|San Andreas Fault
a fracture in the earth's crust marked
by a zone of disrupted land that extends more
than 750 miles from off the coast of northwestern
California to the southeastern part of the state
near the Mexican border
What Is It?
The Earth's crust is fractured
into a series of "plates" that have
been moving very slowly over the Earth's surface
for millions of years. The San Andreas Fault is
the main "boundary" between the Pacific
Plate (to the west of the fault) and the North
American Plate (to the east). Many smaller faults
branch from and join the San Andreas Fault. The
entire San Andreas fault system is more than 800
miles long and extends to depths of 10 miles or
more within the Earth.
Can You See It?
Over much of its length, a
linear trough reveals the presence of the San
Andreas Fault. From the air, the linear
arrangement of lakes, bays, and valleys in this
trough is easy to spot. On the ground, the fault
is marked by distinctive landforms that include
long straight escarpments and narrow ridges.
How Does It Move?
Blocks on opposite sides of the
fault move horizontally. This means that if you
stood on one side of the fault and looked across
it the block on the opposite side would appear to
be moving to your right (assuming you could
actually see the movement), regardless of which
side you are standing on. Geologists refer to
this type of fault movement as right-lateral
strike-slip. Movement along the entire fault
averages 2 to 2¼ inches yearly.
The movement of the plates
along the San Andreas Fault produces strain on
the rocks near the plate boundary. Along some
segments of the fault the strain is released by
frequent small earthquakes that do little damage.
But along certain other segements the plates are
"locked" in place, and the strain
builds up over many years. The built-up strain
occassionally produces a major earthquake. In
1857, a sudden movement along the San Andreas
Fault in the Transverse Ranges caused a severe
earthquake in southern California. In 1906,
movement along another fault segment resulted in
a major earthquake in San Francisco. During this
latter earthquake, roads, fences, and rows of
trees and bushes that crossed the fault were
offset several yards, and the road across the
head of Tomales Bay was offset almost 21 feet.
During the past 15 to 20
million years, coastal California, along with the
rest of the Pacific Plate, has moved about 190
miles in a northwesterly direction with respect
to North America. In other words, way back in
geological history, San Francisco would have been
about half the distance to Los Angeles than it is
now, and Los Angeles would have been south of San
United States Geological Survey pubs.usgs.gov/gip/earthq3/safaultgip.html
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