Clear Lake

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  • 38.97°N
  • 122.77°W

  • 1439 m
    4720 ft

  • 323100
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Most Recent Bulletin Report: June 1992 (BGVN 17:06)


50 small seismic events triggered by M 7.5 earthquake hundreds of km away

Southern California's largest earthquake since 1952, M 7.5 on 28 June, appeared to trigger seismicity at several volcanic centers in California. It was centered roughly 200 km E of Los Angeles. In the following, David Hill describes post-earthquake activity at Long Valley caldera, and Stephen Walter discusses the USGS's seismic network, and the changes it detected at Lassen, Shasta, Medicine Lake, and the Geysers.

In recent years, the USGS northern California seismic network has relied upon Real-Time Processors (RTPs) to detect, record, and locate earthquakes. However, a film recorder (develocorder) collects data from 18 stations in volcanic areas, primarily to detect long-period earthquakes missed by RTPs. The film recorders proved useful in counting the post-M 7.5 earthquakes, most of which were too small to trigger the RTPs.

The film record was scanned for the 24 hours after the M 7.5 earthquake, noting the average coda duration for each identified event. Some events may have been missed because of seismogram saturation by the M 7.5 earthquake. Marked increases in microseismicity were observed at Lassen Peak, Medicine Lake caldera, and the Geysers (table 1). No earthquakes were observed at Shasta, but the lack of operating stations on the volcano limited the capability to observe small events.

Table 1. Number of earthquakes at northern California volcanic centers during 24-hour periods following major earthquakes on 25 April (40.37°N, 124.32°W; M 7.0) and 28 June (34.18°N, 116.47°W; M 7.5) 1992. Events with coda durations less than or equal to 10 seconds and greater than 10 seconds are tallied separately. Earthquakes were identified from film records of seismograms from nearby stations. Courtesy of Stephen Walter.

                        Lassen       Shasta   Medicine Lake    Geysers
    Codas (seconds)   <=10  >10    <=10  >10    <=10  >10     <=10  >10
    25 April             0    0       0    1       0    0        7    2
    28 June              8   14       1    5      12    0       46    4

Film was also scanned for the 24 hours following the M 7.0 earthquake at 40.37°N, 124.32°W (near Cape Mendocino) on 25 April. Although smaller than the 28 June earthquake, its epicenter was only 20-25% as far from the volcanoes. Furthermore, both the 25 April main shock and a M 6.5 aftershock were felt at the volcanic centers, but no felt reports were received from these areas after the 28 June earthquake. Only the Geysers showed any possible triggered events after the 25 April shock. However, background seismicity at the Geysers is higher than at the other centers, and is influenced by fluid injection and withdrawal associated with intensive geothermal development.

Geysers geothermal area report. Film records showed 50 small events in the 24 hours following the M 7.5 earthquake, 46 of which had coda durations <=10 seconds. The increased activity began within minutes of the large earthquake, with at least 16 small shocks recorded within the first hour. Although the Geysers (~750 km NW of the epicenter) is one of the most seismically active areas in N California, this represents a clear increase over the normal daily rate.

Information Contacts: Stephen Walter and David Hill, MS 977, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 USA.

The Global Volcanism Program has no Weekly Reports available for Clear Lake.

Index of Bulletin Reports


Reports are organized chronologically and indexed below by Month/Year (Publication Volume:Number), and include a one-line summary. Click on the index link or scroll down to read the reports.

06/1992 (BGVN 17:06) 50 small seismic events triggered by M 7.5 earthquake hundreds of km away




Bulletin Reports

All information contained in these reports is preliminary and subject to change.


06/1992 (BGVN 17:06) 50 small seismic events triggered by M 7.5 earthquake hundreds of km away

Southern California's largest earthquake since 1952, M 7.5 on 28 June, appeared to trigger seismicity at several volcanic centers in California. It was centered roughly 200 km E of Los Angeles. In the following, David Hill describes post-earthquake activity at Long Valley caldera, and Stephen Walter discusses the USGS's seismic network, and the changes it detected at Lassen, Shasta, Medicine Lake, and the Geysers.

