the

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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