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Definition, causes and areas of formation

An earthquake is an earth tremor whose natural origins are below the surface of the earth. A distinction is made between tectonic, volcanic, and collapse earthquakes, depending on the cause. Collapse earthquakes, involving the collapse of subterranean cavities, are the least common and account for only 3% of all earthquake events. Volcanic earthquakes, i.e. tremors due to magma movements or subterranean explosions in volcanic areas, account for 7%. Tectonic factors are by far the commonest cause (90%) and produce the strongest earthquakes. Seismology is the study of earthquakes.

Areas of formation

More than 90% of all earthquakes occur in regions where large tectonic plates meet. Many active volcanic areas are also to be found in the same regions. A distinction is made between three boundary types, depending on the relative movement of the plates:

 

  • Convergent plate boundaries, where plates collide with each other. If an oceanic plate and a continental plate converge, the heavy oceanic crust pushes its way beneath the continental plate and descends into the mantle, where the rock melts. The area in which this happens is called a subduction zone (example: the Pacific coast of South America).
  • Divergent plate boundaries, where plates move away from each other (example: the Mid-Atlantic Ridge).
  • Transform plate boundaries, where plates move past each other horizontally (example: San Andreas Fault in California).


Owing to the different degrees of stress build-up, convergent zones produce the strongest earthquakes (Chile, 1960; Sumatra, 2004), followed by transform zones (San Francisco, 1906), and divergent zones. Since the hypocentre of quakes on convergent plate boundaries is often in the sea, these strong quakes account for less than 30% of overall global earthquake losses. There is a significant risk they will trigger a tsunami.


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