Plate margin details
The San Andreas Fault marks the junction between the North American and Pacific Plates. The fault is 1300 km long, extends to at least 25 km in depth, and has a north west-south east trend. It is classified as a right lateral (dextral) strike-slip fault.
Although both plates are moving in a north westerly direction, the Pacific Plate is moving faster than the North American Plate, so the relative movement of the North American Plate is to the south east. Movement along the fault is not smooth and continual, but sporadic and jerky. Frictional forces lock the blocks of lithosphere together for years at a time. When the frictional forces are overcome, the plates slip suddenly and shallow focus earthquakes are generated. Landscape and manmade features (eg rivers, fences and roads) are displaced across the fault as movement occurs. San Francisco has historically suffered significant earthquakes, notably in 1906 and 1989.
Seismicity warnings/details
Since 2008, millions of people in California and elsewhere have participated in yearly disaster drills in which they practice diving under a table and learn other preparedness tips. If you’re outdoors when the ground moves, experts recommend bracing against a wall.
Despite a century of research, earthquake prediction remains elusive. Scientists can’t predict when a jolt is coming and are generally pessimistic about ever having that ability. Every warning sign scrutinized – animal behavior, weather patterns, electromagnetic signals, atmospheric observations, levels of radon gas in soil or groundwater – has failed. The latest focus has been on creating early warning systems that give residents and businesses a few seconds heads up after a quake hits, but before strong shaking is felt. Japan has the most advanced seismic alert system in the world while the U.S. is currently testing a prototype.
History of the plate movement
The largest historical earthquakes that occurred along the San Andreas fault were those in 1857 and 1906. The earthquake of January 9, 1857, in southern California apparently was about the same magnitude as the San Francisco earthquake of 1906. According to newspaper accounts, ground movement in both cases was roughly the same type. An account of the 1857 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 1857.
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. The earthquake was felt as far away as Oregon and central Nevada. The 1906 earthquake, which has been estimated at a magnitude 8.3 on the Richter Scale, caused intensities as high as XI on the Modified Mercalli Scale. Surface offsets occurred along a 250- mile length of the fault from San Juan Bautista north past Point Arena and offshore to Cape Mendocino.
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.
Potential future movement and dangers
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 1,400 to 1,500 years large earthquakes have occurred at about 150-year intervals on the southern San Andreas fault. As the last large earthquake on the southern San Andreas occurred in 1857, 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 100 years have passed since the great 1906 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 and changes in elevation.
The San Andreas Fault marks the junction between the North American and Pacific Plates. The fault is 1300 km long, extends to at least 25 km in depth, and has a north west-south east trend. It is classified as a right lateral (dextral) strike-slip fault.
Although both plates are moving in a north westerly direction, the Pacific Plate is moving faster than the North American Plate, so the relative movement of the North American Plate is to the south east. Movement along the fault is not smooth and continual, but sporadic and jerky. Frictional forces lock the blocks of lithosphere together for years at a time. When the frictional forces are overcome, the plates slip suddenly and shallow focus earthquakes are generated. Landscape and manmade features (eg rivers, fences and roads) are displaced across the fault as movement occurs. San Francisco has historically suffered significant earthquakes, notably in 1906 and 1989.
Seismicity warnings/details
Since 2008, millions of people in California and elsewhere have participated in yearly disaster drills in which they practice diving under a table and learn other preparedness tips. If you’re outdoors when the ground moves, experts recommend bracing against a wall.
Despite a century of research, earthquake prediction remains elusive. Scientists can’t predict when a jolt is coming and are generally pessimistic about ever having that ability. Every warning sign scrutinized – animal behavior, weather patterns, electromagnetic signals, atmospheric observations, levels of radon gas in soil or groundwater – has failed. The latest focus has been on creating early warning systems that give residents and businesses a few seconds heads up after a quake hits, but before strong shaking is felt. Japan has the most advanced seismic alert system in the world while the U.S. is currently testing a prototype.
History of the plate movement
The largest historical earthquakes that occurred along the San Andreas fault were those in 1857 and 1906. The earthquake of January 9, 1857, in southern California apparently was about the same magnitude as the San Francisco earthquake of 1906. According to newspaper accounts, ground movement in both cases was roughly the same type. An account of the 1857 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 1857.
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. The earthquake was felt as far away as Oregon and central Nevada. The 1906 earthquake, which has been estimated at a magnitude 8.3 on the Richter Scale, caused intensities as high as XI on the Modified Mercalli Scale. Surface offsets occurred along a 250- mile length of the fault from San Juan Bautista north past Point Arena and offshore to Cape Mendocino.
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.
Potential future movement and dangers
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 1,400 to 1,500 years large earthquakes have occurred at about 150-year intervals on the southern San Andreas fault. As the last large earthquake on the southern San Andreas occurred in 1857, 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 100 years have passed since the great 1906 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 and changes in elevation.