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Tension weakens the crust until the rock fractures, and one block of rock moves downward relative to the other. A reverse fault is usually associated with plates that are colliding. Compression forces a fault block upward.
A thrust fault is a special kind of reverse fault where one or more plates are under the ocean. At a thrust fault, a plate below the sea is moving under another plate, thrusting its edge upward.
The process of one plate diving under the other is called subduction. Thrust faults can produce larger earthquakes than strike-slip faults. 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.
Local geologic conditions strongly influence the intensity of an earthquake. Commonly, sites on soft ground or alluvium have intensities 2 to 3 units higher than sites on bedrock. 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.
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 accounts, ground movement in both cases was roughly the same type.
An account 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. 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 earthquake was felt as far away as Oregon and central Nevada. The earthquake, which has been estimated at a magnitude 8.
Surface offsets occurred along a mile length of the fault from San Juan Bautista north past Point Arena and offshore to Cape Mendocino. On May 18, , an earthquake of magnitude 7. 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.
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 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.
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.
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.
As the last large earthquake on the southern San Andreas occurred in , that section of the fault is considered a likely location for an earthquake within the next few decades. 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.
A great earthquake very possibly will not occur unannounced. Such an earthquake may be preceded by an increase in seismicity for several years, possibly including several foreshocks of about magnitude 5 along the fault.
Before the next large earthquake, seismologists also expect to record changes in the Earth's surface, such as a shortening of survey lines across the fault, changes in elevation, and effects on strainmeters in wells. A key area for research on methods of earthquake prediction is the section of the San Andreas fault near Parkfield in central California, where a moderate-size earthquake has occurred on the average of every years for about the last years.
This section describes where earthquakes have occurred in the past and where they may likely occur in the future, how the ground will shake when they do, and what may happen in a plausible "big one" on the San Andreas.
To become familiar with earthquake vocabulary, you may want to read the " What Should I Know " section first. The earthquakes of California are caused by the movement of huge blocks of the earth's crust- the Pacific and North American plates.
The Pacific plate is moving northwest, scraping horizontally past North America at a rate of about 50 millimeters 2 inches per year. About two-thirds of this movement occurs on the San Andreas fault and some parallel faults- -- the San Jacinto, Elsinore, and Imperial faults see map. Over time, these faults produce about half of the significant earthquakes of our region, as well as many minor earthquakes.
Larger image Earthquakes plotted on this map at their epicenters include: significant earthquakes since as red numbered circles corresponding to the table at left; earthquakes larger than magnitude 5. While there are thousands of earthquakes shown, this is only a very small window on the earthquake history of southern California! For maps of recent earthquake information see here.
Vertical faults such as the San Andreas red band from top left to bottom right are shown as a thin strip.
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