Fractal clustering of induced seismicity in The Geysers geothermal area, California

We analyse earthquakes recorded at The Geysers geothermal field in California, an area where industrial activity induces seismicity. The seismicity is characterized by the seismic b-value and D, the fractal dimension of earthquake hypocentres measured from sliding windows containing 200 events. We study a group of events strongly clustered around an injection well. Over most of the time period examined we find a positive correlation between b and D. However, during the initiation of injection into a new well we find instead a negative correlation. The differences in correlation are statistically significant at the 1 sigma level but only marginally so at the 2 sigma level. These results provide evidence for a transient change in the seismic mechanisms operating, and may be explained by a change from conditions of slow stress loading to rapid loading as a result of the build-up of the rate of water injection into the reservoir.