When do earthquakes typically occur

Since then, scientists have verified and refined this theory, and now have a much better understanding of how our planet has been shaped by plate-tectonic processes. We now know that, directly or indirectly, plate tectonics The deadliest earthquake of the year was a magnitude 7. According to the U.

Geological Survey USGS , was the deadliest year for earthquakes since the Renaissance Age, making it the second most fatal in recorded history, with more than , deaths reported from the magnitude 9. A magnitude 7. EDT local time on Sumatra a. The epicenter was about miles southeast of Panang or miles southwest of Singapore. At p. PDT , the magnitude 6. The epicenter was located at Third Edition Published By Tom Simkin, 1 Robert I. Tilling, 2 Peter R. Vogt 3,1 Stephen H.

Kirby, 2 Paul Kimberly, 1 and David B. Villagers in Kerauja, Nepal standing below a large rock slide that resulted in one fatality. This map shows earthquakes above magnitude 4. There are earthquakes recorded. An earthquake of magnitude 4. The circle sizes correspond to earthquake magnitude, ranging from 4. Skip to main content. Search Search. Natural Hazards.

Earthquakes can strike any location at any time, but history shows they occur in the same general patterns year after year, principally in three large zones of the earth: The world's greatest earthquake belt, the circum-Pacific seismic belt , is found along the rim of the Pacific Ocean, where about 81 percent of our planet's largest earthquakes occur.

It has earned the nickname "Ring of Fire". Why do so many earthquakes originate in this region? The belt exists along boundaries of tectonic plates, where plates of mostly oceanic crust are sinking or subducting beneath another plate.

Earthquakes in these subduction zones are caused by slip between plates and rupture within plates. Earthquakes in the circum-Pacific seismic belt include the M9. The Alpide earthquake belt extends from Java to Sumatra through the Himalayas, the Mediterranean, and out into the Atlantic.

Less than 10 percent of all earthquakes occur within plate interiors. As plates continue to move and plate boundaries change over geologic time, weakened boundary regions become part of the interiors of the plates. These zones of weakness within the continents can cause earthquakes in response to stresses that originate at the edges of the plate or in the deeper crust. The New Madrid earthquakes of and the Charleston earthquake occurred within the North American plate.

Department of the Interior U. Friction , however, holds the rocks in place, causing stresses to build. Finally, the mounting pressure overcomes the friction and a sudden movement occurs along the fault, releasing a large amount of energy. This is an earthquake. While the vast majority of earthquakes occur along faults at Earth's plate boundaries, occasionally, a quake occurs in the middle of a plate, far from any boundary.

Such quakes make up less than 10 percent of all earthquakes. Although rare and not well understood, these earthquakes are no less devastating than those that occur along plate boundaries. Earthquakes along the New Madrid Fault, along the Mississippi River in the United States, in — were among the strongest quakes ever recorded. More recently, in , an intraplate earthquake in the Gujarat region of northwesternn India killed more than 20, people. Astronauts who traveled to the moon in the late s and early s installed seismographs, devices used to measure and record vibrations, on the lunar surface.

Also called lithospheric plate. The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit.

The Rights Holder for media is the person or group credited. Tyson Brown, National Geographic Society. These swarms can be recorded by seismometers and tiltmeters a device that measures ground slope and used as sensors to predict imminent or upcoming eruptions.

A tectonic earthquake begins by an initial rupture at a point on the fault surface, a process known as nucleation. The scale of the nucleation zone is uncertain, with some evidence, such as the rupture dimensions of the smallest earthquakes, suggesting that it is smaller than m while other evidence, such as a slow component revealed by low-frequency spectra of some earthquakes, suggest that it is larger.

Once the rupture has initiated, it begins to propagate along the fault surface. The mechanics of this process are poorly understood, partly because it is difficult to recreate the high sliding velocities in a laboratory. Also the effects of strong ground motion make it very difficult to record information close to a nucleation zone.

Rupture propagation is generally modeled using a fracture mechanics approach, likening the rupture to a propagating mixed mode shear crack. The rupture velocity is a function of the fracture energy in the volume around the crack tip, increasing with decreasing fracture energy.

The velocity of rupture propagation is orders of magnitude faster than the displacement velocity across the fault. A small subset of earthquake ruptures appear to have propagated at speeds greater than the S-wave velocity. These supershear earthquakes have all been observed during large strike-slip events.

