1857 Tejon Quake Explained

David Lynch (top), an astrophysicist researching with the U.S. Geological Survey, shows how the tectonic plates of the San Andreas Fault slide along each other, then suddenly shiver into a massive earthquake. The San Andreas Fault shown in the map (bottom right) runs directly through Gorman, Frazier Park and Pine Mountain then heads across the Carrizo Plain (bottom left).

The Big One Was 'Our' Fault

Walk outside and take a long look at the San Emigdio Mountains that form the wall between our communities and the San Joaquin Valley. "Made in Mexico" is a stamp on a wide variety of items, but it comes as a surprise to most that San Emigdio Mountain actually formed near Tijuana, Mexico.

Astrophysicist and San Andreas fault researcher David Lynch says the thrusting 7,500 foot San Emigdio has been pushed all the way to its present location by the northwest motion of the Pacific tectonic plate as it moves against the southeast motion of the North American plate—traveling at about the same rate of speed as your fingernails grow. The visible fracture line between the two is the San Andreas Fault.

-Editor

By Sue Nicoll

On Tuesday, Sep. 25 at the Frazier Mountain Park Community Center, David K. Lynch, Ph.D. gave a presentation called "Field Guide to the San Andreas Fault," sponsored by the Ridge Route Communities Museum, in honor of the 150th anniversary of the 1857 Fort Tejon quake.

Lynch's informative and amusing talk, was peppered with maps and pictures that gave us a tour of the San Andreas Fault, which runs right under the Frazier Park post office and through the pond in Frazier Mountain Park. The sag ponds in Cuddy Valley are there because the fault runs through there. It continues directly through the community of Pine Mountain before heading out through the Carrizo Plain [see photo bottom right].

Fault lines are the dividing lines between tectonic plates. Most of these plate boundaries are in the ocean, but we are "lucky," Lynch said, that the San Andreas fault is "on dry land so we can study it." Most of the 700 miles of the San Andreas fault is obscured, buried underground and covered by trees and growth, but in the Carrizo Plain there is a long, visible scar of the fault's motion.

Lynch used props (pieces of bread and pieces of foam) to illustrate the motion of the fault, which moves about two inches per year (the same rate at which your fingernails grow).

If you place two pieces of bread on a table and slowly slide them past each other, this represents the way the two tectonic plates move. The Pacific Plate is moving northwest and the North American Plate is moving southeast. The fault is approximately 25 kilometers deep, going down to what is known as "plastic rock," the base of the plates sliding on top of the lava of the mantle.

We are fortunate here in Southern California that the plates slide past each other. We do not have what are called "subduction zones," where one plate is sliding underneath the other. These locations feature the biggest earthquakes in the world as well as volcanoes (as in the Cascade Mountains).

The 1857 Fort Tejon quake, 150 years ago, is estimated to have been 7.9 or 8.0 on the Richter Scale (which did not exist back then), and was centered just north of our Mountain Communities. Our San Emigdio mountains are one of the "transverse ranges," because they run east-west instead of north-south.

You can easily spot signs of fault activity in our area. On Peace Valley Road, by Frazier Mountain High School, there you can see "gouge," which is ground up rock. Watch for a cliff or pass where the rock is a different color on one side than on the other. This occurs because the rock was brought there by fault activity rather than having been formed there. According to Lynch, it is impossible for two different kinds of rock like that to form in the same place.

Lynch left a parting note: if you live on the "sunny side" in Frazier Park, you're on the North American Plate; those who live on the "shady side" are on the Pacific Plate.

This is part of the October 5, 2007 online edition of The Mountain Enterprise.

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