermal systems which could help assess their energy and economic potential.