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The San Andreas Fault
Submitted by Tim Elam


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The San Andreas Fault is the most-studied fault in the world. From a strictly geologic point of view, understanding the San Andreas Fault is key to piecing together the geologic history of much of California. From an engineering and urban planning point of view, assessing risks and safety planning are critical to the well being of millions of Californians. Understanding near-surface conditions, ground motion, and earthquake frequency on the San Andreas and adjacent faults will lead to constructing the safest possible earthquake-resistant structures.

The San Andreas Fault was named by geologist A.C. Lawson in 1895 for San Andreas Lake, a "sag pond," on the fault trace about 20 miles south of San Francisco. San Andreas Lake is now occupied by one of two reservoirs that are major water storage areas for San Francisco.

In part because of California's burgeoning population, recent large earthquakes and historically major earthquakes, the San Andreas Fault has attracted significant research efforts in the past few decades. Results of some of these studies has led to there being generally accepted basics regarding the San Andreas Fault:

  • It is the surface manifestation of the boundary between two tectonic plates of the Earth's crust: the Pacific Plate and the North American Plate; in a historical sense, it is more of a zone of faulting rather than a single trace.
  • It is 700-750 miles long, extending from Imperial County in southern California northwest to an area of the Pacific Ocean west of the Mendocino County coastline.
  • It is a right-lateral, predominantly strike-slip fault, along which at least 190 miles of right-lateral motion has occurred in the last 15 million years.
  • The surface expression of the fault has a variety of geomorphic forms, including shutter ridges, sag ponds, gaps, saddles, scarps, offset streams, valleys, hills and mountains.
  • The fault can be subdivided into many segments; the boundaries of these segments are defined by the rate of creep or offset due to earthquakes.
  • The fault is poorly understood below the first few feet, but it is certainly complex. Most likely, near vertical slabs of rock grind past each other and converge at depth.
  • Earthquakes generated along the fault are defined as "shallow." Their epicenters lie only 4 to 12 miles beneath the Earth's surface.
  • Two major earthquakes have occurred along the San Andreas Fault in recorded history: the 1857 Fort Tejon earthquake, and the 1906 San Francisco earthquake.

Much of the study of the San Andreas Fault in recent years has been directed toward figuring out the recent history of earth movement along the fault. Numerous major earthquakes other than the 1857 and 1906 temblors can be documented with detailed examination of soil layers using radiometric dating. Defining the frequency and estimating the magnitude of these quakes can help scientists get the big picture for defining when the next earthquake may occur along any one segment of the fault.

It doesn't say "when the next major quake will occur," but rather "here is the approximate year or decade or century when a quake of a specific magnitude should be expected to occur." Moderate earthquakes have occurred along the Parkfield (central California) segment of the San Andreas every 22 years or so. However even the Parkfield segment is now about a decade "overdue" for an earthquake.

Much effort has also gone toward monitoring creep rates, and comparing those rates to overall displacement on the fault. Where creep rates are higher, the stress accumulation may be less than areas where the fault is locked, shows little or no creep, and thus may be subject to rare singular catastrophic movements. Also monitored are precursor events such as small earthquakes, water table changes, and gas emissions. Detection of these may give scientists and officials help in warning of imminent earthquake danger.
T.D. Elam 10/10/00

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Link to Sharktooth Hill located in Kern County, California
Link to The San Joaquin Valley Through Time - Submitted by Tim Elam
Link to The McKittrick Tar Seeps - Submitted by Tim Elam
Link to Mount St. Helens - 20 Years Later - Submitted by Tim Elam
Link to The San Andreas Fault
Link to Yosemite Valley