Abstract

At The Geysers geothermal reservoir in northern California, evidence strongly suggests that activities associated with production of electric power cause an increase in the number of small earthquakes. First-degree dynamic moment tensors are used to investigate the relationship between induced earthquakes and injection of water into a well as part of a controlled experiment in the northwest Geysers. The estimation of dynamic moment tensors in the complex shallow crust at The Geysers is challenging, so the method is described in detail with particular attention given to the uncertainty in the results. For seismic events in the moment magnitude range of 0.9–2.8, spectral moduli of dynamic moment tensors are reliably recovered in the frequency range of 1–100 Hz, but uncertainty in the associated spectral phases limits their use to a few simple results. A number of different static moment tensors are investigated, with the preferred one obtained from parameters of a model fitted to the spectral modulus of the dynamic moment tensor. Moment tensors estimated for a group of 20 earthquakes exhibit a range of source mechanisms, with over half having significant isotropic parts of either positive or negative sign. Corner frequencies of the isotropic part of the moment tensor are about 40 % larger than the average of the deviatoric moment tensor. Some spatial patterns are present in source mechanisms, with earthquakes closely related in space tending to have similar mechanisms, but at the same time, some nearby earthquakes have very different mechanisms. Tensional axes of displacement in the source regions are primarily horizontal, while the pressure axes range from near horizontal to vertical. Injection of water into the well in the center of the study area clearly causes an increase in the number of earthquakes per day, but an effect upon source mechanisms is not evident.

Highlights

  • The Geysers geothermal reservoir in northern California is the world’s largest producer ofThe Geysers geothermal reservoir is located in the coast ranges of northern California and is contained within the broad transcurrent boundary between the North American and Pacific plates

  • A general objective of this study is to investigate source mechanisms of earthquakes at The Geysers by solving for their first-degree dynamic moment tensors

  • The manner in which uncertainty in the observational data and Green functions gets transformed into uncertainty in the dynamic moment tensors is best considered in the frequency domain

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Summary

Introduction

The Geysers geothermal reservoir in northern California is the world’s largest producer of. The general conclusion is that most of the seismicity at The Geysers is induced by activities associated with production of geothermal power (MAJER et al 2007). While there seems to be general agreement that geothermal operations play a role in inducing seismic activity at The Geysers, the physical mechanisms involved in this process are still not understood. Source mechanisms of a group of earthquakes that occurred during a controlled experiment are characterized by estimating their dynamic force moment tensors. The interpretation of these moment tensors can help constrain the physical processes taking place in the source regions of the earthquakes

Previous Studies
Moment Tensor Method
Dynamic Moment Tensor Results
Static Moment Tensor Results
General Results
Discussion and Conclusions
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