I've been following a series of small earthquakes in northern California near The Geysers. This area is a popular place for health spas, mud baths and massages. I used to partake when I lived in San Francisco.
Screen shot from Google Earth showing Clear Lake, the volcanic area and all the earthquakes that are happening in the area.
From Wikipedia about Clearlake:
Mt Konocti Volcano
Brief overview of the Clear Lake Volcanic Field
Mt Cobb Volcano
The Clear Lake Volcanic Field is a volcanic field beside Clear Lake in California's northern Coast Ranges. The site of late-Pliocene to early Holocene activity, the volcanic field consists of lava domes, cinder cones, and maars with eruptive products varying from basalt to rhyolite.[2] Cobb Mountain and Mount Konocti are the two highest peaks in the volcanic field, at 4,724 feet (1,440 m)[1] and 4,285 feet (1,306 m)[4] respectively.
The field's magma chamber also powers a geothermal field called The Geysers, which hosts the largest complex of geothermal power plants in the world.[5] These can generate approximately 2000 megawatts, enough to power two cities the size of San Francisco.[3]
From 2009 - Earthquake Swarms
http://scienceblogs.com/eruptions/2009/05/earthquake_swarm_at_the_geyser.php
Earthquake map for today from Intellicast. The round circles at the bottom of the image shows the area where the swarms are massing.
earthquake maps from USGS - The red arrow points to the area where the swarms are taking place.
This map shows the swarm of earthquakes that are taking place near The Geyers.
Brief overview:
The Clear Lake Volcanic Field is a volcanic field beside Clear Lake in California's northern Coast Ranges. The site of late-Pliocene to early Holocene activity, the volcanic field consists of lava domes, cinder cones, and maars with eruptive products varying from basalt to rhyolite.[2] Cobb Mountain and Mount Konocti are the two highest peaks in the volcanic field, at 4,724 feet (1,440 m)[1] and 4,285 feet (1,306 m)[4] respectively.
The field's magma chamber also powers a geothermal field called The Geysers, which hosts the largest complex of geothermal power plants in the world.[5] These can generate approximately 2000 megawatts, enough to power two cities the size of San Francisco.[3]
The Clear Lake volcanic field ( late Pliocene to Holocene) lies in a tectonically active, complex geologic setting within the San Andreas transform fault system in northern Coast Ranges of California. Clear Lake and the volcanic field are located within a fault-bounded, locally extensional basin. The lake is the largest freshwater lake entirely within California; it is probably volcano-tectonic in origin, but is not a caldera lake. The volcanic field is the northernmost of a series of young Cenozoic volcanic fields in the Coast Ranges. Within the field, eruptive loci have migrated northward through the last 2.1 million years. Eruptive centers are lacking. Volcanism appears to be related to extension in a pull-apart basin within the San Andreas fault system and is not directly related to subduction, which ceased off the California coast at this latitude around 3 million years ago.
The Clear Lake volcanics range from basalt through rhyolite in composition. Basalt is rare, and the dominant composition is dacite. Four eruptive episodes separated by time gaps of 0.15 - 0.2 million years were characterized by different compositional ranges. Total erupted volume probably exceeds 70 cubic kilometers. Volcanism has been largely non-explosive with only one major silicic air-fall tuff and no ash-flow tuffs. Numerous young locally vented deposits of palagonitic mafic tuff occur around the southeast shore of Clear Lake. ...
Clear Lake is a popular recreation center with resorts, camping, and boating facilities. State Highway 20 skirts the east side of the lake, and numerous smaller roads provide access to the volcanic features.
Mount Konocti
From: Smithsonian Institution Website, Global Volcanism Program, 2004
The late-Pliocene to early Holocene Clear Lake volcanic field in the northern Coast Ranges, contains lava dome complexes, cinder cones, and maars of basaltic-to-rhyolitic composition. The westernmost site of Quaternary volcanism in California, the Clear Lake field is located far to the west of the Cascade Range in a complex geologic setting within the San Andreas transform fault system. Mount Konocti, a composite dacitic lava dome on the south shore of Clear Lake, is the largest volcanic feature. Volcanism has been largely non-explosive, with only one major airfall tuff and no ash flows. The latest eruptive activity, forming maars and cinder cones along the shores of Clear Lake, continued until about 10,000 years ago. A large silicic magma chamber provides the heat source for the Geysers, the world's largest producing geothermal field.
From: Wood and Kienle, 1990, Volcanoes of North America: United States and Canada: Cambridge University Press, 354p., p.226-229, Contribution by Julie M. Donnelly-Nolan
... the 1-kilometer-high composite dacite volcano, Mount Konocti ... Mount Konocti, largest edifice of the Clear Lake volcanic field ...
The Geysers Geothermal Field -
Sulphur Bank Mine - McLaughlin Mine
From: Wood and Kienle, 1990, Volcanoes of North America: United States and Canada: Cambridge University Press, 354p., p.226-229, Contribution by Julie M. Donnelly-Nolan
Gravity and teleseismic studies suggest that a large silicic magma chamber around 14 kilometers in diameters, lies 7 kilometers and deeper beneath the volcanic field. This reservoir is thought to be the heat source for The Geysers geothermal field (on the southwest side of the volcanic field), which is the largest producing geothermal field in the world, with installed electrical generating capacity of around 2,000 megawatts in 1988, enough electricity for about two cities the size of San Francisco. Numerous thermal springs occur along northwest to north-northwest-trending faults that are subparallel to the main San Andreas fault. Associated epithermal deposits of mercury and gold include the Sulphur Bank Mine (still the site of active mercury deposition) and the McLaughlin Mine (a major disseminated gold deposit in an outlier of the Clear Lake volcanics), and associated hot springs deposits. ...
From: Kious and Tilling, 1996, This Dynamic Earth: The Story of Plate Tectonics: USGS General Interest Publication
The Geysers geothermal field near Santa Rosa, in Northern California produces enough electricity to meet the power demands of San Francisco. The Geysers area is the largest geothermal development in the world.
From this USGS page.
http://vulcan.wr.usgs.gov/Volcanoes/California/ClearLake/description_clear_lake.html