Earthquake

An earthquake is the shaking or vibration of Earth's surface as the result of sudden movement along a fault, the movement of molten rock within the Earth, or human activities. The terms temblor and seism are often used as synonyms for earthquake. The location of an earthquake source within the Earth is known as its focus, and the point on the Earth's surface directly above the focus is known as the epicenter.

Earthquakes are common events. The United States Geological Survey estimates that more than three million earthquakes occur on Earth each year, which is equivalent to more than 9,000 earthquakes per day. Virtually all of these are too small to be noticed by humans and many occur in remote areas far from seismometers. Since Aerial view of Interstate 5, a California highway damaged by the Northridge earthquake in January 1994. Photograph by Robert A. Eplett. Governor's Office of Emergency Services.

1900, there has been on average about 1 magnitude 8 earthquake, 18 magnitude 7.0 to 7.9 earthquakes, 120 magnitude 6.0 to 6.9 earthquakes, and 800 magnitude 5.0 to 5.9 earthquakes on Earth each year.

Earthquakes can range in severity from small events that are imperceptible to humans to devastating shocks that level cities and kill thousands. The world's most destructive earthquake, which occurred in China during the year 1556, killed 830,000 people. Twenty other earthquakes in Europe, Asia, and the Middle East are known to have resulted in more than 50,000 deaths each. The most devastating earthquake to strike the United States was the 1906 San Francisco earthquake, which killed about 3,000 people as a result of shaking and resulting fires. Modern engineering and construction methods have significantly reduced the danger posed by earthquakes in developed countries. In the United States, for example, only five earthquakes since 1950 have killed more than 60 people. The great Alaskan earthquake of 1964, the second largest earthquake ever recorded by seismologists, killed only 15 people. An additional 110 perished, however, in earthquake triggered tsunamis that struck coastal Alaska, Oregon, and California. Most of the fatal earthquakes occurring in the United States since 1950 have killed only one or two people, and the vast majority of earthquakes do not kill anyone.

The size of an earthquake is described by its magnitude, which reflects the amount of energy released by the temblor. There are many different ways of calculating earthquake magnitude, the most famous of which was proposed in the 1930s by the American seismologist Charles Richter (1900–1985). The Richter magnitude is the base 10 logarithm of the largest seismic wave amplitude recorded on a particular kind of seismograph located 62 mi (100 km) from the earthquake epicenter. Adjustments must be made if other kinds of seismographs are used or if they are located at a different distance from the epicenter. An earthquake of a given magnitude will produce waves 10 times as large as those from an earthquake of the next smaller magnitude. The energy released increases by a factor of about 30 from one magnitude to the next. The Richter scale is open-ended, meaning that it has no mathematical upper or lower limits. In reality, however, there are no faults on Earth large enough to produce a magnitude 10 earthquake. The two largest recorded earthquakes were the magnitude 9.5 Chilean earthquake of 1956 and the magnitude 9.2 Prince William Sound, Alaska, earthquake of 1964.

The effects of an earthquake are measured by its intensity. Unlike magnitude, earthquake intensity varies from place to place. The most common measure of intensity is the modified Mercalli scale, which ranges from an intensity of I (not felt except by a few people under especially favorable circumstances) to XII (total destruction, with objects thrown in the air and lines of sight distorted). Surveys and interviews after a large earthquake can be used to create an isoseismic map, which shows the distribution of reported earthquake intensities. Most isoseismic maps show a distorted bull's eye pattern of concentric rings of equal intensity area centered around the epicenter.

Tectonic plate movements

Some earthquakes occur in areas where the tectonic plates comprising Earth's lithosphere move horizontally past each other along large faults or zones of faults. Examples of this type include earthquakes along the San Andreas and Hayward faults in California. Earthquakes also occur in places where a continental plate subducts an oceanic plate, for example along the western coast of South America, the northwest coast of North America (including Alaska), and in Japan. If two continental plates collide but neither is subducted, as in Europe and Asia from Spain to Vietnam, earthquakes occur as the rocks are lifted to form mountain ranges.

In other parts of the world, for example the Basin and Range physiographic province of the western United States and the East African Rift, continental plates are being stretched apart by tectonic forces. The result is that some parts of the Earth's crust are lifted to form mountain ranges while neighboring blocks subside to form basins that collect sediment eroded from the mountains. Earthquakes can occur when movement occurs along faults developed as a result of the stretching.

Faults are planes of weakness, across which rock has moved in opposite directions, within the Earth's crust. They can range in size from continental scale features such as the San Andreas fault in California to small features across which only a few millimeters or centimeters of movement has occurred. Tectonic plate motions increase the level of stress within Earth's crust, which is accommodated as elastic strain energy, until the stress exceeds the strength of the fault. Then, the energy is suddenly released as the rocks on each side of the fault slip past each other to create an earthquake. This process is analogous to a rubber band snapping when it was been stretched to the breaking point. Because there is a frictional resistance to movement along faults, rapid seismic slip can generate enough heat to melt the adjacent rocks and form a glassy rock known as pseudotachylyte. In other cases, the elastic strain energy is slowly and quietly dissipated through a process known as aseismic creep.

Magma movement

Rhythmic earthquakes known as harmonic tremors, which are caused by magma and volcanic gas moving through conduits in the Earth's crust just as air moves through a pipe organ, can foreshadow or accompany volcanic eruptions. Recent studies have also suggested that very large earthquakes, such as the magnitude 9.0 earthquake that affected the west coast of the United States in 1700, may trigger volcanic activity for several decades after their occurrence as the Earth's crust slowly adjusts to the initial movement. Seismologists can also use earthquake activity to infer the presence of magma that has not yet erupted and formed a volcano. Swarms of small earthquakes near Socorro, New Mexico, for example have helped scientists to locate a mass of molten rock about 12 mi (20 km) beneath the Earth's surface. Detailed measurements have shown that the surface is being lifted by about 2 mm per year in that area, but there are no obvious signs that a pool of molten rock lies beneath the surface.

Human activity

Explosions, especially from underground nuclear bomb testing, can produce small earthquakes. Earthquakes caused by explosions produce vibrations different than those caused by movement along faults, and seismic monitoring is an important part of nuclear test ban treaty verification. The implosive demolition of the Kingdome, a sports stadium in Seattle, in the year 2000 produced a magnitude 2.3 earthquake. Seismologists were able to deploy seismometers before the demolition and use the manmade earthquake to learn more about the geology of the area by studying how seismic waves were reflected and refracted beneath Earth's surface. Another well-known example of earthquakes due to human activity occurred at the Rocky Mountain Arsenal near Denver, Colorado, during the 1960s. The pressure of hazardous waste being injected deep into the Earth through disposal wells was large enough to trigger a series of earthquakes. A subsequent experiment in an oilfield near Rangely, Colorado, showed that earthquakes could be triggered at will be injecting water under pressure.