The unusually wide zone of coseismic damage caused by the Kunlun earthquake has been attributed to the effects of the sonic boom developed in such earthquakes. Some earthquake ruptures travel at unusually low velocities and are referred to as slow earthquakes. A particularly dangerous form of slow earthquake is the tsunami earthquake, observed where the relatively low felt intensities, caused by the slow propagation speed of some great earthquakes, fail to alert the population of the neighboring coast, as in the Sanriku earthquake.

Most earthquakes form part of a sequence, related to each other in terms of location and time. Most earthquake clusters consist of small tremors that cause little to no damage, but there is a theory that earthquakes can recur in a regular pattern.

An aftershock is an earthquake that occurs after a previous earthquake, the mainshock. An aftershock is in the same region of the main shock but always of a smaller magnitude. If an aftershock is larger than the main shock, the aftershock is redesignated as the main shock and the original main shock is redesignated as a foreshock. Aftershocks are formed as the crust around the displaced fault plane adjusts to the effects of the main shock.

Earthquake swarms are sequences of earthquakes striking in a specific area within a short period of time. They are different from earthquakes followed by a series of aftershocks by the fact that no single earthquake in the sequence is obviously the main shock, therefore none have notable higher magnitudes than the other. An example of an earthquake swarm is the activity at Yellowstone National Park.

Sometimes a series of earthquakes occur in what has been called an earthquake storm, where the earthquakes strike a fault in clusters, each triggered by the shaking or stress redistribution of the previous earthquakes.

Similar to aftershocks but on adjacent segments of fault, these storms occur over the course of years, and with some of the later earthquakes as damaging as the early ones.

Such a pattern was observed in the sequence of about a dozen earthquakes that struck the North Anatolian Fault in Turkey in the 20th century and has been inferred for older anomalous clusters of large earthquakes in the Middle East. Most, but not all, earthquakes occur at or near plate boundaries.

A great deal of stress is concentrated and a great deal of strain, much of it in the form of rupture of the earth, takes place at locations where two plates diverge, transform, or converge relative to each other. Tension is the dominant stress at divergent plate boundaries. Normal faults and rift valleys as the predominant earthquake-related structures at divergent plate boundaries. Earthquakes at divergent plate boundaries are usually relatively shallow, and, though they can be damaging, the most powerful earthquakes at divergent plate boundaries are not nearly as powerful as the most powerful earthquakes at convergent plate boundaries.

Transform plate boundaries are zones dominated by horizontal shear, with strike-slip faults the most characteristic fault type. Most transform plate boundaries cut through relatively thin oceanic crust, part of the structure of the ocean floor, and produce relatively shallow earthquakes that are only rarely of major magnitude.

However, where transform plate boundaries and their strike-slip faults cut through the thicker crust of islands or the even thicker crust of continents, more stress may need to build up before the thicker masses of rock will rupture, and so the magnitudes of earthquakes can be higher than in transform plate boundary zones confined to thin oceanic crust. This is evident in such places as the San Andreas fault zone of California, where a transform fault cuts through continental crust and earthquakes there sometimes exceed 7.

Convergent plate boundaries are dominated by compression. The major faults found in convergent plate boundaries are usually reverse or thrust faults, including a master thrust fault at the boundary between the two plates and typically several more major thrust faults running roughly parallel to the plate boundary.

The most powerful earthquakes that have been measured are subduction earthquakes, up to greater than 9. All subduction zones in the world are at risk of subduction earthquakes with magnitudes up to or even greater than 9. This includes the Cascadia subduction zone of northern California and coastal Oregon and Washington, the Aleutian subduction zone of southern Alaska, the Kamchatka subduction zone of Pacific Russia, the Acapulco subduction zone of southern Pacific Mexico, the Central American subduction zone, the Andean subduction zone, the West Indian or Caribbean subduction zone, and subduction zones of Indonesia, Japan, the Phillipines, and several more subduction zones in the western and southwestern Pacific Ocean.

Some earthquakes take place far away from plate boundaries. For example, Hawaii is thousands of km thousands of miles from any plate boundary, but the volcanoes that compose the islands have built up so rapidly that they are still undergoing gravitational stabilization.