Earthquake

The seismicity, or seismic activity, of an area is the frequency, type, and size of earthquakes experienced over a period of time.

Seismicity is a measure encompassing earthquake occurrences, mechanisms, and magnitude at a given geographical location.

The word tremor is also used for non-earthquake seismic rumbling.

Episodic tremor and slip (ETS) is a seismological phenomenon observed in some subduction zones that is characterized by non-earthquake seismic rumbling, or tremor, and slow slip along the plate interface.

Earthquakes can also trigger landslides and occasionally, volcanic activity.

In many cases, the landslide is triggered by a specific event (such as a heavy rainfall, an earthquake, a slope cut to build a road, and many others), although this is not always identifiable.

An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves.

Seismic waves are waves of energy that travel through the Earth's layers, and are a result of earthquakes, volcanic eruptions, magma movement, large landslides and large man-made explosions that give out low-frequency acoustic energy.

There are three main types of fault, all of which may cause an interplate earthquake: normal, reverse (thrust), and strike-slip.

An interplate earthquake is an earthquake that occurs at the boundary between two tectonic plates.

Reverse faults, particularly those along convergent plate boundaries, are associated with the most powerful earthquakes, megathrust earthquakes, including almost all of those of magnitude 8 or more.

Since 1900, all earthquakes of magnitude 9.0 or greater have been megathrust earthquakes.

Earthquakes are caused mostly by rupture of geological faults but also by other events such as volcanic activity, landslides, mine blasts, and nuclear tests.

Energy release associated with rapid movement on active faults is the cause of most earthquakes.

This process of gradual build-up of strain and stress punctuated by occasional sudden earthquake failure is referred to as the elastic-rebound theory.

In geology, the elastic-rebound theory is an explanation for how energy is released during an earthquake.

This is demonstrated by earthquake focal mechanisms.

The focal mechanism of an earthquake describes the deformation in the source region that generates the seismic waves.

The sides of a fault move past each other smoothly and aseismically only if there are no irregularities or asperities along the fault surface that increase the frictional resistance.

In geology, aseismic creep or fault creep is measurable surface displacement along a fault in the absence of notable earthquakes.

In subduction zones, where older and colder oceanic crust descends beneath another tectonic plate, deep-focus earthquakes may occur at much greater depths (ranging from ).

A deep-focus earthquake in seismology (also called a plutonic earthquake) is an earthquake with a hypocenter depth exceeding 300 km.

An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves.

Along these plate boundaries, earthquakes, volcanic activity, mountain-building, and oceanic trench formation can occur.

In subduction zones, where older and colder oceanic crust descends beneath another tectonic plate, deep-focus earthquakes may occur at much greater depths (ranging from ).

Subduction zones are sites that usually have a high rate of volcanism and earthquakes.

Earthquakes often occur in volcanic regions and are caused there, both by tectonic faults and the movement of magma in volcanoes.

Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along these plate boundaries (or faults).

These supershear earthquakes have all been observed during large strike-slip events.

A supershear earthquake is an earthquake in which the propagation of the rupture along the fault surface occurs at speeds in excess of the seismic shear wave (S-wave) velocity.

Strike-slip faults, particularly continental transforms, can produce major earthquakes up to about magnitude 8.

Slip along transform faults does not increase the distance between the ridges it separates; the distance remains constant in earthquakes because the ridges are spreading centers.

Some earthquake ruptures travel at unusually low velocities and are referred to as slow earthquakes.

A slow earthquake is a discontinuous, earthquake-like event that releases energy over a period of hours to months, rather than the seconds to minutes characteristic of a typical earthquake.

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.

A foreshock is an earthquake that occurs before a larger seismic event (the mainshock) and is related to it in both time and space.

These seismically active areas of subduction are known as Wadati–Benioff zones.

Differential motion along the zone produces numerous earthquakes, the foci of which may be as deep as about 670 km.

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, so none has a notable higher magnitude than another.

An aftershock is a smaller earthquake that follows a larger earthquake, in the same area of the main shock, caused as the displaced crust adjusts to the effects of the main shock.

Prior to the development of strong-motion accelerometers that can measure peak ground speed and acceleration directly, the intensity of the earth-shaking was estimated on the basis of the observed effects, as categorized on various seismic intensity scales.

Seismic intensity scales categorize the intensity or severity of ground shaking (quaking) at a given location, such as resulting from an earthquake.

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 1896 Sanriku earthquake.

Such events are a result of relatively slow rupture velocities.

The first scale for measuring earthquake magnitudes was developed by Charles F. Richter in 1935.

Richter is most famous as the creator of the Richter magnitude scale, which, until the development of the moment magnitude scale in 1979, quantified the size of earthquakes.

Prior to the development of strong-motion accelerometers that can measure peak ground speed and acceleration directly, the intensity of the earth-shaking was estimated on the basis of the observed effects, as categorized on various seismic intensity scales.

An accelerograph can be referred to as a strong-motion instrument or seismograph, or simply an earthquake accelerometer.

Examples are the earthquakes in Alaska (1957), Chile (1960), and Sumatra (2004), all in subduction zones.

In Alaska, the earthquake caused severe damage to roads and buildings on Adak Island, but no lives were lost.