San Andreas Transform Fault Zone

By Irene K. Nester

April 25, 2008

ES 767 Global Tectonics, Spring 2008

Table of ContentsIntroduction
Evolution of the San Andreas Fault Zone
Characteristics of the San Andreas Fault Zone
Image Gallery

Figure Courtesy of USGS

The San Andreas fault zone is a transform boundary between two tectonic plates, the North America plate and the Pacific plate. The northwest-southeast trending fault zone extends from the East Pacific rise in the Gulf of California (between Baja California and the Mexican mainland) to the Mendocino fracture zone offshore of northern California. The Mendocino fracture zone is an approximately east-west trending fracture zone and transform boundary between the Pacific plate to the south and the Gorda plate to the north. Its eastern end meets the San Andreas fault and the Cascadia subduction zone at the Mendocino triple junction adjacent to the coast of California. The current relationship of the San Andreas fault zone to the tectonic plates and mid-ocean ridges in the vicinity of the fault is illustrated on the figure (at left) from the USGS web site, Geologic History of the San Andreas Fault System.

The San Andreas fault is one of only a few examples of a transform boundary between two tectonic plates that is exposed on a continent. Some other continental transforms are the Alpine fault in New Zealand and the Dead Sea Transform fault system in the Middle East. The San Andreas fault zone includes the main fault trace and many other major and minor fault strands. The width of the fault zone varies from a few meters to a few kilometers. The relative rate of motion between the North American plate and the Pacific plate is approximately 3.5 to 4.6 cm per year, most of which (2.0 to 3.5 cm per year) is accounted for by horizontal displacement along the San Andreas fault zone. The remainder is expressed by displacement along other, subparallel faults such as the Imperial and the San Jacinto fault zones in southern California.

Figure Courtesy of USGS
Evolution of the San Andreas Fault Zone

Prior to 30 million years ago (Ma), the eastern margin of the westward moving Farallon tectonic plate met the western margin of the North American plate at a convergent plate boundary. The oceanic Farallon plate was subducted beneath the continental North American plate at this boundary. At the same time, the opposite margin of the Farallon plate was diverging from the Pacific plate, located to the west, along a mid-ocean ridge. The rate of convergence at the subduction zone was greater than the rate of spreading along the ridge, which caused the ridge to move toward the subduction zone.

Approximately 30 Ma, during the middle Oligocene, the mid-ocean ridge between the Farallon plate and the Pacific plate began to be subducted beneath the western margin of the North American plate along the north-south trending Cordilleran subduction zone. As a consequence, the Farallon plate was split into two smaller plates. The northern plate is termed the Juan de Fuca plate and the southern plate is the Cocos plate. Two triple plate junctions formed and separated from each other along the subduction zone as the Pacific, Juan de Fuca, and Cocos plates continued to converge with the North American plate. The Mendocino triple junction is the northern junction (Juan de Fuca, Pacific, and North American plates) and the southern junction is the Rivera triple junction (Cocos, Pacific, and North American plates).

As the North American plate encountered the Pacific plate the relative motion between them caused the convergent boundary along this portion of the margin of North America to evolve into a transform boundary. The Pacific plate is moving to the northwest relative to the North American plate and the Juan de Fuca and Cocos plates are moving to the southeast as they separate from the Pacific plate along mid-ocean ridges. This relative motion is expressed along the San Andreas transform fault as right-lateral strike-slip offset. The total offset between the Pacific and North American plates since the fault formed is approximately 470 km, but the amount of offset varies along the different segments of the San Andreas and multiple subparallel faults that also accommodate the relative plate motion.

The sequence of events beginning approximately 30 Ma is presented graphically on two diagrams from the USGS, the Evolution of the San Andreas Fault (at left) and the San Andreas Fault Sequential Diagram.

Back to Top of Page

Characteristics of the San Andreas Fault Zone

The surface expression of the San Andreas transform fault zone is a right-lateral strike-slip fault. The fault zone is approximately 1300 km long and is made up of three major segments, each with characteristic types, magnitudes and frequencies of earthquake activity. These differences are primarily related to differing geometries between the fault trace and plate motions, and the physical properties of the rock formations that the fault passes through.

The southern segment (also called the Mojave segment) of the fault lies between its southern end at the East Pacific Rise in the Gulf of California and Parkfield, California. There is a significant northward bend in the fault trace along this segment in the vicinity of Frazier Park. Compression along the fault zone at this bend has resulted in the formation of a group of mountain ranges called the Transverse Ranges. Infrequent but relatively large earthquakes are characteristic of this segment. The average recurrence interval for earthquakes on the southern segment is approximately 140 to 160 years. The last earthquake that occurred was in 1857 near Fort Tejon.

