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Standing on the Fault that
is Splitting California
The San Andreas Fault,
an Active Part of the Pacific Ring of Fire
In June 2008 at
Gualala. Northern
California with Bridie
and my sister Sr,
Antonia I spent a
weekend as guests of
Miguel Torrado, who
proudly showed us
his giant redwoods
and his own private
section of the famous
San Andreas Fault.
This was a surprise to
me as I had always
associated the fault
with the southern and
San Francisco areas
of California. From the
air, the track of the
fault could be seen as
a treeless division in
the generally wooded
countryside, travelling
northwards and
turning west into the
Pacific Ocean, a few
miles north of the
small town of Gualala.
By Tony Killian
Figure 1. The tectonic plates of the world as mapped
in the second half of the 20th century
Plate tectonics
Plate tectonics is a scientific
theory which describes
the large scale motions of
the lithosphere, which is
broken up into what are
called tectonic plates. In the
case of the Earth, there are
currently seven to eight major
and many minor plates. The
lithospheric plates ride on
the fluid-like (visco-elastic
solid) asthenosphere.
These plates move in relation
to one another with motions
ranging up to a typical
10–40 mm/.a. The location
where two plates meet is
called a plate boundary,
and plate boundaries are
commonly associated with
geological events such as
earthquakes and the creation
of topographic features such
as mountains, volcanoes, midocean ridges, and oceanic
trenches. The majority of
the world's active volcanoes
occur along plate boundaries,
with the Pacific Plate's Ring
of Fire being most active
and most widely known.
Tectonic plate
interaction
Basically, three types of plate
boundaries exist characterised
by the way the plates move
relative to each other:
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(Conservative) occur where
plates slide or, perhaps more
accurately, grind past each
other along transform faults;
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(Constructive) occur where
two plates slide apart
from each other; and
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margins) occur where two
plates slide towards each
other commonly forming
either a subduction zone, if
one plate moves underneath
the other, or a continental
collision, which often results
in the raisingof earth levels.
to the North American Plate
(on the east). The boundary
between them is the San
Andreas Fault, the "master"
fault of an intricate fault network
that cuts through rocks of the
California coastal region, and
is an example of a transform
boundary exhibiting dextral
motion. The entire San Andreas
Fault system is more than
800 miles long and extends
to depths of at least 10 miles
within the Earth. In detail, the
fault is a complex zone of
crushed and broken rock from a
few hundred feet to a mile wide.
Tectonic Plates.
Two of these moving plates
meet in western California;
The Pacific Plate (on the west)
moves northwestward relative
Figure 2 –Regional Schematic
of the San Andreas Fault
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Southern segment
Myths and Facts
The southern segment of the
fault begins on the Eastern
edge of the Salton Sea about
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and runs northwestward
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Hot Springs, skirting to the
east of Los Angeles through
Palmdale before it begins
a slow bend to the west.
After crossing through
Frazier Park, the fault begins
to bend northwestward.
Northwest of Frazier Park,
the fault runs through the
Carrizo Plain, a long, treeless
plain, where the track of
the fault is most visible.
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! It won’t happen and it
can’t happen. The San Andreas
is a continental transform fault,
which forms a sliding tectonic
boundary between the Pacific
Plate and the North American
Plate, The Pacific Plate, on
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and the Big Sur stand is slowly
moving northwards and the
North American Plate moving
Southwards, carrying San
Francisco, Sacramento and
the Sierra Nevada’s with it. 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 shock waves in all
directions and it is the waves
that we feel as earthquakes.
Central segment
The central segment of the
San Andreas Fault runs in
a northwestern direction
from Parkfield to Hollister,
which is some 75 miles
southwest of San Francisco.
The creeping
segment
Photo 1: Image of the
San Andreas Fault
Northern segment
The northern segment of the
fault runs from Hollister, up
the San Francisco Peninsula,
then offshore at Pacifica
at Mussel Rock... The fault
returns onshore at Bolinas
Lagoon just north of Stinson
Beach in Marin County.
The fault runs on and off the
coast to Fort Ross, continuing
overland to pass to the west
of Santa Rosa forming part
of the linear valley through
which the Gualala River flows.
It goes offshore at Point Arena
just north of Gualala, running
underwater parallel to the
coast, coming ashore for a
short distance near Cape
Mendocino, where it begins
to bend offshore to the west,
leaving California entirely.
64
The creeping segment
of the San Andreas Fault
extends from San Juan
Bautista, near Monterey, to
the short Parkfield segment
deep in the Coast Ranges.
