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Southern California Geology


Southern California's major faults

trace of a fault on the desert floor

Selected Bibliographic References


General statement

Examine the history of Southern California's major fault zones:

Faults of the Inland Empire Region
Faults of the Mojave Desert Region
Eastern California Shear Zone
Garlock Fault
Faults of the Eastern Transverse Ranges
Faults of the Peninsular Ranges Province
Contractional Faults of the Transverse Ranges
Faults of the Los Angeles Basin Region
Faults of the San Diego-Oceanside Region
Faults of the Offshore Continental Borderland

General statement

As you can see by examining a fault map of southern California or a satellite image showing superimposed fault lines, southern California is traversed by numerous faults. Some of these, like the San Andreas Fault, are master players in the southern California fault mosaic; others are minor and not so well known.

satellite image of southern California Click to enlarge satellite image of southern California showing superposed fault lines (image sources: (1) faults from Jennings, 1994; (2) Landsat image from Jet Propulsion Laboratory of the California Institute of Technology). A Southern California Earthquake Center link contains the original image
Jennings, C.W., 1994, Fault activity map of California and adjacent areas: California Division of Mines and Geology, California Geologic Data Map Series, Map No. 6, scale 1:750,000.
San Andreas Fault Click to enlarge a diagram of southern California showing the San Andreas Fault as a master player in a tectonic setting that includes other faults and compressional fold belts (image source: Southern California Earthquake Center)

Many of these faults are capable of generating earthquakes-some small and local, others major and capable of impacting human activities throughout southern California. In order to understand how and where earthquakes occur and to prepare for their effects, geologists and seismologists need to address several issues:


  • geologic relations among the various faults (do their geographic distributions and geologic characteristics mean anything in terms of earthquake potential?)
  • the long-term geologic history of these faults (what is the earthquake history of each fault [paleo-earthquake history] and how have they interacted with each other over time?)
  • the role each fault plays in the strain budget of southern California (how are the minute stress and strain forces that result from southern California's location at an active plate margin distributed among the various faults and folds, and have there been any changes in this stress strain budget throughout the last several hundred thousand years?)

A fault is "a fracture or zone of fractures [in the earth's crust] along which there has been [movement] of the sides relative to one another" (modified from the Glossary of Geology, Bates and Jackson, 1987, p. 223). A fault forms where rock of the earth's crust literally is broken as the result of accumulated stresses operating on the crust. The resulting fault plane will have a dip direction and dip amount, and its intersection with the ground surface will define a trace that will have a compass trend (azimuth).

Faults extend from the ground surface downward into the earth's crust; this downward dimension is called a fault's "subsurface" extent. Faults usually extend for several kilometers into the subsurface, until the brittle conditions that allow for rock-breakage give way to plastic (ductile) conditions that lead to rock- and crystal-flowage. The photograph below shows a profile (cross-sectional) view of a fault in a Guatemala roadcut. Were it not for this roadcut, geologists would have no direct information about the fault-plane dip or about the amount of movement (displacement).

Faulted sedimentary rock layers Faulted sedimentary rock layers exposed in a roadcut in Guatemala. The sedimentary rocks originally were deposited as horizontal layers of sand and gravel in streams and debris flows. Photo copyright J.K. Nakata, used with permission. Click here for more info and enlarged image .

Faults not only extend downward into the earth's crust, they also can be traced laterally across the landscape. The fault plane in the preceding photograph could be traced to the top of the roadcut, and if conditions permitted, its trace on the ground surface could be identified and mapped away from the roadcut. In the desert regions of southern California it is easy to recognize the geographic extent of faults that form clear traces across the desert landscape. Such is the case for the strand of the San Andreas Fault zone pictured in the next photograph.

trace of a fault on the desert floor Trace of a fault on the desert floor of southern California; view looking east at the Coachella Valley trace of the Banning Fault in the northern Coachella Valley. Photo by J.C. Matti, USGS. Click here for more info and enlarged image.

If you are not familiar with geologic faults and their relationship to earthquakes, navigate to the following sites for a review of fault terminology, structure, and geometry:

  • An online description of California faults published originally in the California Geological Survey journal "California Geology" (Pridmore, 1992)
  • The Geology Basics Glossary discussion of faults and earthquakes (maintained by the National Park Service and the USGS)
  • A Southern California Earthquake Center (SCEC) site on the basics of faults and earthquakes

How are strain budgets determined for southern California?


  • paleoseismological studies that attempt to identify evidence for the timing and character of earthquakes over the last few thousand years by examining the detailed geologic record of trench excavations (Sieh, 1978, 1984; Fumal and others, 1993; Seitz and Weldon, 1994; McGill and others, 1998a, b). Studies like these lead to estimates of fault-slip rate and faulting recurrence, which in turn lead to probabilistic estimates of seismic risk
  • regional geologic-mapping investigations like those of the Southern California Areal Mapping Project (SCAMP) that attempt to identify long-term movement histories for various fault zones (Crowell, 1962, 1981; Ehlig, 1981; Powell and Weldon, 1992; Powell, 1993; Matti and Morton, 1993; Weldon and others, 1993; Dillon and Ehlig, 1993; Dickinson, 1997). These investigations map out the distribution of rock masses that have been displaced by the faults, and work out the timing of movements among faults that may have been active for a time and then abandoned as the strain distribution in southern California shifted with time to other faults.

From these kinds of studies, a picture is emerging of how faults in southern California interact with each other. In this scenario, the master San Andreas Fault and its relatives are all components of a widespread fault-and-fold system that has developed in southern California in response to plate tectonic interactions along the western margin of North America.

plate-tectonic setting of southern California Click on this thumbnail to see a diagram illustrating the plate-tectonic setting of southern California (image source: USGS general-interest publication "This dynamic earth: the story of plate tectonics" (Kious and Tilling, 1996). The image was downloaded from the online version of the publication)

Many kinds of geologic structures occur within the San Andreas System, including:


  • Right-lateral strike-slip faults develop where blocks of Earth's crust are sliding past each other in a right-handed sense (as the observer looks across the fault)
  • Left-lateral strike-slip faults develop where blocks of Earth's crust are sliding past each other in a left-handed sense (as the observer looks across the fault)
  • Normal dip-slip faults develop where the Earth's crust is pulling apart (extending) and crustal blocks drop and rise vertically past each other
  • Reverse dip-slip faults develop where the Earth's crust is being squeezed together (contracting) and crustal blocks ramp upward over adjacent blocks
  • Fold belts develop where the crust is being squeezed and warped rather than broken by faults, thereby yielding folds in certain kinds of rock

deformed sedimentary rock layers Severely deformed sedimentary rock layers exposed in a roadcut of the Antelope Valley Freeway within the San Andreas Fault zone near Palmdale, California. Photo copyright J.K. Nakata, used with permission. Click here for more info and enlarged image .
Aerial photograph of folded sedimentary rock Click to enlarge aerial photograph of folded sedimentary rock layers in the foothills of the Himalaya Mountains. Fold belts like this occur in southern California, although not as geographically extensive (image source: University of British Columbia Keck Geology Consortium Structural Geology Slide Set , photograph © Kevin Pogue)

The San Andreas system in southern California thus should be viewed as an integrated system of right-slip, left-slip, contractional, and extensional fault zones and associated fold belts. Long-term movements on these geologic structures have had a profound impact on southern California's landscape and geologic setting over the last 5 million years or more.

Selected Bibliographic References

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