EGS: The Geysers: What is the history of seismicity at The Geysers?
Prior to the beginning of geothermal exploitation at The Geysers, seismicity was very low. However, the seismic coverage of stations was also very low. Nevertheless, according to USGS historical records for Northern California, no events were detected (above magnitude 2) before 1969— in a two degree box centered on The Geysers. Current studies of The Geysers Geothermal Field seismicity have reached the conclusion that deep-well injection in the field produces mostly microseismic events, with magnitudes on the Richter Scale (M) mostly between 0.5 and 3.0. As can be seen in the figure below, the seismicity between magnitude 3.0 and 4.6 (the largest event recorded in The Geysers field, which was in 1973) is on the order of a few magnitude 4 events per year and on the order of 20 to 30 magnitude 3 events per year. Although the magnitude 4 events have been increasing, the number of magnitude three events has been relatively constant since the mid 1980s, despite the rate of injection increasing since then.
Figure 1. The Geysers annual steam production (red line) plotted with the earthquake activity (M > 1.5 events), and water injection (blue line). Also shown are the M> 3.0 events (green solid line at bottom of plot) and M > 4 events (stars at top of chart).
The second figure shows the typical distribution of seismicity over the entire Geysers field for a typical year’s time.
Figure 2. A plan view of The Geysers geothermal field. The triangles are the locations of the seismic monitoring stations and the blue boxes are the locations of some of the injection wells.
Note that worldwide, EGS-project-induced-seismicity data indicate that the largest event linked to EGS activities to date was M3.7, in the Cooper Basin of Australia. However, research based on maximum fault lengths in the region indicates that a magnitude 5.0 is the largest possible (but not probable) event in The Geysers.
The largest seismic events of the past 12 years in the NCPA area occurred within the Big Sulphur Creek Fault Zone, a fault not considered active at the surface but which does form the southwestern boundary of The Geysers steam reservoir. Future EGS Demo Project activities have been designed to avoid this zone. A concern to the residents is not only the amount of seismicity but the magnitude of the largest seismic event likely to occur. Although no one can accurately predict earthquakes, the magnitude of an earthquake is dependent on the surface area that can slip, i.e., the length times the depth or width of the fault. Therefore, a large earthquake could occur only on a large fault. There are no mapped faults of large length in The Geysers, so it is extremely unlikely that induced seismicity caused by activities in The Geysers would lead to a large earthquake. There are some zones that may be faults, but which have no surface expression; the Big Sulfur Creek Fault Zone is the closest possible fault of this kind. Though the risk associated with this feature is very small, a buffer zone has been established around it to further reduce the probability that seismicity caused by water injection (hydroshearing) will migrate towards the fault.