In recent years, the USGS northern California seismic network has relied upon Real-Time Processors (RTPs) to detect, record, and locate earthquakes. However, a film recorder (develocorder) collects data from 18 stations in volcanic areas, primarily to detect long-period earthquakes missed by RTPs. The film recorders proved useful in counting the post-M 7.5 earthquakes, most of which were too small to trigger the RTPs.

The film record was scanned for the 24 hours after the M 7.5 earthquake, noting the average coda duration for each identified event. Some events may have been missed because of seismogram saturation by the M 7.5 earthquake. Marked increases in microseismicity were observed at Lassen Peak, Medicine Lake caldera, and the Geysers (table 1). No earthquakes were observed at Shasta, but the lack of operating stations on the volcano limited the capability to observe small events.

Table 1. Number of earthquakes at northern California volcanic centers during 24-hour periods following major earthquakes on 25 April (40.37°N, 124.32°W; M 7.0) and 28 June (34.18°N, 116.47°W; M 7.5) 1992. Events with coda durations less than or equal to 10 seconds and greater than 10 seconds are tallied separately. Earthquakes were identified from film records of seismograms from nearby stations. Courtesy of Stephen Walter.

                        Lassen       Shasta   Medicine Lake    Geysers
    Codas (seconds)   <=10  >10    <=10  >10    <=10  >10     <=10  >10
    25 April             0    0       0    1       0    0        7    2
    28 June              8   14       1    5      12    0       46    4

Film was also scanned for the 24 hours following the M 7.0 earthquake at 40.37°N, 124.32°W (near Cape Mendocino) on 25 April. Although smaller than the 28 June earthquake, its epicenter was only 20-25% as far from the volcanoes. Furthermore, both the 25 April main shock and a M 6.5 aftershock were felt at the volcanic centers, but no felt reports were received from these areas after the 28 June earthquake. Only the Geysers showed any possible triggered events after the 25 April shock. However, background seismicity at the Geysers is higher than at the other centers, and is influenced by fluid injection and withdrawal associated with intensive geothermal development.

Geysers geothermal area report. Film records showed 50 small events in the 24 hours following the M 7.5 earthquake, 46 of which had coda durations <=10 seconds. The increased activity began within minutes of the large earthquake, with at least 16 small shocks recorded within the first hour. Although the Geysers (~750 km NW of the epicenter) is one of the most seismically active areas in N California, this represents a clear increase over the normal daily rate.

Information Contacts: Stephen Walter and David Hill, MS 977, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 USA.

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.

The Global Volcanism Program is not aware of any Holocene eruptions from Clear Lake. If this volcano has had large eruptions (VEI >= 4) prior to 10,000 years ago, information might be found on the Clear Lake page in the LaMEVE (Large Magnitude Explosive Volcanic Eruptions) database, a part of the Volcano Global Risk Identification and Analysis Project (VOGRIPA).

This compilation of synonyms and subsidiary features may not be comprehensive. Features are organized into four major categories: Cones, Craters, Domes, and Thermal Features. Synonyms of features appear indented below the primary name. In some cases additional feature type, elevation, or location details are provided.


Cones

Feature Name Feature Type Elevation Latitude Longitude
Cobb Mountain Cone 1439 m 38° 49' 0" N 122° 44' 0" W
Round Mountain Cone 39° 3' 0" N 122° 38' 0" W
Roundtop Mountain Cone 38° 55' 0" N 122° 40' 0" W

Domes

Feature Name Feature Type Elevation Latitude Longitude
Hannah, Mount Dome 1212 m 38° 53' 0" N 122° 45' 0" W
Konocti, Mount Dome 38° 59' 0" N 122° 46' 0" W

Thermal

Feature Name Feature Type Elevation Latitude Longitude
Geysers, The Geyser 38° 48' 0" N 122° 48' 0" W
Mount Konocti, a twin-peaked complex dacitic volcano of Pleistocene age on the south-central shore of Clear Lake, is the most prominent feature of the Clear Lake volcanic field. Wright Peak (left) is a rhyodacitic and dacitic lava dome with associated lava flows, and Buckingham Peak (right) is composed of dacitic lava flows capped by an eroded basaltic-andesite cinder cone. The flat ridge extending across the photo in front of Mount Konocti is a peninsula of Cretaceous-Jurrasic rocks of the Franciscan formation.

