Geology And Geophysics

Understanding the San Andreas Fault



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To understand the San Andreas Fault, we must first understand what a fault is. No, it is not the same definition as the word 'blame'. A fault is a fracture or a zone of fractures between two blocks of rock. These fractures allow the blocks to move, either rapidly, in the form of an earthquake, or more slowly, in the form of a creep. A creep is a more or less continuous movement occurring on faults due to ongoing tectonic deformation. You can usually not feel the effects of a creep as you would an earthquake.

The San Andreas Fault itself stretches about 800 miles along southwestern California. It was first discovered in 1895 by Andrew Lawson, a geology professor at UC Berkley. It is estimated to be about 15-20 million years old. He named it after a small lake called the Laguna de San Andreas (The Lagoon of San Andreas) located in a linear valley formed by the fault just south of San Francisco.

The San Andreas Fault is separated into three sections. The first is the Mojave segment in the south. Geologists study this portion of the fault more than any other in the world because on top of it lies a cutout of the fault where the second largest city on the fault, Palmdale, is located. Also, the Antelope Valley Freeway runs through it, and from there you can see the deep layers of "shifted" crust. Another interesting fact about the Mojave is that the San Gabriel Mountains were created by the Fault.

The central portion of the San Andreas fault runs from Parkfield to Hollister in a northwestern direction. While the southern section of the fault and the parts through Parkfield experience earthquakes, the rest of the central section of the fault displays an aseismic creep.

The northern segment of the fault runs from Hollister, through the Santa Cruz Mountains. Here the epicenter of the 1989 Loma Prieta earthquake took place. From the mountains, it goes to the San Francisco Peninsula, where it was first identified by Lawson. It then travels offshore at Pacifica at Mussel Rock, where the approximate epicenter of the 1906 earthquake took place. Eventually, it runs underwater along the coast until it reaches Cape Mendocino, where it begins to bend to the west, ending at a triple junction with the Mendocino Fracture Zone and the Cascadia subduction zone.

According to the USGS (United States Geological Survey), the largest historical earthquakes that occurred along the San Andreas fault were those in 1857 and 1906. The San Francisco earthquake and fire of April 18, 1906, took about 700 lives and caused millions of dollars worth of damage in California from Eureka southward to Salinas and beyond. The earthquake was felt as far away as Oregon and central Nevada. The 1906 earthquake, which has been estimated at a magnitude 8.3 on the Richter Scale, caused intensities as high as XI on the Modified Mercalli Scale.

Unfortunately California has probably not seen the end of devastating earthquakes. Geologists have noted that over the past 1,500 or so years, large earthquakes have occurred about every 150-years on the southern San Andreas fault. The last large earthquake happened in 1857 on the southern portion of the San Andreas Fault and we can expect another within the next few decades. However, with all the technology and preparation that California has done, hopefully if another earthquake does hit, it will not have the same toll as others in the past.

Works Cited:
Schulz, Sandra S. and Wallace, Robert E. "The San Andreas Fault". http://pubs.usgs.gov/gip/earthq3/safaultgip.html. USGS. Last modified 06-24-97. 3/25/07.


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