|This article is outdated. (August 2012)|
The Geysers is a complex of 22 geothermal power plants, drawing steam from more than 350 wells, located in the Mayacamas Mountains 72 mi (116 km) north of San Francisco, California. The largest in the world, the Geysers has 1517 MW of active installed capacity with an average production factor of 63% (955 MW). Calpine Corporation operates and owns 19 of the 22 active plants in the Geysers and is currently the United States' largest producer of geothermal energy. Two other plants are owned jointly by the Northern California Power Agency and the City of Santa Clara's municipal Electric Utility (now called Silicon Valley Power). The Bottle Rock Power plant owned by the US Renewables Group was expected to be reopened in 2007. Another plant is under development by Ram Power Corp, formerly Western Geopower, with operation set to begin in 2010. Since the activities of one geothermal plant affects those nearby, the consolidation of plant ownership at The Geysers has been beneficial because the plants operate cooperatively instead of in their own short-term interest.
The Geysers geothermal development spans an area of around 30 square miles (78 km2) in Sonoma, Lake and Mendocino counties in California, located in the Mayacamas Mountains. Power from The Geysers provides electricity to Sonoma, Lake, Mendocino, Marin, and Napa counties. It is estimated that the development meets 60% of the power demand for the coastal region between the Golden Gate Bridge and the Oregon state line.
Steam used at The Geysers is produced from a greywacke sandstone reservoir, that is capped by a heterogeneous mix of low permeability rocks and underlaid by a silicic intrusion. Gravity and seismic studies suggest that the source of heat for the steam reservoir is a large magma chamber over 4 mi (7 km) beneath the ground, and greater than 8 mi (14 km) in diameter. Unlike most geothermal resources, the Geysers is a dry steam field, which means it mainly produces superheated steam. Because the power plant turbines require a vapor phase input, dry steam resources are generally preferable. Otherwise, a two-phase separator is required between the turbine and the geothermal wells to remove condensation that is produced with the steam. The Geysers complex is now recharged by injecting treated sewage effluent from the City of Santa Rosa and the Lake County sewage treatment plant. This effluent, which used to be discharged into waterways such as the Laguna de Santa Rosa, is now piped to the geothermal field where it replenishes the steam reservoir.
Induced seismicity 
Current studies of The Geysers Geothermal Field seismicity have reached the conclusion that deep-well injection in the field produces mostly microseismic events between magnitude 0.5-3.0 on the Richter Scale (M).
Worldwide, the largest induced seismic event to date linked to Enhanced Geothermal System (EGS) activity was M3.7 in the Cooper Basin of Australia. However, research based on maximum fault lengths indicates that a M5.0 is the largest possible (but not probable) event in the Geysers.
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 — the length times the depth or width of the fault. Therefore, a large earthquake can 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 will lead to a large earthquake.
The first recorded discovery of The Geysers was in 1847 during John Fremont's survey of the Sierra Mountains and the Great Basin by William Bell Elliot. Elliot called the area "The Geysers," although the geothermal features he discovered were not technically geysers, but fumaroles. Soon after, in 1852, The Geysers was developed into a spa for The Geysers Resort Hotel, which attracted the likes of Ulysses S. Grant, Theodore Roosevelt, and Mark Twain.
It was here that Pacific Gas and Electric began operation of the first successful geothermal electric power plant in the United States in 1960. The original turbine lasted for more than 30 years and produced 11 MW net power.
The Geysers electrical plant reached peak production in 1987, at that time serving 1.8 million people. Since then, the steam field has been in gradual decline as its underground water source decreases. Currently, the Geysers produce enough electricity for 1.1 million people.
Techniques developed from Enhanced Geothermal Systems research will increase the production of the region in the future. By reinjecting greywater from the nearby city of Santa Rosa, existing wells will be recharged. This water will be naturally heated in the geothermal reservoir, and be captured by the existing power plants as steam. The project should increase electrical output by 85 MW, enough for about 85,000 homes.
See also 
- Geothermal energy in the United States
- Geothermal power
- AltaRock Energy
- List of largest power stations in the world
- List of geothermal power stations
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|Wikimedia Commons has media related to: The Geysers|
- Calpine to Increase Renewable Energy Production at The Geysers; Calpine and Santa Rosa to Expand Geysers Recharge Project
- USGS map of frequent earthquakes in the Geyser area
- Interactive, Real-Time Map of Earthquakes at The Geysers
- The short film A Geysers Album is available for free download at the Internet Archive [more]. History of The Geysers Geothermal Field, produced by the California Department of Conservation, Division of Oil, Gas, and Geothermal Resources (1992)