Source Mechanisms of Induced Earthquakes at The Geysers Geothermal Reservoir

Abstract

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.

A Source Model for Induced Earthquakes at the Geysers Geothermal Reservoir

“…In some cases, the time history of source quantities shows the behavior expected for a process that starts with the failure of a shear crack and grows into the extension of wing cracks, but in other cases it is difficult to give a meaningful physical interpretation of the results. The general conclusion, similar to that of JOHNSON (2014), is that the uncertainty in the dynamic moment tensors prevents a detailed time domain description of the source processes. However, all of the results of Appendix B can be applied to static moment tensors merely by ignoring the time dependence of the expressions.…”

“…A fairly complete discussion of the background of geophysical studies at the Geysers, the demonstration project, the method used in estimating moment tensors, and the moment tensor results can be found in JOHNSON (2014) and the references contained therein. The only earthquakes considered in that and this study are those with epicenters located within a 1.5 km square area centered on well P32.…”

“…1 and 2 summarize some of the results obtained by JOHNSON (2014) for earthquakes associated with the demonstration project at well P32. These results support a number of conclusions, many of which are consistent with other studies of earthquakes at the Geysers.…”

“…A discussion of this deformation and the references that describe it can be found in JOHNSON (2014). What complicates an attempt to relate deformation to seismicity is the fact that the measured deformation is a trend observed over a 20 year period in a large, producing part of the field, while the seismicity of this study is for a period of \1 year in a small, non-producing part of the field.…”

“…The maximum slip on the shear crack is listed in Table 3 and plotted versus the scalar moment in The scalar moments m s , m w , and m u are given as the signed percentages of the sum of their absolute values In JOHNSON (2014), it is pointed out that, for the 20 events listed in Tables 1 and 2, there are significant differences between the corner frequency of the isotropic part of the dynamic moment rate tensor f ci and the average corner frequency of the deviatoric dynamic moment rate tensor f 0 cd : The results are listed in Table 2, with a more complete discussion along with plots and linear regression analysis found in JOHNSON (2014). A basic result is that the average ratio of these two corner frequencies is…”