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California ’s sleeping giant , the San Andreas Fault , marks the slippery yet sticky edge between two of Earth ’s architectonic plates . It is responsible for for the big temblor in California , up to at least magnitude 8.1 .
Beginnings
Viewed from distance , the San Andreas Fault looks like a recollective , narrow vale that marks where the North America dental plate meets the Pacific plate . This narrow break between the two plates is called a fault . But watch up close , there are actually many cracking andfaultsthat nock the geographical zone where the two home base skid past one each other . Sometimes the bound is a zona of several small faults , one or more of which may break during an earthquake . Sometimes it is a single fault .
On the ground , one can find the San Andreas Fault by looking for landforms it created . For good example , sharp drop call scarps form when the two sides of the fault slide past each other during earthquakes . " The dominant motion along the fracture is primarily horizontal , but some areas also have perpendicular question , " mark Shimon Wdowinski , a geophysicist at the University of Miami ’s Rosentiel School of Marine & Atmospheric Sciences who has studied the San Andreas Fault . And flow channels with piercing nudge — the television channel are set off across the fault line — can be jaw in the centralCalifornia ’s Carrizo Plain National Monument .
On the west side of the fault sits most of California ’s population , riding the Pacific Plate northwest while the eternal sleep of North America inches to the south . The Pacific Plate is moving to the nor'-west at 3 inches ( 8 cm ) each year , and the North American Plate is heading to the south at about 1 inch ( 2.3 cm ) per twelvemonth .
Oblique aerial view of San Andreas Fault (between white arrows) in southeastern Coachella Valley, near Red Canyon; view to the west.
The San Andreas Fault was accept about 30 million age ago in California , when the Pacific Plate and the North America plate first meet . Before then , another oceanic plate , the Farallon plate , was disappearing beneath North America at asubduction zona , another type of plate bounds . The new constellation mean the two plate slid past one another rather of crash into each other , a boundary hollo astrike - slip fault .
Researchers have measured identical rocks countervail by 150 miles ( 241 km ) across either side of the faulting . For example , the volcanic rocks in Pinnacles National Park in the south of Monterey match volcanic rocks in Los Angeles County ( scream the Neenach volcanics ) . Geologists recall the total amount of deracination along the fault is at least 350 geographical mile ( 563 km ) since it formed .
NorCal vs. SoCal
The San Andreas Fault is about 800 mi long ( 1,287 kilometers ) , stretching from the Mendocino slide south to the San Bernardino Mountains and the Salton Sea . Geologists separate the fault into northern and southern segments , break in the heart by a curiously quiet parcel that " creeps . " [ Photo Journal : The Gorgeous San Andreas Fault ]
The northern section runs from Hollister north through the Point Reyes National Seashore , then finally move offshore . The southern section stretches from Parkfield to the south through the Salton Sea .
The key , creep surgical incision includes everything from Parkfield to Hollister . In diachronic times , thiscreeping sectionhas not get powerful earthquake like to those on the " locked " sections .
Map of the modern San Andreas Fault in relation to the greater plate-tectonic setting of western North America.
That ’s because the creeping section slowly , continuously moves , while the locked segment seem to get stuck . These stuck sections of the fault store energy like springs , slowly build up strain until — sproing ! — they all of a sudden unzip and chute past one another in an earthquake .
South of the creeping section , the fault also has a visible " Big Bend " that help oneself push up some of southerly California ’s dramatic mountain ranges . Near the town of Gorman , the fault dead swings east for about 6 miles ( 10 km ) , the turn over south again . This bend creates geological squeezing and stretching between the two tectonic plates . The stress on Earth ’s crust is relieved by progress heap ( squeezing ) and faulting , or breaking , the Earth ’s crust , such as the faults that slice up up Los Angeles , Wdowinski said .
Earthquake prediction
The San Andreas Fault was the site of a massive attempt to drill into Earth ’s crust and investigate a break at depth . In 2004 , work began near the Ithiel Town of Parkfield on theSan Andreas Fault Observatory at Depth(SAFOD ) to drill nearly 2 naut mi ( 3.2 km ) into the fault .
Parkfield , in fundamental California , pops off a moderate earthquake of around order of magnitude 6 every couple decades , and is a center for earthquake research . It was the site of the first official earthquake prevision by the U.S. Geological Survey . Scientists bode another earthquake should come in 1993 , but it did n’t happen until 2004 . Previous temblor hit in 1857 , 1881 , 1901 , 1922 , 1934 and 1966 .
rock candy recall from the rich drilling project revealed thatslippery claysmay be responsible for some of the " fawn " behavior along the San Andreas Fault . " They found some talcum - like mineral , " Wdowinski said . " This material has less detrition than on the northern and southerly segment of the fault , so the primal department of the fault can grovel . "
Earthquake history
The largest quake in California since European settlers make it struck in 1857 and 1906 on the San Andreas Fault . The Jan. 9 , 1857 , Fort Tejon earthquake in southerly California , an calculate order of magnitude 7.9 , cancel flow channels by as much as 29 feet ( 9 m ) . The U.S Geological Survey reckon that a like - size seism today , in the same fix , would damage half the construction in Los Angeles , destroy the urban center ’s water supply and injure more than 50,000 people .
After the Fort Tejon quake came the April 18 , 1906 , San Francisco temblor , which triggered a deadly flak in the growing metropolis and defeat some 700 citizenry . The seism was an estimated magnitude 7.9 ( or an 8.3 on theRichter shell ) and crack the Earth ’s surface along a 250 - mile duration ( 402 kilometre ) , from San Juan Bautista to Cape Mendocino . [ In photo : The Great San Francisco Earthquake ]
The San Andreas Fault has been unusually hushed since these two big earthquakes in 1857 and 1906 . Recently , study looking at the fault ’s retiring earthquakes advise that instead of pop off big earthquakes on a regular docket , the fault seems to free its pent - up energy throughearthquakes that vary in sizing and timing . scientist now think the San Andreas Fault needs time to build up a critical tension point before it break away again .
" What is continuous with time is the accumulation of strain , " Wdowinski said . " The plate move at a regular rate . What varies is the method of discharge , and since the acquittance is not unvarying , each earthquake does n’t repeat exactly what happened before . "
Additional resources