While elsewhere the fault is
locked and moves in major
earthquakes, here there is
constant steady movement of
about 3 centimetres per year
and only minor quakes. This
kind of fault motion is called a
seismic creep, and it is rather
rare around the world. Yet this
segment, the related Calaveras
fault and its neighbour, the
Hayward fault, all exhibit creep,
which slowly bends roadways
and pulls buildings apart.
looked across it, the block
on the opposite side would
appear to have moved to
the right. Geologists refer to
this type fault displacement
as right-lateral strike-slip.
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in the San Francisco region,
roads, fences, and rows of
trees and bushes that crossed
the fault were offset by
several yards, and the road
across the head of Tomales
Bay was offset almost 21
feet, the maximum offset
recorded. In each case, the
ground west of the fault
moved relatively northward.
Sudden offset that initiates
a great earthquake occurs
on only one section of the
fault at a time. Total offset
accumulates through time in
an uneven fashion, primarily
by movement on first one,
and then another section of
the fault. The sections that
produce great earthquakes
remain "locked" and quiet over
a hundred or more years while
strain builds up; then, in great
lurches, the strain is released,
producing great earthquakes.
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.
Geologists believe that
the total accumulated
Although it is difficult to
imagine this great amount
of shifting of the Earth's
crust, the rate represented
by these ancient offsets
is consistent with the rate
measured in historical time.
Assuming the plate
boundary does not change
as hypothesized, projected
motion indicates that the
landmass west of the San
Andreas Fault, including Los
Angeles, will eventually slide
past San Francisco then
continue northwestward
toward the Aleutian Trench,
over a period of perhaps
twenty million years.
Earthquakes
Earthquakes are a fact
of life in California with
tremors recorded daily
throughout the state.
Although it is certain that
many major earthquakes have
occurred over the centuries
in the vicinity of the fault, the
earliest earthquake recorded
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Further quakes
were recorded in
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Listed here are a few of
the largest events recorded
in the last century:
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Earthquake 1906
Location: 2 miles offshore
from San Francisco
What Kind of
Permanent
Displacement
has occurred
along the Fault?
Blocks on opposite sides of
the San Andreas Fault move
horizontally. If a person stood
on one side of the fault and
displacement from
earthquakes and creep is
at least 350 miles along the
San Andreas fault since it
came into being about 15-20
million years ago. Studies
of a segment of the fault
between Tejon Pass and
the Salton Sea revealed
geologically similar terrains
on opposite sides of the fault
now separated by 150 miles,
and some crustal blocks may
have moved through more
than 20 degrees of latitude.
Magnitude: 7.8
Figure 3 –Displacement of
the San Andreas Fault
The Great San Francisco
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Francisco and the coast of
northern California. It ruptured
along the San Andreas Fault
both northward and southward
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San Francisco Earthquake 1906
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to Los Angeles, and inland
as far as central Nevada. The
earthquake and resulting fire
would be remembered as one
of the worst natural disasters
in the history of the United
States. The death toll has been
revised to today's conservative
estimate of at least 3000;
some estimates have put it
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225,000 and 300,000 people
were left homeless out of a
population of about 400,000.
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destroyed by the earthquake
and fire. It is thought that
more damage was done by
the resulting fire that occurred
after the earthquake than
by the earthquake itself.
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Earthquake 1952
Location: 23 miles
south of Bakersfield
Magnitude: 7.5
The largest earthquake in
southern California since
the Fort Tejon earthquake
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Valley earthquake of 1872,
the Kern County earthquake
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and widespread damage.
It was felt in Reno, Nevada,
and required a construction
effort in Las Vegas to
realign structural steel.
This quake and its aftershocks
(at least 20 were of magnitude
5.0 or greater) were
responsible for damaging
hundreds of buildings in the
Kern County area, at least 100
of which had to be torn down.
It devastated a section of
the Southern Pacific Railroad
line near Bear Mountain.
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Earthquake 1971
Location:¡h
N, 118° 24.04' W
Magnitude:
Also known as the Sylmar
Earthquake, this earthquake
occurred on the San Fernando
fault zone, a zone of thrust
faulting which broke the
surface in the Sylmar-San
Fernando Area. The total
surface rupture was roughly
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maximum slip was up
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earthquake caused over
$500 million in property
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Earthquake 1989
Location: 10 miles
northeast of the city of
Santa Cruz, California
Magnitude: 7.1
The Loma Prieta earthquake
occurred in the greater
San Francisco Bay Area in
California. The earthquake
lasted for 15 seconds and
a portion of the Cypress
Street Viaduct, Interstate
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This was a major earthquake
which caused severe damage
as far as 70 miles away; most
notably in San Francisco,
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Peninsula, and in areas
closer to the epicenter in the
communities of Santa Cruz,
the Monterey Bay, Watsonville
and Los Gatos. There were at
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and 3,757 injuries as a result
of this earthquake. The quake
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billion in property damage,
the costliest natural disaster
in U.S. history at the time. It
was the largest earthquake
to occur on the San Andreas
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San Francisco earthquake.