The central segment of the San Andreas fault zone is about 140 km long and lies between Parkfield and Hollister, California. Relatively frequent moderate earthquakes occur in the Parkfield area, e.g., earthquakes with magnitudes of approximately 6 occurred near Parkfield in 1857, 1881, 1901, 1922, 1934 and 1966. The average recurrence interval on the fault near Parkfield is about 20 years. To the northwest, between Parkfield and Hollister, the fault moves steadily by aseismic creep accompanied by many small earthquakes with magnitudes less than 5. This behavior is thought to be at least partially due to low friction serpentinite rocks present in the fault zone.

The northern segment extends from Hollister northwest to the triple junction with the Mendocino fracture zone and the Cascadia subduction zone. The northern segment passes through the Santa Cruz Mountains, where the 1989 Loma Prieta earthquake was centered, and the San Francisco Peninsula. The devastating 1906 San Francisco earthquake occurred on this segment of the fault. The northern segment experiences infrequent large earthquakes with an average recurrence interval of approximately 240 years.

The following table presents the date, location, magnitude and other physical characteristics of historical earthquakes that have occurred along the fault zone.

Major Historical Earthquakes on the San Andreas Fault
(Nearest City/Town)
December 1812WrightwoodSouthernNot Available7.5 (estimated)Not Available12 - 170
January 1857Fort TejonSouthernNot Available7.9 (approximate)4.5 (average)
9 (maximum)
300 - 400
April 1906San FranciscoNorthernNot Available7.83 - 4.5 (average)
6.4 (maximum)
June 1966ParkfieldCentral/Southern8.66.00.10 - 0.5027
October 1989Loma PrietaNorthern186.9235
September 2004ParkfieldCentral/Southern7.96.00.58 (maximum)25

Data compiled from multiple sources, primarily the SCEC web site.

Back to Top of Page

Image Gallery: Surface Features of the San Andreas Fault Zone

Barrows, A.G. 1987. Roadcut Exposure of the San Andreas Fault Zone along the Antelope Valley Freeway near Palmdale, California. Centennial Field Guide Volume 1: Cordilleran Section of the Geological Society of America: Vol. 1, No. 0 pp. 211-212.

Dreger, D. and Kim A. 2006. Rupture Process of the 2004 Parkfield Earthquake Utilizing Near-Fault Seismic Records and Implications for ShakeMap. SMIP06 Seminar on Utilization of Strong-Motion Data, p. 1-18. California Earthquake Map Collection. Isoseismal Maps for Selected California Earthquakes. World Wide Web page <> [accessed in April 2008].

Goldfinger, C., Grijalva, K., Burgmann, R., Morey, A. E., Johnson, J. E., Nelson, H. C., Gutierrez-Pastor, J., Ericsson, A., Karabanov, E., Chaytor, J. D., Patton, J. and Gracia, E. 2008. Late Holocene Rupture of the Northern San Andreas Fault and Possible Stress Linkage to the Cascadia Subduction Zone. Bulletin of the Seismological Society of America, Vol. 98, No. 2. pp. 861-889.

Jacoby, G. C. Jr., Sheppard, P. R. and Sieh, K. E. 1988. Irregular Recurrence of Large Earthquakes Along the San Andreas Fault: Evidence from Trees (Abstract). Science, Vol. 241. no. 4862, pp. 196-199.

NASA Visible Earth. A Catalog of NASA Images and Animations of our Home Planet. World Wide Web page <> [accessed in April 2008].

Southern California Earthquake Data Center. World Wide Web homepage <> [accessed in April 2008].

United States Geological Survey (USGS). 1999. The October 17, 1989, Loma Prieta, California, Earthquake - Selected Photographs. World Wide Web page <> [accessed in April 2008].

USGS Earthquake Hazards Program - Northern California. 2002. Virtual Tour of the San Andreas Fault at the Carrizo Plain using Kite Aerial Photography. World Wide Web page <> [accessed in April 2008].

USGS Western Earth Surface Processes Team. 2006. Geologic History of the San Andreas Fault System. World Wide Web page <> [accessed in April 2008].

USGS Earthquake Hazards Program. 2008. Earthquake Summary Poster of the Parkfield, California Earthquake of 28 September 2004. World Wide Web page <> [accessed in April 2008].

Werner, C. L., Rosen, P., Hensley, S., Fielding, E. and Buckley, S. 1997. Detection of Aseismic Creep Along the San Andreas Fault Near Parkfield, California with ERS-1 Radar Interferometry. European Space Agency Earthnet Online, World Wide Web page <> [accessed in April 2008].
Back to Top of Page

©2008 by Irene Nester. All rights reserved.
Site Design by Irene Nester.