Earth faults

The hard, outer rock layer of Earth's surface has many fractures. A “fault” is the name given to a place where the rocks on either side of such a fracture have experienced forces that have caused them to move in relation to each other. Earthquakes are the most highly visible manifestation of a faulting event, but other effects of faulting include the movement of groundwater, the formation of new surface land features like mountains and volcanoes, and the distribution of minerals and other raw materials.

and Fault Formation

Earth's crust is its hard outermost layer, which ranges in thickness from about 10 kilometers (6.2 miles) to 65 kilometers (40.4 miles) across the world. Scientists long believed that Earth's crust was composed of a single, unbroken layer of rock. However, a substantial body of geological evidence now supports the well-accepted theory of plate tectonics, which describes the structure of Earth's crust as being composed of a series of approximately one dozen large, quite rigid pieces known as plates.

These plates are constantly in motion, moving slowly but fluidly in relation to each other over the upper mantle, a layer of molten rock that sits just below the crust. Together, the crust and the upper mantle make up what is known as Earth's lithosphere. When plates are moving in different directions relative to each other, they have a tendency to become stressed by forces that are either stretching or compressing them.

Plate movements take a variety of forms. At divergent boundaries, the plates pull away from each other and generate rifts in the land that are filled by oceans. At convergent boundaries, the plates push toward each other, and one plate dives beneath another, causing the earth's crust to become recycled back into the mantle. At transform boundaries, plates slide sideways past each other in a shearing movement.

If the tension created by each of these types of movements is great enough, it can cause fractures in the earth's crust and can cause the rocks on opposite sides of the fracture to slip and slide relative to each other. These fractures, which can be as small as a few centimeters or as large as hundreds of kilometers in length, are known as faults. It is most common for faults to occur at or near the boundaries between two or more plates, but they also can be formed within a plate.

Fractures are unevenly distributed over the earth's surface. Some regions of the world are riven by huge numbers of faults, making these regions more likely to have earthquake and volcanic activity. Other regions have few or no faults. Most of the time, faults do not occur individually along a single fault line. Instead, numerous small faults tend to occur within a larger belt. The entirety of this fractured area is known as a fault zone. Because plate tectonic movements take place on such a long time line (typically, a plate will move no farther than a few centimeters per year), most of the faults that are present on the earth's surface came from tectonic stresses that took place in past geological eras.

Fault Types and Fault Movements

The dip, or angle, of a fault plane may be completely vertical, completely horizontal, or any angle of inclination in between. The rocks on either side of the fault plane may be displaced in various directions in relation to each other. This second characteristic of a fault is the one geologists use to classify faults into different groups. Two major types of displacement have been identified: strike-slip, which consists of primarily horizontal movement, and dip-slip, which consists of primarily vertical movement.

Strike-Slip Faults. In a strike-slip fault, the fault plane is usually near vertical in orientation, and the rocks on either side of it move sideways past each other. This type of faulting action occurs when forces are pushing the rocks laterally in a process called shear.

There are several types of strike-slip faults, usually classified by describing what would be seen by a hypothetical witness standing on one side of the fault line and watching the movement of the rocks on the far side. If the rocks on the far side appear to have shifted to the left, the fault is known as a left-lateral strike-slip fault, or a sinistral fault. If the rocks on the far side appear to have shifted to the right, the fault is known as a right-lateral strike-slip fault, or a dextral fault. (It does not matter what side of the fault the rock movement is observed from.) Strike-slip faults are widespread, and often associated with earthquake events. The San Andreas fault in California, which happens to be a right-lateral strike-slip fault, is an example of this.

Dip-Slip Faults. In a dip-slip fault, the fault plane is usually relatively horizontal in orientation, and the rocks on either side of it move up and down relative to each other. As with strike-slip faults, there are several types of dip-slip faults depending on the specific direction of the rock movement on either side of the fault plane. A normal fault is a dip-slip fault wherein the hanging wall moves downward relative to the footwall. (Geologists use the term “hanging wall” to describe the rock material that lies above a fault plane and “footwall” to describe the rock material that lies below it.) This type of movement occurs when forces are pulling the rocks apart in a process called extension.

A reverse fault is a dip-slip fault wherein the hanging wall moves upward relative to the footwall. This type of movement occurs when forces are pushing the rocks together in a process called compression. In Southern California, for example, the San Gabriel Mountains are being pushed up and over the San Fernando and San Gabriel valleys because of of reverse fault movement.

Other Fault Types. When a normal fault has a very shallow, gentle, or flat dip, it is known as a detachment fault. The Snake Range mountains in east-central Nevada are home to a typical example of a detachment fault. When a reverse fault has a very shallow, gentle, or flat dip, it is known as a thrust fault. Thrust faults are commonly involved in the creation of large mountain belts; the Glarus thrust in the Alps of eastern Switzerland is an example of a thrust fault.

If two normal faults, which each have a steep dip (a high fault plane angle), occur next to each other, they can displace the crust in an upward direction to form a long, raised ridge called a horst. The Vosges Mountains in France make up a typical horst formation. Conversely, when the earth's crust has been displaced downward between two high-angle normal faults, forming a long valley, the resulting landform is known as a graben. The Owens Valley in east-central California is an example of a graben. If a fault has sections in which rocks are clearly moving vertically in relation to each other and has sections in which rocks are clearly moving horizontally in relation to each other (in other words, if it has significant, measurable components of both strike-slip and dip-slip faults), it is known as an oblique-slip or oblique strike-slip fault.

