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Exploring the beautiful nature of California


California Nature: San Andreas Fault


The San Andreas fault forms a continuous narrow break in the Earth's crust that extends from northern California southward to Cajon Pass near San Bernardino. Southeastward from Cajon Pass several branching faults, including the San Jacinto and Banning faults, share the movement of the crustal plates. In this stretch of the fault zone, the name "San Andreas" generally is applied to the northeastern most branch. Scientists have learned that the Earth's crust is fractured into a series of "plates" that have been moving very slowly over the Earth's surface for millions of years. Two of these moving plates meet in western California; the boundary between them is the San Andreas fault. The Pacific Plate (on the west) moves northwestward relative to the North American Plate (on the east), causing earthquakes along the fault. The San Andreas is the "master" fault of an intricate fault network that cuts through rocks of the California coastal region. The entire San Andreas fault system is more than 800 miles long and extends to depths of at least 10 miles within the Earth.
San Andreas Fault Line through California and its effect on California Mountains The presence of the San Andreas fault was brought dramatically to world attention on April 18, 1906, when sudden displacement along the fault produced the great San Francisco earthquake and fire. During the 1906 earthquake in the San Francisco region, roads, fences, and rows of trees and bushes that crossed the fault were offset 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. This earthquake, however, was but one of many that have resulted from episodic displacement along the fault throughout its life of about 15-20 million years.  In detail, the fault is a complex zone of crushed and broken rock from a few hundred feet to a mile wide. Many smaller faults branch from and join the San Andreas fault zone. Almost any road cut in the zone shows a myriad of small fractures, fault gouge (pulverized rock), and a few solid pieces of rock. On the ground, the fault can be recognized by carefully inspecting the landscape. The fault zone is marked by distinctive landforms that include long straight escarpments, narrow ridges, and small undrained ponds formed by the settling of small blocks within the zone. Many stream channels characteristically jog sharply to the right where they cross the fault.
Blocks on opposite sides of the San Andreas fault move horizontally. If a person stood on one side of the fault and 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. 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. Other stretches of the fault, however, apparently accommodate movement more by constant creep than by sudden offsets that generate great earthquakes. In historical times, these creeping sections have not generated earthquakes of the magnitude seen on the "locked" sections.
up close view of the San Andreas Fault in California  Geologists believe that the total accumulated 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 terranes 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. 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. Surveying shows a drift at the rate of as much as 2 inches per year. The San Andreas Fault is not a straight line. Where there are bends in the fault, blocks of rock get pushed up or drop down, making mountains or basins. The Transverse Ranges and the Coastal Ranges of California have been pushed up as the Pacific Plate moves past the North American Plate. The Los Angeles Basin, the Venture Basin, and the San Francisco Bay are all blocks of rock that have dropped down.
How will California’s landscape change in the future? Scientists can estimate the directions and speeds that the plates are moving. They can predict future plate boundary interactions. For example, the small part of California that is on the Pacific Plate, including Los Angeles, will continue moving northwest along the coast relative to the North American Plate. This means that Los Angeles and San Francisco are approaching each other about as fast as your fingernail grows. The San Francisco earthquake and fire of April 18, 1906, took about 700 lives and caused millions of dollars worth of damage in California from Eureka southward to Salinas and beyond. On May 18, 1940, an earthquake of magnitude 7.1 occurred along a previously unrecognized fault in the Imperial Valley. Similar movement on the Imperial fault occurred during an earthquake in November 1979. The greatest surface displacement was 17 feet of right-lateral strike-slip in the 1940 earthquake. Clearly, this fault is part of the San Andreas system. Other earthquakes of probable magnitudes of 7 or larger occurred on the Hayward fault in 1836 and 1868 and on the San Andreas fault in 1838.
 
 
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