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What is an earthquake and what causes it?

An earthquake is the result of a sudden release of energy stored in the Earth’s crust that creates seismic waves.

Earthquakes are measured accordingly with a seismometer, commonly known as a seismograph. The magnitude of an earthquake is conventionally reported using the Richter scale or a scale related to momentum (with magnitude 3 or lower earthquakes being difficult to notice and magnitude 7 causing serious damage to large areas).

On Earth’s surface, earthquakes can be caused by a shaking or shifting of the ground. Sometimes, they cause tsunamis, which can lead to loss of life and destruction of property. An earthquake is caused by tectonic plates sticking together and placing a strain on the earth.

Earthquakes can occur naturally or as a result of human activities. Smaller earthquakes can be caused by volcanic activity, landslides, mine explosions and nuclear experiments.

In its most generic sense, the word earthquake is used to describe any seismic event whether a natural phenomenon or a man-made event that generates seismic waves. Most naturally occurring earthquakes are related to the tectonic nature of the Earth. Such earthquakes are called tectonic earthquakes.

The Earth’s lithosphere is a slowly but continuously moving plate caused by the release into space of the heat of the Earth’s mantle and core. Plate boundaries lock as the plates move past each other, creating frictional stress. When the frictional stress exceeds a critical value, called the local strength, sudden failure occurs.

The tectonic plate boundary along which failure occurs is called a fault plane. When failure results in a violent displacement of the Earth’s crust, elastic strain energy is released and seismic waves are radiated, thus causing an earthquake.

This process of strain, stress and failure has been referred to as the elastic-rebound theory. It is estimated that only 10 percent or less of the total energy of an earthquake is radiated as seismic energy. Most of the earthquake energy is used to power earthquake fracture growth and is converted to heat, or released in friction.

Most tectonic earthquakes originate at depths that do not exceed tens of kilometers. In subsurface areas, when older, colder oceanic crust descends beneath another tectonic plate, deep-focus earthquakes can occur at much greater depths (up to seven hundred kilometers).

These are earthquakes that occur at a depth at which the underlying lithosphere should no longer be fragile, due to high temperature and pressure. A possible mechanism for the generation of deep-focus earthquakes is olivine-induced faulting undergoing a phase transition to a spinel structure.

Earthquakes can also occur in volcanic regions and are caused there by both tectonic faults and the movement of magma in volcanoes. Such earthquakes can be an early warning of volcanic eruptions.

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