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Title: Seismicity of the Coso Range, California

Abstract

A 16-station seismographic network, approximately 40 km north-south by 30 km east-west, was installed in the Coso Range, California, in September 1975 as part of a geological and geophysical assessment of the geothermal resource potential of range. During the first 2 years of network operations, 4216 local earthquakes (0.5< or =m< or =3.9) defined zones of seismicity that strike radially outward from a Pleistocene rhyolite field located near the center of the Coso Range. Most earthquakes were located in zones showing a general northwest trend across the range. Six earthquake swarms occurred within the area that includes the rhyolite field. Fault plane solutions show regional north-south compression: earthquakes located in northwest striking zones generally had right lateral strike slip focal mechanisms, those in northeast striking zones left lateral strike slip focal mechanisms, and those in north-south striking zones both normal and strike slip focal mechanisms. Earthquake depths showed little variation across the Coso Range; the depth distribution is similar to that of several carefully studied segments of the central San Andreas fault. The b value calculated for the entire range is 0.99 +- 0.08. The rhyolite field has a significantly higher b value of 1.26 +- 0.16; if only themore » shallow events (depth <5 km) are used in the calculation, the b value for this area becomes even higher, 1.34 +- 0.24. The higher b values were interpreted as reflecting the existence of short average fault lengths (<5 km) within the rhyolite field. The seismic data and other data suggest that the fault system lying between the rhyolite field and the adjacent Coso Basin is an important tectonic boundary. Present information is insufficient to determine the geothermal production capability of this fault system, but is does suggest that the system is a good target for further exploration.« less

Authors:
;
Publication Date:
Research Org.:
U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025
OSTI Identifier:
5166030
Resource Type:
Journal Article
Resource Relation:
Journal Name: J. Geophys. Res.; (United States); Journal Volume: 85:B5
Country of Publication:
United States
Language:
English
Subject:
15 GEOTHERMAL ENERGY; 58 GEOSCIENCES; COSO HOT SPRINGS; SEISMICITY; EARTHQUAKE FOCI; EARTHQUAKE MAGNITUDE; GREAT BASIN; SLIP; CALIFORNIA; NORTH AMERICA; USA; WESTERN REGION; 150301* - Geothermal Exploration & Exploration Technology- Geophysical Techniques & Surveys; 580201 - Geophysics- Seismology & Tectonics- (1980-1989)

