The San Andreas Fault

[Music] near 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 muscle rock where the fault dips into the sea San Andreas Lake the central fall region where deep straight valleys Mark the Fall's [Music] path to the naked eye the San Andreas fault is most evident on the barren flatland of central California's cizo plane here wether is scarcely a trace of civilization the vault is visible as a thin well- defined line of deformation crossed by an occasional dry stream Gully in some places the vault is traced by the foot of a gently sloping Hill or by a shallow depression in the landscape in others it is more obscure its presence marked only by a primitive [Music] Road more dominant features can be seen farther south on the cizo pl L where a few desolate farmhouses do the land The Vault appears as a thin Valley that cuts across the general slope of the Plaine where the terrain is steep the centuries have etched their Mark into the land with deep eroded channels that descend from the vicinity of the falline altogether the San Andreas fault slices through some 1100 kilm of California Countryside from Mexico North through the San Francisco Bay area and on into the sea in many places it is not a single tractor but a complex zone of branches called the San Andreas fault system on the southern portion of the Fall system east of the saltan sea rocks have been squeezed and fractured by pressure in The Fault 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 gorund a road cut exposes a massive granite Granite is often used for building normally it is a strong durable Rock but in The Fault Zone 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 zone a 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 the awesome forces that come into play are rarely experienced directly by man but proof of motion along the fault is here in the G ologic record of the land one of the most dramatic examples of this proof can be found in the cizo plain where the dry gullies of drainage Creeks descend to the [Music] falline these streams do not flow in a straight line at the fault they Veer sharply to the north the offset of this stream is the result of motion along the fall [Music] 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 completely interrupted the stream's path eventually the Waters of the stream eroded A New Path into the land but again the new stream was offset by movement along the fall today the stream is displaced about 20 m to the right where it crosses the 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 [Music] time much greater displacements involving much longer periods of time have occurred along the San Andrea's fault over millions of years this movie movement has been sufficient to displace huge blocks of Earth in this Creek bed two different types of rock lie side by side The Rock on the Rock on the right a few thousand years old the fault is evident between the sections where a soft pable substance called fault gou 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 is moved and how long it has 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 Frasier mountains in the desert of Southern California an outcrop of identical rock is found in the orocopia mountains on the Eastern ins side of the Vault this suggests that movement along the fault 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 kmers 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 at top an active branch of the fall zone inside one of the winery warehouses reinforced concrete walls and floors are slowly being pulled apart by creep movement along the fault this simple model shows how the displacement takes place the slow movement of creep can be measured here the Earth is moving at an average of about 13 mm a year the other form of fault movement is sudden and sometimes violent the [Music] earthquake severe earthquakes can tort and Destroy man-made objects of all descriptions in California they often originate along one of the branch fractures that make up the San Andreas Vault system the sudden power of most California earthquakes is released less than 12 kilomet below the surface of the Earth the shock waves that radiate from their centers can damage buildings at distances of 150 km the amount of energy released varies widely 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 plat the Pacific Plate which includes the Western sliver of California is moving slowly northwestward with relation to the North American Plate this model will show how the plates move the white lines are for reference plate Motion in the model causes the buildup of strain The Strain is released in sudden slips in the same way strain builds along the San Andre is 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 Andrea's fault at present few geological phenomena are analyzed by more scientists or monitored by more sophisticated equipment near be Valley south of Hollister one of hundreds of tilt meters measures the slope of land that is 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 vault 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 10 km 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 the helicopter any motion along the fault is detected as a change in the shape of a laser triangle minute fault movements can also be measured directly with sensitive creep 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 instrument size MERS on the fault Zone measure these Tremors and relay the information to data collecting stations by radio this infation translated to seismograms provides a continuous picture of tremor activity in The Fault Zone some types of earthquake research can be conducted under controlled laboratory conditions in this experiment a sample of rock is subjected to sheer 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 through electronic sensors Tremors in the rock sample are measured they result in seismogram that closely resemble those produced by real earthquakes this experiment can be repeated while varying the pressure temperature and water content of the rock 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 menow Park California data is received from hundreds of instruments All Along The Fall system statistics on tilt strain creep and Tremor activity are collected and stored current conditions are monitored and data is fed into computers the computers combine data compiled over many years and translate into visual form 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 fault system the fault appears to be locked elastic strain is probably building up geologists estimate that the strain in the fault's northern region is already approaching the level that preceded the devastating Earthquake of 1906 if that earthquake were to be repeated today its tragedy would be increased manyfold in modern California geologists and Engineers have tried to estimate the effects of such an earthquake in one estimate property damage exceeds 24 billion 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 million are centered around Metropolitan Los Angeles it has been estimated that by the end of the century 17th 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 Andrea's fault some geologists say that California's next major earthquake could come tomorrow others say it won't come for a 100 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 but the San Andreas fault and the motion along it are only small parts of a 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 Tire