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Earthquake 1994
Location: 20 miles westnorthwest of Los Angeles
Magnitude:
This was the first earthquake
to strike directly under an
urban area of the United
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Long Beach earthquake.
The earthquake occurred
on a blind thrust fault, and
produced the strongest ground
motions ever instrumentally
recorded in an urban setting
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was wide-spread, sections
of major freeways collapsed,
parking structures and office
buildings collapsed, and
numerous apartment buildings
suffered irreparable damage.
The next “Big One”
Along the Earth's plate
boundaries, such as the San
Andreas Fault, segments exist
where no large earthquakes
have occurred for long
intervals of time. Scientists
term these segments "seismic
gaps" and, in general, have
been successful in forecasting
the time when some of the
seismic gaps will produce
large earthquakes. Geological
studies show that over the
past 1,400 to 1,500 years
large earthquakes have
occurred at about 150-year
intervals on the southern San
Andreas Fault. As the last large
earthquake on the southern
San Andreas occurred in
1857, that section of the
fault is considered a likely
location for an earthquake
within the next few decades.
A study completed by the
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has demonstrated that the
Southern segment of the San
Andreas fault, which stretches
from Parkfield in Monterey
County, California all the way
down to the Salton Sea, is
now capable of a Richter
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scale 8.1 earthquake. This
segment (which, at its closest,
is 40 miles away from Los
Angeles) had been stressed to
a level sufficient for the next
“big one,” as it is commonly
called. Such an event would
be felt all throughout much of
Southern California, including
densely populated areas of
metropolitan Los Angeles,
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Ensenada and Tijuana and
could kill thousands of people
and cause hundreds of
millions of dollars in property
and economic damage. The
information available suggests
that the fault is ready for
the next big earthquake
but when the triggering will
happen no one can tell. It
could be tomorrow or it could
be 10 years from now.
The San Francisco Bay area
has a slightly lower potential
for a great earthquake, as
less than 100 years have
passed since the great
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moderate-sized, potentially
damaging earthquakes could
occur in this area at any time.
Warnings
A great earthquake very
possibly will not occur
unannounced. Such an
earthquake may be preceded
by an increase in seismicity
for several years, possibly
including several foreshocks
of about magnitude 5 along
the fault. Before the next large
earthquake, seismologists also
expect to record changes in
the Earth's surface, such as
a shortening of survey lines
across the fault, changes
in elevation, and effects
on strainmeters in wells.
A key area for research on
methods of earthquake
prediction is the section of
the San Andreas Fault near
Parkfield in central California,
where a moderate-size
earthquake has occurred
on the average of every
20-22 years for about the
last 100 years. Since the
last sizeable earthquake
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has a high probability for a
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to occur within the next
decade. The U.S. Geological
Survey has placed an array of
instruments in the Parkfield
area and is carefully studying
the data being collected;
attempting to learn what
changes might precede an
earthquake of about that size.
What Can Be Done
About the Faults
and Earthquakes?
Even though people cannot
stop earthquakes from
happening, they can learn
to live with the problems
caused by earthquakes.
Three major lines of defense
against earthquake hazards
are being developed.
Buildings in earthquake-prone
areas should be designed
and constructed to resist
earthquake shaking. Building
codes that require attention
to earthquake shaking have
been improving in recent
decades and constitute a
first line of defense. In some
cities, programs are underway
to strengthen or tear down
older buildings most likely to
collapse during earthquakes.
A second line of defense
involves the selective use
of land to minimize the
effects of hazardous ground.
High- occupancy or critical
structures, for example,
should not be placed astride
the San Andreas Fault or on
landslide-prone areas. The
third line of defense will be
the accurate prediction of
earthquakes. When such
prediction becomes possible,
it will permit timely evacuation
of the most hazardous
buildings. A major program
aimed at learning how to
predict earthquakes and to
assess and minimize their
hazards was initiated following
the Earthquake Hazards
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is being carried out by the
U.S. Geological Survey, other
Federal Agencies, universities,
and private groups.
References
Sandra S. Schulz and Robert
E. Wallace U.S. Geological
Survey Information Services
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of SanAndreasFault.org
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CARLOW
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