Photo by Lee Siebert, 1967 (Smithsonian Institution).
Mount Konocti, a dacitic complex volcano rising above the southern shore of Clear Lake, is the largest feature of the late-Pliocene to early Holocene Clear Lake volcanic field in the northern Coast Ranges. The volcanic field contains lava dome complexes, cinder cones, and maars of basaltic-to-rhyolitic composition. It is located far to the west of the Cascade Range in a complex geologic setting within the San Andreas transform fault system. The latest eruptive activity took place about 10,000 years ago.

Photo by Lee Siebert, 1997 (Smithsonian Institution).
Elongated Rattlesnake Island near the NE tip of Clear Lake is seen here from the NW. Most of the flat-lying western side of the island is a basaltic-andesite lava flow of late-Pleistocene age. The flow was erupted to the NE and NW from a cinder cone that is the gently rounded ridge that forms the far southern tip of the island at the left. The scoriaceous aa flow was erupted subaerially above the lake level.

Photo by Lee Siebert, 1997 (Smithsonian Institution).
Mount Konocti, a composite dacitic volcano, rises above the southern shore of Clear Lake. The arcuate escarpment facing the lake on the the NW-most peak, Buckingham Peak (right), is a landslide scarp produced when part of the dome collapsed. A series of maars cuts the southern shore of Clear Lake, giving it a scalloped shoreline. Clear Lake itself, the largest natural freshwater lake entirely within California, is an elongated, irregular body of water more than 30 km long. It has a volcano-tectonic origin.

Photo by Lee Siebert, 1997 (Smithsonian Institution).
Little Borax Lake, a small maar near the south-central shore of Clear Lake, is one of the youngest features of the Clear Lake volcanic field. The maar was erupted through landslide deposits on the SE flank of Buckingham Peak, the NW-most of the twin dacitic domes forming Mt. Konocti. The forested ridge in the background is the SE rim of the landslide scarp, and sunlight reflects off the surface of Clear Lake at the extreme left. Other maars in this area occur directly along the margin of the lake, producing a scalloped shoreline.

Photo by Lee Siebert, 1997 (Smithsonian Institution).
Brilliantly colored, bright-red oxidized scoria deposits from a cinder cone near the SE end of Clear Lake are quarried for road aggregate. The basaltic-andesite cinder cone is one of a N-S-trending chain of young cones at the eastern margin of the Clear Lake volcanic field. This the northernmost of two quarried cinder cones bissected by the highway leading into the town of Clearlake Oaks.

Photo by Paul Kimberly, 1997 (Smithsonian Institution).
Round Mountain (center), seen here from the WNW with the hills of the California Coast Range in the background, is one of a series of basaltic-andesite cinder cones erupted along a N-S-trending line at the eastern margin of the Clear Lake volcanic field. These cones form some of the youngest products of the volcanic field. Lava flows from Round Mountain extend west into High Valley (lower right) and east to the North Fork Cache River. The background hills are composed of Cretaceous-Jurrasic metamorphic rocks of the Franciscan formation.

Photo by Paul Kimberly, 1997 (Smithsonian Institution).
Round Mountain, seen here from the west, rises above the floor of High Valley. Round Mountain is the northernmost of a N-S-trending chain of basaltic-andesite cinder cones at the eastern end of the Clear Lake volcanic field. A lava flow from Round Mountain underlies part of the High Valley. Many of the basaltic-andesite and andesitic lava flows of the Clear Lake field are contaminated with crustal materials. They often contain xenocrysts of quartz up to several cm in size that are known locally as "Lake County Diamonds."

Photo by Paul Kimberly, 1997 (Smithsonian Institution).
The Geysers steam field, located at the SW end of the Clear Lake volcanic field in the Coast Ranges of western California, is the world's largest producing geothermal field. A large silicic magma chamber, about 14 km in diameter and 7 km and more in depth, provides the heat source for the geothermal field. The Geysers produced about 2000 megawatts of electricity in 1988, enough to power two cities the size of San Francisco.

Photo by Paul Kimberly, 1997 (Smithsonian Institution).

The following references have all been used during the compilation of data for this volcano, it is not a comprehensive bibliography. Discussion of another volcano or eruption (sometimes far from the one that is the subject of the manuscript) may produce a citation that is not at all apparent from the title.

Anderson C A, 1936. Volcanic history of the Clear Lake area, Calif. Geol Soc Amer Bull, 47: 629-664.

California Div. Mines and Geology, 1958-69. Geologic atlas of California, 1:250,0000 scale.. Calif Div Mines Geol.

Donnelly-Nolan J M, Hearn B C Jr, Curtis G H, Drake R E, 1981. Geochronology and evolution of the Clear Lake volcanics. In: McLaughlin R J, Donnelly-Nolan J M (eds) Research in the Geysers-Clear Lake geothermal area, northern California. {U S Geol Surv Prof Pap}, 1141: 47-60.

Garrison L E, 1972. Geothermal steam in the Geysers-Clear Lake region, California. Geol Soc Amer Bull, 83: 1449-1468.

Hammersley L, DePaolo D J, 2006. Isotopic and geophysical constraints on the structure and evolution of the Clear Lake volcanic system. J Volc Geotherm Res, 153: 331-356.

Hearn B C Jr, Donnelly-Nolan J M, Goff F E, 1981. The Clear Lake volcanics: tectonic setting and magma sources. In: McLaughlin R J, Donnelly-Nolan J M (eds) Research in the Geysers-Clear Lake geothermal area, northern California. {U S Geol Surv Prof Pap}, 1141: 25-45.

Hearn B C Jr, Donnelly-Nolan J M, Goff F E, 1995. Geologic map and structure sections of the Clear Lake volcanics, Northern California. U S Geol Surv Map, I-2362.

Luedke R G, Smith R L, 1981. Map showing distribution, composition, and age of late Cenozoic volcanic centers in California and Nevada. U S Geol Surv Map, I-1091-C.

Miller C D, 1989. Potential hazards from future volcanic eruptions in California. U S Geol Surv Bull, 1847: 1-17.

Wood C A, Kienle J (eds), 1990. Volcanoes of North America. Cambridge, England: Cambridge Univ Press, 354 p.

Volcano Types

Volcanic field

Tectonic Setting

Subduction zone
Continental crust (> 25 km)

Rock Types

Major
Dacite
Andesite / Basaltic Andesite
Rhyolite
Basalt / Picro-Basalt

Population

Within 5 km
Within 10 km
Within 30 km
Within 100 km
52,801
52,801
71,791
1,059,280

Affiliated Databases

Large Eruptions of Clear Lake Information about large Quaternary eruptions (VEI >= 4) is cataloged in the Large Magnitude Explosive Volcanic Eruptions (LaMEVE) database of the Volcano Global Risk Identification and Analysis Project (VOGRIPA).
WOVOdat WOVOdat is a database of volcanic unrest; instrumentally and visually recorded changes in seismicity, ground deformation, gas emission, and other parameters from their normal baselines. It is sponsored by the World Organization of Volcano Observatories (WOVO) and presently hosted at the Earth Observatory of Singapore.
EarthChem EarthChem develops and maintains databases, software, and services that support the preservation, discovery, access and analysis of geochemical data, and facilitate their integration with the broad array of other available earth science parameters. EarthChem is operated by a joint team of disciplinary scientists, data scientists, data managers and information technology developers who are part of the NSF-funded data facility Integrated Earth Data Applications (IEDA). IEDA is a collaborative effort of EarthChem and the Marine Geoscience Data System (MGDS).
Smithsonian Collections Search the Smithsonian's NMNH Department of Mineral Sciences collections database. Go to the "Search Rocks and Ores" tab and use the Volcano Name drop-down to find samples.