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. Since the compression waves travel faster through the Earth, they arrive first at a distant point; they are known as primary or "P" waves. Other stretches of the fault, however, apparently accommodate movement more by constant creep than by sudden offsets that generate great earthquakes.
The Pacific Plate on the west moves northwestward relative to the North American Plate on the eastcausing earthquakes along the fault. Some of the world's largest recorded earthquakes--on January 31,off the coast of Colombia and Ecuador, and on March 2,off the east coast of Honshu, Japan--had magnitudes of 8. Schulz and Robert E. Wallace The presence of the San Andreas fault was brought dramatically to world attention on April 18,when sudden displacement along the fault produced the great San Francisco earthquake and fire.
Some of the vibrations are of very low frequency, with many seconds between waves, whereas other vibrations are of high enough frequency to be in the audible range. Blocks on opposite sides of the San Andreas fault move horizontally. The earthquake, which has been estimated at a magnitude 8. Similar movement on the Imperial fault occurred during an earthquake in November The greatest surface displacement was 17 feet of right-lateral strike-slip in the earthquake.
In detail, the fault is a complex zone of crushed and broken rock from a few hundred feet to a mile wide. What Is It? 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. The energy released by an earthquake of M 7, however, is approximately 30 times that released by an earthquake of M 6; an earthquake of M 8 releases times 30x30 the energy of an earthquake of M 6. In an earthquake, people may note first a sharp thud, or blast-like shock, that marks the arrival of the P wave.
As the last large earthquake on the southern San Andreas occurred inthat section of the fault is considered a likely location for an earthquake within the next few decades. Local geologic conditions strongly influence the intensity of an earthquake. The ground first bends, then, upon reaching a certain limit, breaks and "snaps" to a new position.
Two of these moving plates meet in western California; the boundary between them is the San Andreas fault. The scale is logarithmic; a recording of 7, for example, ifies a disturbance with ground motion 10 times as large as a recording of 6.
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The San Andreas is the "master" fault of an intricate fault network that cuts through rocks of the California coastal region. Almost any road cut in the zone shows a myriad of small fractures, fault gouge pulverized rockand a few solid pieces of rock. Where Is It? 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.
Earthquakes with a Richter value of 5 or higher are potentially damaging. In each case, the ground west of the fault moved relatively northward.
What is it?
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. A few seconds later, they may feel a swaying or rolling motion that marks the arrival of the S wave. As the Richter scale does not adequately differentiate between the largest earthquakes, a new "moment magnitude" scale is being used by seismologists to provide a better measure.
An of the earthquake describes a sheep corral cut by the fault that was changed from a circle to an "S"-shape--movement clearly representative of right-lateral strike-slip. The vibrations are of two basic types, compression waves and transverse or shear waves.
The transverse waves arriving later are referred to as shear or "S" waves.
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Many smaller faults branch from and the San Andreas fault zone. Earthquakes Along the Fault Literally thousands of small earthquakes occur in California each year, providing scientists with clear indications of places where faults cut the Earth's crust.
Studies of offset stream channels indicate that as much as 29 feet of movement occurred in The San Francisco earthquake and fire of April 18,took about lives and caused millions of dollars worth of damage in California from Eureka southward to Salinas and beyond. The Richter Scale, named after Charles F. Richter of the California Institute of Technology, is the best known scale for the measuring of magnitude M of earthquakes. This earthquake, however, was but one of many that have resulted from episodic displacement along the fault throughout its life of about million years.
An earthquake of magnitude 2 is the smallest earthquake normally felt by humans. 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 miles, and some crustal blocks may have moved through more than 20 degrees of latitude.
On the ground, the fault can be recognized by carefully inspecting the landscape.
In this stretch of the fault zone, the name "San Andreas" generally is applied to the northeastern most branch. Magnitude is a measure of the size of an earthquake. The Modified Mercalli Scale represents the local effect or damage caused by an earthquake; the "intensity" reported at different points generally decreases away from the earthquake epicenter.
Clearly, this fault is part of the San Andreas system. The crustal plates of the Earth are being deformed by stresses from deep within the Earth. A great earthquake very possibly will not occur unannounced.
Total offset accumulates through time in an uneven fashion, primarily by movement on first one, and then another section of the fault. Geologic studies show that over the past 1, to 1, years large earthquakes have occurred at about year intervals on the southern San Andreas fault. What Surface Features Characterize It? Over much of its length, a linear trough reveals the presence of the San Andreas fault; from the air, the linear arrangement of lakes, bays, and valleys in this trough is striking.
Surveying shows a drift at the rate of as much as 2 inches per year. 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. Other earthquakes of probable magnitudes of 7 or larger occurred on the Hayward fault in and and on the San Andreas fault in Along the Earth's plate boundaries, such as the San Andreas fault, segments exist where no large earthquakes have occurred for long intervals of time.
Viewed from the ground, however, the features are more subtle. For example, an earthquake of intensity II barely would be felt by people favorably situated, while intensity X would produce heavy damage, especially to unreinforced masonry. Geologists believe that the total accumulated displacement from earthquakes and creep is at least miles along the San Andreas fault since it came into being about million years ago.
Many stream channels characteristically jog sharply to the right where they cross the fault. 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.
On the moment magnitude scale, the San Francisco earthquake is estimated at magnitude 7.
The earthquake was felt as far away as Oregon and central Nevada. On May 18,an earthquake of magnitude 7. What Is an Earthquake? During the earthquake in the San Francisco region, ro, 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. The San Francisco Bay area has a slightly lower potential for a great earthquake, as less than years have passed since the great earthquake; however, moderate-sized, potentially damaging earthquakes could occur in this area at any time.
Southeastward from Cajon Pass several branching faults, including the San Jacinto and Banning faults, share the movement of the crustal plates. Scientists term these segments "seismic gaps" and, in general, have been successful in forecasting the time when some of the seismic gaps will produce large earthquakes. The entire San Andreas fault system is more than miles long and extends to depths of at least 10 miles within the Earth.
In the process of breaking or "faulting," vibrations are set up that are the earthquakes. Geologists refer to this type fault displacement as right-lateral strike-slip. Intensity is a measure of the strength of shaking experienced in an earthquake.
Commonly, sites on soft ground or alluvium have intensities 2 to 3 units higher than sites on bedrock. Sudden offset that initiates a great earthquake occurs on only one section of the fault at a time. The largest historical earthquakes that occurred along the San Andreas fault were those in and The earthquake of January 9,in southern California apparently was about the same magnitude as the San Francisco earthquake of According to newspaper s, ground movement in both cases was roughly the same type.
Surface offsets occurred along a mile length of the fault from San Juan Bautista north past Point Arena and offshore to Cape Mendocino. In historical times, these creeping sections have not generated earthquakes of the magnitude seen on the "locked" sections.