Desert and regions further east. Archaeologists recognize at least four different geochemical subsources: Sugarloaf, West Sugarloaf, Joshua Ridge, and West Cactus Peak. These different subsources were used to varying degrees in different time periods, but the Sugarloaf and West Sugarloaf were the most commonly exploited geochemical types (Eerkens and Rosenthal 2004; Ericson and Glascock 2004). Coso Geothermal Power Project The Coso Volcanic Field is well known as a geothermal area. Fumaroles are present along faults bounding the rhyolite-capped horst and locally within the rhyolite field. A multi-disciplinary program of geothermal assessment carried out in the 1970s defined a potential resource of 650 megawatts electric with a nominal life span of 30 years. Judged by the youthfulness of the rhyolite lavas and by a zone of low seismic velocity crust roughly beneath the rhyolite, a magma body may be the source of thermal energy for the geothermal system. Commercial power development began in the 1980s. Located within the Naval Air Weapons Station China Lake Station near Ridgecrest, California, power plants at the Coso Geothermal were operated by Caithness Energy, LLC (Reno, Nevada) and sold in 2006. It currently produces 270 MW from four geothermal power plants. More than 100 wells have been drilled throughout the field, with production depths from 2,000 to 12,000 feet (610 to 3,700 m), and temperatures from 200° to 350 °C. Coso began generating electricity in 1987. Improvements have resulted in more efficient use of the resource. Seismicity The Coso Volcanic Field is one of the most seismically active regions in the United States, producing dozens of tremors in the M1 and M2 range each week. Tremors in the M3 range occur at a rate of 2–6 per month and M4 quakes occur two-three times each year. Recent activity in the M5 range happened in 1996 and 1998 when tremors of M5.3, M5.1, M5.2, and M5.0 occurred within a day of each other. These tremors were actually recorded along the eastern side of the Coso Volcanic Field, 15 miles (24 km). September 30, 2009 to October 6, 2009 there have been 429 earthquakes ranging from 0.1 to 5.2. Some days have activity at about one tremor every minute. On October 2, 2009 there were three earthquakes, (5.2, 4.7, and a 4.9), all within one hour of each other. On July 4th and 5th, 2019 two large earthquakes sparked a surge of activity in the area in and