Why is the san andreas fault important
The San Andreas Fault - Full Documentary
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every year thousands of people gather
here to enjoy the sport of rodeo in San
Juan Bautista California
the grandstand on which they say is
located on one of the most active
geological phenomena known to man the
base of this Bank or scarf marks the
place were two huge portions of the
Earth's crust are colliding grinding
past each other with a force that
reduces Hard Rock to powder with a power
that can create mountains and trigger
violent tremors that are felt over
hundreds of miles of country sacks the
same portion sell at work 125 kilometers
north near the metropolitan area of San
Francisco in 1906 this city was
devastated by one of the greatest
earthquakes in California more than 700
people lost their life for modern San
Francisco this disaster had one positive
effect it prompted study into the
origins of earthquakes
soon after the 1906 earthquake it became
evident that a single geological feature
dominated California's long history of
seismic activity the feature was named
after the lake that it forms just south
of San Francisco
today there's known as the San Andreas
Fault
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in San Francisco urbanization has plowed
over the fault and nearly obliterated
its trace but the fault becomes clearly
visible in a radar image of the San
Francisco Bay Area mussel rock where the
fault dips into the sea San Andreas Lake
the central fault region where deep
straight valleys mark the false paths to
the naked eye the San Andreas Fault is
most evident on the Barrens flatland of
central California's Carrizo Thane
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here whether it's scarcely a trace of
civilization the fault is visible as a
thin well-defined line of defamation
tossed by an occasional dry stream gully
in some places the fault is traced by
the foot of a gently sloping hill or by
a shallow depression in the land here in
others it is more obscure its presence
marked only by a primitive Road
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more dominant features can be seen by
their south from the Carrizo Plain where
a few desolate farmhouses doubt the land
the vault appears as a thin valley that
cuts across the general slope of the
plain
where the terrain is steep the centuries
have etched their mark into the lab with
deep eroded channels that descend from
the vicinity of the fall five
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altogether the San Andreas Fault slices
through some 1,100 kilometers of
California country from Mexico north
through the San Francisco Bay Area and
our on into the sea in many places it is
not a single factor but a complex zone
of branches called the San Andreas Fault
system on the southern portion of the
fall system east of the Salton Sea rocks
have been squeezed and fractured by
pressure in the fall zone as evidence of
its powerful motion The Fault has left
slick highly polished rock surfaces
evidence of this sort is often found
where road construction has torn away
part of the Earth's surface near the
town of Gorman a road cut exposes a
massive grant granite is often used for
building normally it is a strong durable
Rock but in the fall film this granite
has been crushed almost to a powder the
softening is the result of millions of
years of grinding action farther south
in the fall film highway passes through
terrain that has been forced upward by
pressure around the fault where the road
intersects the fault zone rock on the
face of the road cut is contorted and
twisted by the compression of two huge
sections of earth squeezing together
changes like this take place very slowly
they awesome forces that come into play
a rarely experienced directly by man but
proof of motion along the fault is here
in the geologic record of the land
one of the most dramatic examples of
this proof can be found in the Carrizo
Plain where the dry gullies of drainage
creeks descend to the fault line
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these streams do not flow in a straight
line at the fault they veered sharply to
the north the offset of this street is
the result of motion along the fault
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this model shows how the offset was
produced land on the western side of the
fault has been moving slowly northward
with relation to the eastern side where
the stream crossed the fault trace this
horizontal motion diverted the flow
further motion completely interrupted
the streams fat
eventually the waters of the stream
eroded a new path into the land but
again the new stream was offset by
movement along the fault today
the stream is displaced about 20 metres
to the right where it crosses a fault
line this offset is probably the result
of several hundred years of fault motion
in geologic terms several hundred years
is not a very long time much greater
displacement involving much longer
periods of time have occurred along the
San Andreas Fault
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over millions of years this movement has
been sufficient to displace huge blocks
of earth in this creek bed two different
types of rock life side-by-side the rock
on the left is about three million years
old
the rock on the right a few thousand
years old the fault is evident between
the sessions where a soft pliable
substance called fault couch has been
formed by the Earth's relentless
squeezing and grinding action matching
of rocks on opposite sides of the fault
zone gives an indication of how much the
earth has moved and how long it is been
moving a sample of an unusual and easily
recognizable type of rock is selected
from the western side of the fault zone
in central California's Fraser mountains
in the desert of Southern California an
outcrop of identical rock is found in
the oil copia mountains on the eastern
side of the fault
this suggests that movement along the
problem had separated one massive rock
into two probably over a period of about
20 million years
the separation is shown by the points
marked a on this map the matching rocks
are about 300 kilometers apart
geologists have mapped many distinctive
rocks that seem to have been separated
by fault motion the oldest rocks are the
farthest apart the motion that occurs
along the fault takes two distinct forms
one is the very slow nearly continuous
movement that geologists call creep this
fence has been offset by creep the
motion is slow but persistent normally
