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Translations of Encyclopedia about Geology

 

Cause of Earthquakes

During the times of the Greek philosopher Aristotle, people believed that earthquakes are caused by underground fires and storms. In Japan, people thought that the earth was carried by a gigantic fish and that its movements were causing the earth to shake.

Today, our knowledge concerning the causes of earthquakes is fairly accurate. In 90 percent of the cases, the cause of the earthquakes is directly linked to the tectonic processes in the lithosphere. Most often, the earthquakes occur in the region of plate boundaries.

These plates move in parallel, collide, or fracture. If the plates drift apart, only slight shocks will occur. The causes of stronger shocks vary. When one of the plates slides over the other, it creates great stress, and when this stress is finally released, it causes an earthquake. One such earthquake occurred in 1985 in Mexico City, killing 10.000 people.

Horizontal movements have been observed on the San andreas fault. This fault runs on the boundary between the North American and the Pacific plates. In 1989, during the Loma Prieta earthquake, the North American plate rose one metre, while the Pacific plate moved two metres to the northwest. Fortunately, the major part of the shock waves was captured under ground.

The most common cause of earthquakes are the movements on vertically situated faults, which was the case of the earthquake in Armenia, in 1988. The earthquake hypocentre was 10 kilometres below the surface of the earth, however, the fault penetrated all the way to the surface

Earthquakes with hypocentres originating in depths of more than 100 kilometres occur most often on continental edges or island crests, which are located in the vicinity of deep-ocean trenches or young volcanic mountains, such as the island arc of Japan or the west coast of South America. Here, the plates collide in such a way, that the plate which is bent down descends under the overlapping plate into the earth's mantle.

Earthquake hypocentres occur also on surfaces sloping from the ocean towards the land. They occur generally in the depth of up to 700 kilometres. This region is called the Wadati-Benioff zone. In addition, in these regions there may be also horizontal movement. In 1995, parts of the Port Island and Rokko Island moved 4 metres! On average, the movement ranges between 10 and 20 centimetres.

Regions further away from the tectonic plates experience great earthquakes less frequently. These regions include, among others, northern Europe and the eastern portion of the United States.

Contrary to this rule, however, in 1750 there was an earthquake in London, and in 1811 a series of earthquakes occurred in Missouri (USA). There is no clear explanation for the occurrence of these shocks in the centre of continental plates. It is possible that deep down in the earth's crust there are uncharted plate boundaries, which cause these earthquakes.

Seven percent of all shocks are of volcanic origin. They are the so-called volcanic earthquakes, the shocks are linked in time, place, and cause to the volcanic activity. In general, these are localised shocks. Three percent of shocks can be classified as landslide earthquakes. They are caused by crushed ceilings of underground cavities which may have formed in many different ways. These are localised, short-range earthquakes.

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