The Geysers Geothermal Field is located in a tectonically active region of Northern California. The major seismic hazards in the region are from large earthquakes occurring along regional faults that are located miles away from the geothermal field, such as the San Andreas and Healdsburg-Rodgers Creek faults. However, activities associated with the withdrawal of steam for producing electric power cause or induce small quakes to occur in the field. These smaller quakes are frequently felt by those who work at the field and by nearby residents.
Seismicity at The Geysers was poorly documented when power generation commenced in the 1960's, but since 1975 high-quality seismic monitoring data has been available, and it has been demonstrated that increased steam production and fluid injection correlates positively with changes in earthquake activity. The level of seismicity has been fairly stable since the mid-1980s, even though power production has declined in the field with the depletion of the steam reservoirs. Sheet 3 of Seismicity map of Santa Rosa, California 1969-1995 illustrates how seismicity has spatially and temporally expanded with geothermal production.
Seismologists have proposed several mechanisms to explain why earthquakes are being induced. The operators of the geothermal field are withdrawing mass (steam boiled from water) and heat, both of which cause the surrounding rock to contract, which in turn can induce earthquakes as a result of the contractional stresses. In addition, the operators condense the extracted steam and flow the water back into the steam reservoir at depths of one to three km in order to extend the life of the field. Furthermore, reclaimed water from nearby Lake County and Santa Rosa is pumped to The Geysers and flowed into the steam reservoir. The condensed steam and reclaimed water is cold, whereas the rock is hot, and it appears that this thermal contrast is a significant factor in inducing the earthquakes. It is also possible that the hydraulic pressure of the injected fluid finds its way into faults and facilitates fracturing due to increased fluid pressures.
To date, the largest quake recorded at The Geysers is approximately M4.5. It is possible that a magnitude 5 could occur, but larger earthquakes are thought to be unlikely. In order for a larger earthquake to occur, it is necessary that a large fault exist. For example, the 1906 magnitude (M) 7.8 San Francisco earthquake ruptured nearly 300 miles of the San Andreas Fault. At The Geysers no such continuous fault is known to exist. Rather, there are numerous small fractures in the rock located near the many steam and injection wells. The activities described above result in local stress changes which, when added to the prevailing regional tectonic stresses, induce the quakes on these small faults.
Studies were conducted in the 1980's to image the magma body using seismic data from distant earthquakes and other geophysical data. These studies have limited resolution, and some of the interpretations are controversial. The magma body, if correctly modeled, is mostly in the Earth's crust. It is thought that basaltic intrusions have intruded the crust, melting the host rock and forming a silicic magma chamber more than 7 km below the earth's surface. Steam wells have encountered granitic material at depths of 1 to 4 km, and at the surface volcanic rocks form Cobb Mountain on the eastern edge of the field. However, all these igneous rocks have been dated at over 1 million years old, and are therefore too old to be the heat source for the active geothermal system at The Geysers. Volcanic rocks as young as 10,000 years are found further north in the Clear Lake region, and scientists believe that similarly young intrusive rocks underlie The Geysers as well, but much deeper than the deepest steam wells. All of the earthquakes at The Geysers locate at depths above 6 km and above the presumed magma body.
At this time, there is no evidence that magmatic pressures are involved in generating earthquakes. Our main methods for detecting volcanic unrest are seismic and geodetic. The Geysers field is densely monitored by seismic instrumentation operated by both Calpine Corporation and the USGS, and both data streams are integrated in real time into the near real-time earthquake map. The seismic instrumentation would record signs of volcanic unrest such as harmonic tremor or long-period quakes, and scientists would quickly recognize any deepening of quakes. The USGS, UC Berkeley, and the NSF Plate Boundary Observatory also operate regional GPS networks that could be densified should the situation warrant.