creep can be observed only through its
effects in Hollister California the
Almaden winery sits directly atop an
active branch of the fault zone inside
one of the winery warehouses reinforced
concrete walls and floors are slowly
being pulled apart by Creek movement
along the fault this simple model shows
how the displacement takes place
the slow movement of Crete can be
measured here the earth is moving at an
average of about 13 millimeters a year
the other form of fault movement is
sudden and sometimes violent the
earthquake severe is Detroit and
destroying man-made objects of all
descriptions in California they often
originate along one of the branch
fractures that make up the San Andreas
Fault system the sudden power of most
California earthquakes is released less
than 12 kilometers below the surface of
the earth the shock waves that radiate
from their centers in damaged buildings
at distances of 150 kilometers the
amount of energy released varies widely
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the energy of this earthquake was less
than 1% of that released near San
Francisco in 1906 it is obvious that
earthquakes are closely related to
movements of the Earth's crust
geologists believe that the San Andreas
Fault may be the border between two huge
portions of the Earth's crust
called plates the Pacific plate which
includes the western sliver of
California is moving slowly north
westward with relation to the North
American plate this model will show how
two plates move the white lines are for
reference plate motion in the model
causes the build-up of stream the strain
is released in sudden slits in the same
way strain builds along the San Andreas
Fault generally friction between plates
causes the locking of rocks along the
fault when the locking rocks give way
the strain built up over long periods of
time is released in a matter of seconds
the result is an earthquake to learn
more about when and where earthquakes
might occur geologists study many kinds
of movement along the San Andreas Fault
at present few geological phenomena are
analyzed by more scientists or monitored
by more sophisticated equipment near
Bear Valley south of Hollister one of
hundreds of tiltmeters measures the
slope of land that has warped by elastic
strain near the fault zone even the
slightest change is recorded by the
meter it is so sensitive that it reacts
to the earth wave created by the
footsteps of an approaching man
the fault is also studied with laser
beams that measure horizontal movement
over long distances a narrow intense
beam of light is aimed from one side of
the fault to a reflector ten kilometers
away on the other side meanwhile a
helicopter flies close to the line of
sight to collect atmospheric data that
could affect the reading the travel time
of the light is recorded and corrected
for the atmospheric distortion measured
from a helicopter any motion along the
fault is detected as a change in the
shape of a laser triangle minut fault
movements can also be measured directly
with sensitive treat meters this pit is
the terminal point of an underground
wire that stretches diagonally across
the fault if there is any movement the
wire is pulled the displacement is
measured and recorded earthquakes too
are constantly recorded hundreds of
minor tremors occur along the fault
every year most of them so slight that
they can be detected only by very
sensitive instruments seismometers on
the fault zone measure these tremors and
relay the information to data collecting
stations by radio this information
translated to seismograms
provides a continuous picture of tremor
activities in the false show
some types of earthquake research can be
conducted under controlled laboratory
conditions in this experiment a sample
of rock is subjected to shear stress the
object is to duplicate the kind of
locking and slipping behavior that is
characteristic of rocks deep in the San
Andreas Fault zone
truly electronic sensors tremors in the
rock sample are measured they result in
seismograms that closely resemble those
produced by wheel earthquake this
experiment can be repeated while varying
the pressure temperature and water
content of the rocks geologists can thus
learn if any of these factors are
important in determining when and where
earthquake activity may occur at the
National Center for earthquake research
in Menlo Park California data is
received from hundreds of instruments
all along the falsest autistics on tilt
stream treat and tremor activity are
collected in storage
current conditions are monitored and
data is fed into computers
the computers combine data compiled over
many years and translate it into visual
forms
month by month tremor by tremor a
computer provides a graphic readout of
seismic activity in the San Francisco
Bay Area
a close look at this readout shows very
little tremor activity in the northern
portion of the fall system the fault
appears to be locked elastic strain is
probably building up geologists estimate
that the strain in the faults northern
reaches is already reaching the level
that preceded the devastating earthquake
of 1906 if that earthquake were to be
repeated today its tragedy would be
increased many fold in modern California
geologists and engineers have tried to
estimate the effects of such a network
in one estimate property damage exceeds
24 billion dollars more than 100 bridges
and freeway overpasses collapse 10,000
residents of the San Francisco Bay Area
are killed the San Francisco Bay Area is
today the home of more than 3 million
people another 7 billion are centered
around metropolitan Los Angeles it has
been estimated that by the end of the
century one seventh of the United States
population will live in the state of
California most of these people will be
living in the shadow of the San Andreas
Fault some geologists say that
California's next major earthquake could
come tomorrow others say it won't come
for a hundred years
but all agree that it is coming the
question of when and where the next
major earthquake will strike is of great
importance to the people of California
what the San Andreas Fault and the
motion along it are only small parts of
much larger story somewhere deep within
the planet
an unknown force is causing huge
sections of the crust to move gradually
rearranging the surface of the earth
what is the nature of this force where
does it originate and what effect will
it have on the geography of the earth
over millions of years of geologic time
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