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

We analyse earthquakes recorded at The Geysers geothermal field in Californ ia, an area where industrial activity induces seismicity. The seismicity is characterized by the seismic b-value and D, the fractal dimension of earth quake hypocentres measured from sliding windows containing 200 events. We s tudy a group of events strongly clustered around an injection well. Over mo st 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 ins tead a negative correlation. The differences in correlation are statistical ly significant at the 1 sigma level but only marginally so at the 2 sigma l evel. These results provide evidence for a transient change in the seismic mechanisms operating, and may be explained by a change from conditions of s low stress loading to rapid loading as a result of the build-up of the rate of water injection into the reservoir.