• fractal;
  • geothermal;
  • seismicity.

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 1σ level but only marginally so at the 2σ 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.