The Geysers
The Geysers is a complex of 22 geothermal power plants, drawing steam from more than 350 wells, located in the Mayacamas Mountains 116 km ( 72 mi) 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 has only recently been reopened. 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.

Description
The Geysers geothermal development spans an area of around 78 km² ( 30 mi²) 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 Felsite intrusion. Gravity and seismic studies suggest that the source of heat for the steam reservoir is a large magma chamber over 7 km ( 4 mi) beneath the ground, and greater than 14 km ( 8 mi) 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 sewage effluent used to be dumped into rivers and streams and is now piped to the geothermal field where it replenishes the steam produced for power generation.

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). As can be seen in the figure, 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 M4 events per year and on the order of 20 to 30 M3 events per year. Although the M4 events have been increasing, the number of M3 events have been relatively constant since the mid 1980s. 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.

History
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.

Future
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.