Faults and Earthquakes

When the forces of tension, compression, and shear along a fault plane become so great that they are capable of overcoming the resistance provided by friction, the rocks on either side will move. Creep describes those rocks that move slowly and continuously. Creep can cause visible effects, similar to a slight offset between the edge of a curb and a sidewalk, but it is not likely to have dramatic consequences.

If the rocks move suddenly and violently, however, either a tremor (also known as a tremblor) or an earthquake ensues. An earthquake occurs because of the seismic waves, or vibrations in the earth's crust, produced by the pressure of rocks on either side of a fault plane pushing against each other. The earthquake itself is the name given to the breakage of the rocks. The place underground where this breakage takes place is known as the earthquake's focus. The epicenter of an earthquake is the place on the earth's surface that is located directly above the focus.

An earthquake has occurred at some point in the history of most faults. However, as the plates that make up the earth's crust shift, the stresses they undergo also change. For this reason, most faults—because they were formed many millions of years ago and because the earth's crust has shifted through time—are no longer influenced by the same geological forces they once were. Faults that have not experienced any breakages or ruptures within the last 1.8 million years are considered unlikely to cause an earthquake. Faults that have experienced breakages or ruptures within the last 11,500 years, in contrast, are considered likely sources of earthquake activity.

To estimate the age of the most recent rupture associated with a particular fault, geologists dig trenches across fault lines and look at the layers of sediment on either side. If a layer appears relatively even across the fault, rather than being bent or cut by the fault line, then it can be assumed that it was laid down after the most recent earthquake associated with that fault. (Scientists use radiometric dating of uranium and plutonium isotopes in rock to estimate the age of layers of sediment.)

The San Andreas Fault and the

Despite its name and visible fault line appearing strikingly straight, the San Andreas fault is actually a fault zone, or a system of branching fractures. The San Andreas was formed by the stresses associated with the movement along a transform margin between two tectonic plates: the Pacific and the North American. The movement of the Pacific plate is in a northwest direction along the western edge of the North American plate. Since its formation (parts of the San Andreas fault zone are believed to be as old as 30 to 40 million years, while others are believed to date back only 5.5 million years or so), it has been the source of innumerable earthquakes, some very violent, at various places along its length. Not until the infamous 1906 San Francisco earthquake and associated fires did the San Andreas begin to be the focus of scientific attention.

On the morning of April 18, 1906, an earthquake ruptured approximately 477 kilometers (296 miles) of the surface of the earth in the northernmost region of the San Andreas fault zone. The epicenter of the earthquake—which lasted about forty-five seconds to one minute—was located near the city of San Francisco, which also was the site of most of the resulting casualties, deaths, and property destruction. Shockwaves were felt as far away as southern Oregon and central Nevada. About 6.1 meters (20 feet) of offset, or rock displacement, was observed at the surface, and the slip is believed to have been even greater than that underground. Imperfections in the quality of data collected at the time and differences in the ways in which measurements are calculated have led to some dispute about the magnitude of the San Francisco earthquake, but seismologists generally describe it as being between 7.7 and 8.3 on the surface-wave magnitude scale used to describe the size of an earthquake.

In 1906, American seismological research was in its infancy. In the wake of the devastating effects of this earthquake, however, the then-governor of California created the State Earthquake Investigation Commission to look into the underlying geological causes of the movement along the San Andreas fault zone. The resulting studies, compiled in a document known as the Lawson report (1908), represented the first truly comprehensive scientific description of earthquake activity in the United States. Its surveys demonstrated, for example, that buildings constructed on soft sedimentary soils were far more likely to be strongly shaken and to sustain severe damage. Triangulation data—measurements of changes in the angles between lines that connect fixed observation points—also revealed that the displacement of rock was the greatest at the fault line itself and decreased in size as distance from the fault increased.

Based on these measurements, geophysicist Harry Fielding Reid developed his influential theory of elastic rebound. According to Reid's theory, Earth's crust is gradually distorted through time by long periods of slow and steady plate movement. When a fault rupture occurs, the accumulated pressure from these movements is released, causing seismic waves that allow the crust to “rebound” into its original position. It is this sudden rebound that constitutes an earthquake. The theory of elastic rebound would go on to serve as the basis of most modern earthquake science.

Principal Terms

dip: the angle that describes the steepness of a fault plane

fault line: the two-dimensional trace that is visible where a fault plane meets the earth's surface

fault plane: the planar, or flat, surface where the strata, or layers of rock making up the earth's crust, has broken

fault scarp: a steep cliff that is formed when tectonic plates move against each other along a fault line

fault zone: the area around a particular fault plane within which numerous small fractures in the earth's surface have occurred

footwall: the rocky material that lies beneath a fault plane

hanging wall: the rocky material that rests above a fault plane

lithosphere: the hard outermost layer of the earth, consisting of both the crust, made up of a collection of rigid plates, and the upper mantle

slip: a measurement that describes both the direction and the magnitude of the movement of the rocks on either side of a fault plane, in relation to each other

subduction: the movement of the edge of one plate of the earth's crust sideways and downward, beneath another plate, so that it enters the mantle

tectonic: of or relating to the large-scale geological processes that both shape the structure of the earth's crust and take place within it; from the ancient Greek word meaning “builder”

transform margin: a boundary between two or more tectonic plates where one plate slides horizontally past another; also called a transform boundary or, simply, a fault

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