Citation Formats

Walter, A.W., and Weaver, C.S. Seismicity of the Coso Range, California. United States: N. p., 1980. Web. doi:10.1029/JB085iB05p02441.
Walter, A.W., & Weaver, C.S. Seismicity of the Coso Range, California. United States. doi:10.1029/JB085iB05p02441.
Walter, A.W., and Weaver, C.S. Sat . "Seismicity of the Coso Range, California". United States. doi:10.1029/JB085iB05p02441.
@article{osti_5166030,
title = {Seismicity of the Coso Range, California},
author = {Walter, A.W. and Weaver, C.S.},
abstractNote = {A 16-station seismographic network, approximately 40 km north-south by 30 km east-west, was installed in the Coso Range, California, in September 1975 as part of a geological and geophysical assessment of the geothermal resource potential of range. During the first 2 years of network operations, 4216 local earthquakes (0.5< or =m< or =3.9) defined zones of seismicity that strike radially outward from a Pleistocene rhyolite field located near the center of the Coso Range. Most earthquakes were located in zones showing a general northwest trend across the range. Six earthquake swarms occurred within the area that includes the rhyolite field. Fault plane solutions show regional north-south compression: earthquakes located in northwest striking zones generally had right lateral strike slip focal mechanisms, those in northeast striking zones left lateral strike slip focal mechanisms, and those in north-south striking zones both normal and strike slip focal mechanisms. Earthquake depths showed little variation across the Coso Range; the depth distribution is similar to that of several carefully studied segments of the central San Andreas fault. The b value calculated for the entire range is 0.99 +- 0.08. The rhyolite field has a significantly higher b value of 1.26 +- 0.16; if only the shallow events (depth <5 km) are used in the calculation, the b value for this area becomes even higher, 1.34 +- 0.24. The higher b values were interpreted as reflecting the existence of short average fault lengths (<5 km) within the rhyolite field. The seismic data and other data suggest that the fault system lying between the rhyolite field and the adjacent Coso Basin is an important tectonic boundary. Present information is insufficient to determine the geothermal production capability of this fault system, but is does suggest that the system is a good target for further exploration.},
doi = {10.1029/JB085iB05p02441},
journal = {J. Geophys. Res.; (United States)},
number = ,
volume = 85:B5,
place = {United States},
year = {Sat May 10 00:00:00 EDT 1980},
month = {Sat May 10 00:00:00 EDT 1980}
}
  • Major volcanic episodes occurred in the Coso Range at about 6 m.y., 4-2.5 m.y., and later than 1 m.y. Petrographic features, such as quartz xenocrysts in basalt, sieved plagioclase phenocrysts, broad compositional ranges of phenocrysts within single samples, and occurrence of commingled bombs and lava flows, indicate that many of the intermediate-composition rocks formed by mixing of basaltic magma with silicic material. A large volume, highly silicic center near Haiwee Ridge evolved 3.1-2.5 m.y. ago. This system erupted high-silica rhyolite air-fall and ash-flow tuff and in its later stages rhyodacite lava flows. The evolution of the Coso volcanic field, frommore » early basalt to polygenetic intermediate-composition volcanoes to a comparatively large-volume silicic center, is thought to reflect systematic changes in the least principal stress (S/sub 3/) and the related tectonic extension rate. Beginning about 4 m.y. ago, basalt was erupted onto a surface of little relief, ponding in a shallow north-trending basin. Intermediate-composition magma was produced mainly between 3.5 and 3.3 m.y., at a time when S/sub 3/ may have decreased and the extension rate increased. Thus, Pliocene volcanism progressed through a sequence of compositions and eruptive patterns that reflect the transition from tectonic stability to extension.« less
  • The tectonics of the Coso Range has been described as having arcuate and ring faults both suggesting the presence of a circumscribed subsidence bowl or calderalike feature. New information suggests the Coso Range is situated in an area of transition between the stress of the right slip San Andreas fault-plate interaction and the extensional tectonics of the Basin and Range. Arcuate faults in the Coso Range are interpreted to have been produced by the regional stress field rather than to have been of volcanogenic origin. Focal mechanisms of small-magnitude earthquakes support the stress directions indicated by local fault patterns. Fumerolesmore » in the area are primarily associated with oblique slip faults rather than with arcuate or ring faults. The geothermal reservoir is therefore much different from that of a caldera or subsidence bowl, and the overall geothermal potential is probably less than earlier estimates.« less
  • The effect of an underlying magma reservoir cannot be identified within the complex gravity pattern in the Coso Range, California. Rather, linear gravity contours, which suggest a regional tectonic origin, enclose the location of most of the volcanic activity of the Coso Range. Faults along the edges of northwest trending, magnetic blocks probably provided paths of minimum resistance to the ascending viscous magma that was extruded as rhyolite domes. Dense, magnetic rocks associated with a complex mafic pluton 9 km in diameter form a relatively impermeable north border of the Pleistocene volcanic field. A heat flow high nearly coincides withmore » the west half of a 6-km-diameter magnetic low. A 2-km-diameter outcrop of a pre-Cenozoic silicic pluton, which has low magnetization compared to the surrounding metamorphic rocks, presumably typifies the rocks that underlie the magnetic low and heat flow high. Hydrothermal fluids may have destroyed some magnetite in the more magnetic wall rock, further reducing the magnetic intensity.« less
  • Thirty-eight separate domes and flows of phenocryst-poor, high-silica rhyolite of similar major element chemical composition were erupted over the past 1 m.y. from vents arranged in a crudely S-shaped array atop a granitic horst in the Coso Range, California. Most of the extrusions are probably less than about 0.3 m.y. old. The area is one of Quaternary basaltic volcanism and crustal extension. The central part of the rhyolite field is characterized by high heat flow, low apparent resistivity, and substantial fumarolic activity indicative of an active geothermal system. The immediate source of heat for the surficial geothermal phenomena is probablymore » a silicic magma reservoir that may still contain molten or partially molten material at a depth of at least 8 km beneath the central part of the rhyolite field. Outlying rhyolite extrusions probably reflect the presence of feeder dikes emanating from the reservoir beneath the central region. Azimuths of dikes appear to be parallel to the regional tectonic axis of maximum horizontal compression, analogous to some dike-fed flank eruptions on basaltic shields and andesitic strato-volcanoes. The areal extent of a magma reservoir and the present total heat content of the silicic magma system at Coso may be less than was previously estimated. However, the area is still considered to be one of significant geothermal potential.« less