What are earthquakes?

An earthquake, as the word itself describes, is a quake or a movement of the Earth, also known as seism from the Greek word that means shock or tremor. The Earth is a planet that is “alive”, it moves continually under our feet due to its internal dynamics and the tectonic processes. In fact the coasts of America and Europe separate a few centimetres every year. Normally the movements take place in a continuous imperceptible manner, however at times, due to the resistance opposed by the rocks, the thrust and tectonic deformations accumulate progressively like loading a spring. When the resistance of the rocks is exceeded, suddenly there is a break, and a movement along the surface of the fracture: this provokes a sudden release of energy that then spreads inside the Earth, producing a series of vibrations, the seismic waves, till they reach the surface. It is here that we note the often destructive, at times catastrophic effects of the earthquakes. The points where the rocks break and move are known as faults, the surfaces may at times be immense, kilometres long, where different rocks and also entire continents, pushed by the tectonic forces, come into contact with one another, and are deformed up to the final breakage. The size of the faults varies greatly – from enormous scars that cut across our entire planet, marking the points of contact between the various lithospherical plates, to small surfaces of only few square metres. The energy of an earthquake, however, not only depends on the extension of the surface but also on the amount of movement and the amount of energy that had accumulated before the breakage. Generally, however, seisms of a greater intensity are located near the larger faults. Together with these, usually smaller superficial movements occur, which may provoke seisms of a minor intensity, at times as a consequence of a more important seism, as in the case of the ground settling after shocks that follow the main event. The earthquake that occurred some days ago in Indonesia could in fact be of this type, a secondary movement, even though of great intensity, following the very big earthquake in December. Very active faults, that move continually, may seem dangerous because they generate a large number of small earthquakes, however the faults that do not move much and therefore get “loaded” very slowly with large quantities of energy are the ones that must be feared most.

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