Transform Plate Boundaries - Transform Fault
The San Andreas Fault is a continental transform fault that extends roughly 1, kilometers ( mi) through California. It forms the tectonic boundary between the Pacific Plate and the North 2 Plate boundaries; 3 Formation; 4 Study. Early . (December ) (Learn how and when to remove this template message). A smaller number of transform faults cut continental lithosphere. The most famous example of this is the San Andreas Fault Zone of western North America. The San Andreas fault is the border between two tectonic plates—the North American Plate and Pacific The fault is moving at about 2 centimeters (just under an inch) per year. Plate tectonic boundaries are regions where lithospheric plates meet. An example of a divergent plate boundary is the Mid- Atlantic Ridge.
Early years[ edit ] The fault was first identified in Northern California by UC Berkeley geology professor Andrew Lawson in and named by him after the Laguna de San Andreasa small lake which lies in a linear valley formed by the fault just south of San Francisco.
Eleven years later, Lawson discovered that the San Andreas Fault stretched southward into southern California after reviewing the effects of the San Francisco earthquake. Large-scale hundreds of miles lateral movement along the fault was first proposed in a paper by geologists Mason Hill and Thomas Dibblee. This idea, which was considered radical at the time, has since been vindicated by modern plate tectonics. Following recorded seismic events in,andscientists predicted that another earthquake should occur in Parkfield in It eventually occurred in Due to the frequency of predictable activity, Parkfield has become one of the most important areas in the world for large earthquake research.Special look into San Andreas Fault
An array of sensors will be installed to record earthquakes that happen near this area. In particular, scientific research performed during the last 23 years has given rise to about 3, publications. Moreover, the risk is currently concentrated on the southern section of the fault, i.
According to this study, a massive earthquake on that southern section of the San Andreas fault would result in major damage to the Palm Springs - Indio metropolitan area and other cities in San BernardinoRiverside and Imperial counties in California, and Mexicali Municipality in Baja California. Older buildings would be especially prone to damage or collapse, as would buildings built on unconsolidated gravel or in coastal areas where water tables are high and thus subject to soil liquefaction.
The information available suggests that the fault is ready for the next big earthquake but exactly when the triggering will happen and when the earthquake will occur we cannot tell It could be tomorrow or it could be 10 years or more from now. The ability to predict major earthquakes with sufficient precision to warrant increased precautions has remained elusive. That study predicted that a magnitude 7.
Scientists believe quakes on the Cascadia subduction zone may have triggered most of the major quakes on the northern San Andreas within the past 3, years. The evidence also shows the rupture direction going from north to south in each of these time-correlated events. However the San Francisco earthquake seems to have been the exception to this correlation because the plate movement was moved mostly from south to north and it was not preceded by a major quake in the Cascadia zone.
What type of boundary is the San Andreas Fault? In which state is it located?
List of earthquakes in California The San Andreas Fault has had some notable earthquakes in historic times: Though it is known as the Fort Tejon earthquake, the epicenter is thought to have been located far to the north, just south of Parkfield.
Two deaths were reported. Its moment magnitude was 7. The epicenter was near San Francisco. At least 3, people died in the earthquake and subsequent fires.
The magnitude was estimated to be 7. Moment magnitude was about 6. This quake occurred on October 17,at approximately 5: On September 28, at PDT, a magnitude 6. It was felt across the state, including the San Francisco Bay Area. As the Earth rotates, the liquid inner core spins, creating the Earth's magnetic field. Within the crust and mantle, there also are two important mechanical layers—the lithosphere and asthenosphere.
The lithosphere is the outermost of these layers, and comprises the crust and uppermost mantle. The lithosphere is relatively cool, making the rock strong and resistant to deformation. The lithosphere is broken into the moving tectonic or lithospheric plates. Below the lithosphere is a relatively narrow, mobile zone of the mantle called the asthenosphere.
San Andreas Fault
The asthenosphere is a weak zone, formed of mostly solid rock with perhaps a little magma mixed inand flows very slowly, in a manner similar to the ice at a bottom of a glacier. The rigid lithosphere is believed to "float" or move about on the slowly flowing asthenosphere. Plate Tectonic Theory is Developed The plate tectonic theory known today evolved in the s, owing to four major scientific developments: Demonstration of the young age of the ocean floor; Confirmation of repeated reversals of the Earth's magnetic field in the geologic past; Emergence of the seafloor-spreading hypothesis and associated recycling of the oceanic crust; and Precise documentation that the Earth's earthquake and volcanic activity was concentrated along subduction zones and mid-ocean ridges.
Before the nineteenth century, the depth of the open ocean was a matter of speculation, although most scientists believed it to be flat and featureless. Only in did the first bathymetric maps reveal the first evidence of underwater mountains in the central Atlantic.
Inseismologists found that the sediment layer on the floor of the Atlantic was much thinner than previously thought.
San Andreas Transform Fault Zone
Scientists believed that the oceans were over 4 billion years old, and were perplexed by the distinct lack of sediment cover.
The answer to this question would prove vital to advancing the theory of plate tectonics. In the s, scientists began recognizing magnetic variations in the rocks of the ocean floor.
This was not entirely unexpected, since it was known that basalt contained the mineral magnetite, and this mineral was known to locally distort compass readings. In the early part of the twentieth century, geologists recognized that oceanic rocks had normal or reverse polarity i.
This can be explained by the ability of the magnetite grains to align themselves in the molten basalt with the Earth's magnetic field. When the rock cools, these grains are "locked" in, recording the magnetic orientation or polarity normal or reversed at the time of cooling. As more of the ocean floor was mapped, patterns of alternating stripes of normal and reverse polarity were noted; this became known as magnetic striping.
With the discovery of magnetic striping at mid-ocean ridges, scientists began to theorize that mid-ocean ridges mark structurally weak zones where magma from deep within the Earth rises and erupts at the surface. This theory, called seafloor spreading, quickly gained acceptance, but raised an additional question: If new crust is continually being formed at mid-ocean ridges, and the Earth is not increasing in size, what is happening to the old crust?
Harry Hess and Robert Dietz postulated that the old crust must be destroyed in the deep canyon-like oceanic trenches, while new crust if formed at the mid-ocean ridges. This theory explained why the Earth is not expanding, there is little sediment on the ocean floor, and oceanic crust is much younger than continental rocks. The final scientific discovery that cemented the theory of plate tectonics occurred with improvements in seismic detection in the s.
Seismologists identified regions of earthquake activity that coincided with Hess's predicted areas of ocean crust generation mid-ocean ridges and oceanic lithosphere destruction subduction zones. Today scientists know that tectonic plates move, because they can measure their motion directly using the global positioning system GPS.
Plate Tectonic Boundaries Plate tectonic boundaries are regions where lithospheric plates meet. There are three types of plate tectonic boundaries: Divergent boundaries occur along spreading ridges where plates are moving apart and new crust is being created by ascending magma from the mantle.
An example of a divergent plate boundary is the Mid-Atlantic Ridge. This submerged mountain chain extends from the Arctic to the southern tip of Africa, and is one part of the global ridge system that extends around the Earth. Convergent boundaries are regions where lithospheric plates collide.
The type of convergence depends on the types of plates involved: