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Facts about the San Andreas Fault For Kids

The San Andreas Fault is a continental transform fault that extends roughly 1300 km (810 miles) through California. It forms the tectonic boundary between the Pacific Plate and the North American Plate, and its motion is right-lateral strike-slip. The fault divides into three segments, each with different characteristics and a different degree of earthquake risk, the most significant being the southern segment, which passes within about 35 miles of Los Angeles.

  • Following the 1906 San Francisco earthquake, Lawson concluded that the fault extended all the way into southern California.
  • A project called the San Andreas Fault Observatory at Depth (SAFOD) near Parkfield, Monterey County, is drilling into the fault to improve prediction and recording of future earthquakes.
  • The northern segment of the fault runs from Hollister, through the Santa Cruz Mountains, epicenter of the 1989 Loma Prieta earthquake, then up the San Francisco Peninsula, where it was first identified by professor Lawson in 1895, then offshore at Daly City near Mussel Rock.
  • From Fort Ross the northern segment continues overland, forming in part a linear valley through which the Gualala River flows. The fault then runs along the southern base of the San Bernardino Mountains, crosses through the Cajon Pass and continues northwest along the northern base of the San Gabriel Mountains.
  • The Southern segment, which stretches from Parkfield in Monterey County all the way down to the Salton Sea, is capable of an 8.1 magnitude earthquake.
  • A large earthquake on this Southern segment would kill thousands of people in Los Angeles, San Bernandino, Riverside, and surrounding areas, and cause hundreds of billions of dollars in damage.
  • The Pacific Plate, to the west of the fault, is moving in a northwest direction while the North American Plate to the east is moving toward the southwest, but relatively southeast under the influence of plate tectonics.
  • At this time, a spreading center between the Pacific Plate and the Farallon Plate was beginning to reach the subduction zone off the western coast of North America.
  • As the relative motion between the Pacific and North American Plates was different from the relative motion between the Farallon and North American Plates, the spreading ridge began to be “subducted” creating a new relative motion and a new style of deformation along the plate boundaries.
  • The fault was first identified in Northern California by UC Berkeley geology professor Andrew Lawson in 1895 and named by him after the Laguna de San Andreas, a small lake which lies in a linear valley formed by the fault just south of San Francisco.
  • Scientists believe quakes on the Cascadia subduction zone may have triggered most of the major quakes on the northern San Andreas within the past 3,000 years.
  • 1989 Loma Prieta earthquake: About 25 mi were ruptured near Santa Cruz, California, causing 63 deaths and moderate damage in certain vulnerable locations in the San Francisco Bay Area.