Download San Andreas Fault

The San Andreas Fault
The San Andreas Fault is the most famous fault in the world. This is partly because of the
disastrous 1906 San Francisco earthquake, but rather more importantly because it passes through
California, a highly-populated state that is
frequently in the news. And with many
research institutions dedicated to studying
such an accessible fault, the SAF has become
a household name.
The San Andreas Fault is a place
where two tectonic plates touch, the North
American and Pacific Plates. The plates are
rigid (stiff) slabs of rock that comprise the
crust and upper mantle of the Earth. The SAF
is about 800 miles long, and roughly ten
miles deep.
the ground and shake the surface. We know this shaking
as earthquakes. While we think of plates as rigid, they can
stretch a little, like pizza crust. That is why we can have an
earthquake on the SAF in northern California but not on
the SAF in southern California.
People often think of the SAF as a line in the
ground, and that by hopping across it, they can go from
one plate to the other. This is not true. Although the most
recent break in the ground that defines the trace of the
SAF is a narrow line, the actual plate boundary should be
viewed as a fault zone. The width of a fault zone can be
less than a mile wide, or many miles across.
California is famous for its earthquakes on the SAF
(1906 San Francisco Earthquake, 1857 Fort Tejon
earthquake), but the state's quakes are not particularly
large. The largest earthquakes in the world occur near
subduction zones, where one plate is sliding under the
other.
The plates are continually moving but
where the touch each other, they get stuck.
As the rest of the plates moves, the stuck
parts deform like compressing a spring so
they build up stress in the rocks along the
fault. When the rock breaks or slips, the
plates move, causing an earthquake. As they
break and scrape by one another, the plates
produce seismic waves that travel through
The SAF is about 28 million years old. It 'began' when the East Pacific Rise, the boundary between
the Farallon and Pacific Plates, subducted under the North American Plate near what is now Los Angeles.
Spreading northwest and southeast, The SAF gradually grew, and it still growing. The grinding plates and
earthquakes are gradually warping and reshaping California. In a few million years, California will look
very different than it does today. Consider this: California is about 1000 miles long. Consider the
following:
1000 miles x 5280 ft/mile x 12 inches/ft = 63,360,000 inches
At the measured slip rate of about 2.5 inches per year, it will take
63,360,000 inches/ 2.5 inches/year = 25.3 million years
It will take 25.3 million years for the state to be sheared and twisted beyond all recognition. With
similar arithmetic, we can see that 25 million years ago, California shared not the faintest resemblance
with the modern paradise we know today.