The San Andreas Fault is the sliding boundary between the Pacific Plate
and the North American Plate. It slices California in two from Cape
Mendocino to the Mexican border. San Diego, Los Angeles and Big Sur are
on the Pacific Plate. San Francisco, Sacramento and the Sierra Nevada
are on the North American Plate. And despite San Francisco’s legendary
1906 earthquake, the San Andreas Fault does not go through the city.
But communities like Desert Hot Springs, San Bernardino, Wrightwood,
Palmdale, Gorman, Frazier Park, Daly City. Point Reyes Station and
Bodega Bay lie squarely on the fault and are sitting ducks.
The San Andreas Fault is a transform fault. Imagine placing two slices
of pizza on the table and sliding them past one another where they
touch along a common straight edge. Bits of pepperoni from one side
crumble across the boundary onto the anchovy side. The same thing
happens with the fault, and the geology and landforms along the mighty
rift are extremely complicated.
The plates are slowly moving past one another at a couple of inches a
year - about the same rate that your fingernails grow. But this is not
a steady motion, it is the average motion. For years the plates will be
locked with no movement at all as they push against one another.
Suddenly the built-up strain breaks the rock along the fault and the
plates slip a few feet all at once. The breaking rock sends out waves
in all directions and it is the waves that we feel as earthquakes.
In many places like the Carrizo Plain (San Luis Obispo County) and the
Olema Trough (Marin County), the fault is easy to see as a series of
scarps and pressure ridges. In other places, it is more subtle because
the fault hasn’t moved in many years and is covered with alluvium, or
overgrown with brush. In San Bernardino and Los Angeles Counties, many
of the roads along the fault cut through great mountains of gouge, the
powdery, crumbled rock that has been pulverized by the moving plates.
Aerial photo of the San Andreas Fault showing drainage that is offset by movement of the fault.
Photograph copyright by David Lynch
Photo of the San Andreas Fault near Gorman California, showing rocks of the Pacific Plate on
the left and the North American Plate on the right. Photograph copyright by David Lynch.
The hallmark of the San Andreas Fault is the different rocks on either
side of it. Being about 28 million years old, rock from great distances
have been juxtaposed against rocks from very different locations and
origins. The Salinian block of granite in central and northern
California originated in Southern California, and some even say
northern Mexico. Pinnacles National Monument in Monterey County
is only half of a volcanic complex, the other part being 200 miles
southeast in Los Angeles County and is known as the Neenach Volcanics.
There are many myths and legends about the San Andreas Fault, the
biggest being that it will one day crack and California will slide into
the sea. WRONG! It won’t happen and it can’t happen. Nor is there any
thing such as “earthquake weather” or preferred times of day when
earthquakes hit.
The San Andreas Fault is more accessible than any other fault in the
world. With California’s large population and temperate climate, there
are many roads that snake along the fault. They are uncrowded and
peaceful, perfect for family outings. There is abundant camping, bird
watching, wild flowers and wildlife, rock collecting and natural beauty
along the way. State and National parks are strung along the fault like
beads on a string. All it takes is a good map, a comfortable car and a
desire to see the world’s most famous fault.
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David K. Lynch, PhD, is an astronomer and planetary scientist living in
Topanga, CA. When not hanging around the fault or using the large
telescopes on Mauna Kea, he plays fiddle, collects rattlesnakes, gives
public lectures on rainbows and writes books (Color and Light in
Nature, Cambridge University Press) and essays. Dr. Lynch's latest book
is the Field Guide to the San Andreas Fault. The book contains twelve
one-day driving trips along different parts of the fault, and includes
mile-by-mile road logs and GPS coordinates for hundreds of fault
features. As it happens, Dave's house was destroyed in 1994 by the
magnitude 6.7 Northridge earthquake.
