Energy released on the surface

The energy released by an earthquake propagates into the rocks of the Earth’s crust and lithosphere through two types of seismic waves – P waves or primary waves, so-called because they are the first ones to reach the seismographs, and S waves or secondary waves, which are slower. P waves are compression waves, similar to sound waves. When the internal waves reach the Earth’s surface, they are modified and propagate using different mechanisms. These are the waves that we feel and the ones that provoke the greatest damage. Normally undulatory or sussultatory types of vibrations are felt. The latter, which have a strong vertical component, are the ones that potentially cause most damage. Together with the moving waves, often earthquakes are accompanied by strong rumbling: this is the effect of the propagation in the air of the compression waves. These low-frequency sounds, often infra-sounds that are near the limit of the auditory threshold , in fact often produce that particular feeling of alarm and anxiety that can even be felt during mild earthquakes, that makes one immediately distinguish between a seismic tremor and the passage, for example, of a train, with equally intense vibrations. At times particularly sharp ears can hear these sounds of the Earth even many days before: some animals, for example, dogs, pigs, fish and snakes are particularly sensitive to these sounds and can help foresee earthquakes. The duration of the vibrations in general, is a few seconds, at times some minutes (the earthquake of these days in Indonesia, lasted approximately three minutes): the gravity of the destructions that take place depends to a great extent also from the duration of the tremors. Generally an earthquake is not an isolated event, but it is preceded and followed by a series of tremors of minor intensity. Repeated tremors or settling tremors often continue for months. Apart from vibrations, an earthquake generally also produces other effects, that contribute to making the situation even more dramatic. At times large amounts of water vapour are released and perturbations in the electromagnetic field take place: this can produce optical phenomena, such as light “domes”, or electric phenomena such as lightning storms. At times gases are freed, often containing sulphur, and produce bad smelling fumes.  The movements of the Earth’s surface can also violently dislocate large masses of air, thus interacting with the atmospheric phenomena. In 1969, during an earthquake in Japan, a temporary rise, of 1.6 km, of the mass of air was observed, above the epicentre, at a height of 330 km.

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