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Southern California Geology

 

Crustal Strain and Crustal Structure in Southern California

Crustal Strain Studies

Contrary to what intuition might lead you to conclude about the hard rock of the Earth's crust, the hard southern California substrate is continually responding to earth stresses. These stresses lead to crustal strain (deformation).

 

Stress -the force per unit area acting on any solid or on any surface within it (modified from the American Geological Institute Glossary of Geology, Bates and Jackson, 1987, p. 651). In southern California, stress is placed on geologic bodies (rock masses and the particles or crystals within them) by forces induced by the plate-tectonic setting of the region.

 

Strain- change in the shape or volume of a body as a result of stress; change in relative configuration of the particles of a substance. Synonym: deformation (modified from the American Geological Institute Glossary of Geology, Bates and Jackson, 1987, p. 647).

 

Deformation- (a) a general term for the process of folding, faulting, shearing, compression, or extension of rocks as a result of various Earth forces. (b) strain (from the American Geological Institute Glossary of Geology, Bates and Jackson, 1987, p. 647).

 

Bates, R.L., and Jackson, J.A., 1987, Glossary of geology, 3rd ed.: Alexandria, Virginia, American Geological Institute, 788 p.


Throughout southern California, we can see the results of stresses that have acted on geologic materials over long periods of time:

 

  • Faults- those we can observe, like the San Andreas, San Jacinto, Cucamonga, Hollywood, and San Gabriel faults, and those we cannot observe like the blind thrusts that have formed beneath the Los Angeles Basin.

  • Folds- regions of sedimentary rock where once-horizontal layers have been warped into upfolds (anticlines) and downfolds (synclines)

  • Mountain belts- The San Gabriel Mountains, San Bernardino Mountains, Santa Monica Mountains, ranges of the Santa Susanna and Santa Clara Valley areas, and other mountain belts in southern California owe their origin to long-term stresses that have led to uplift of the mountain blocks

  • Valleys and down-dropped blocks- some valleys like Yucaipa Valley, the Fontana-Upland-Ontario plain, the Temecula-Elsinore trough, and the Santa Clara Valley owe their origin ultimately to down-faulting or down-folding


As geologists work out the long-term history of how faults, folds, and mountain belts have evolved in southern California, a question to be asked is:

 

  • do the stresses that created these features exist today, and do they operate in the same directions and with the same magnitudes?

To answer these questions, geologists, seismologists, and geophysicists in the U.S. Geological Survey, Southern California Earthquake Center, and California Division of Mines and Geology have developed various techniques and programs for analyzing real-time crustal strain in southern California. The goal of these investigations is:

 

  • to measure miniscule strains in the southern California crust
  • to determine the direction and orientation of stresses that have induced these strains.

Web sites that discuss the rationale for crustal-strain measurements and techniques for making these measurements include:

 

(1) A USGS link that describes general aspects of crustal deformation (strain) studies , particularly their relevance to seismologic and earthquake-hazards issues

(2) A USGS link that describes crustal-deformation studies in southern California . This site describes the specific instrumental techniques employed, shows the distribution of instrument sites, and provides links to real-time strain data

(3) Various links that provide interactive maps and data on specific crustal-deformation studies in southern California, including:

 

(4) A link to the Southern California Integrate GPS Network , an array of Global Positioning System (GPS) stations distributed throughout southern California with emphasis on the greater Los Angeles metropolitan region;

(5) Two USGS links, one explaining how GPS is used in crustal-strain studies , the other explaining the role of GPS in earthquake hazards

(6) A USGS link that lists the various GPS networks in the United States, including a subset of networks in southern California. Southern California GPS networks include the following sites that have links to real-time GPS velocity data:

 


Selected References
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This site last updated September 3, 2004 (ps)

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