1 agosto 2016

The origin of the earthquake

Movement by the plates, which compose the Earth's crust, causes strong stress to develop which leads to an accumulation of strain and energy deep below the ground. Subject to minor strain, rocks undergo elastic deformation: if the strain increases, they crack. When the deep stress exceeds the shear strength of the rock, rocks along surface cracks (faults) slide rapidly against one another and release the accumulated energy in the form of elastic waves (seismic waves) to create the earthquake.
The volume of rock that is the origin of the earthquake is called the hypocentre. Its projection towards the surface is called the epicentre, in other words the area that, closest to the origin of the earthquake, also suffers the worst effects.

Two kinds of waves can be generated inside the Earth’s crust, depending on the movement that affects the particles of rock during propagation. P or primary waves are longitudinal and move in a series of compressions and dilations along the direction of propagation. S or secondary waves are transverse and propagate by shaking of the particles of earth crosswise to the direction of propagation.
P waves are the fastest to the reach the surface, with S waves arriving afterwards. Then there are seismic waves that generate and propagate only along the Earth’s surface: surface waves. Waves produce either swaying or shaking effects on the surface, depending on which of the two kinds of waves prevails.
Tremors rarely occur in isolation, it is more normal to observe seismic sequences that are usually characteristics for the areas in question. In some areas seismic events are frequent but of weak intensity, so-called “earthquake swarms”. In others, a main tremor may be preceded (foreshock) and/or followed by weaker shocks (aftershocks). In any case, after a big earthquake new weaker shocks should generally be expected.
This is explained by how the phenomenon comes about because, following the rapid slip of the main fault, the lithosphere attains new balance through small aftershocks.

Every day thousands of small earthquakes occur all over the world that cause no damage but are only felt by man or measured by instruments (for example the INGV, Italian National Seismic Network, every year localises from 1700 to 2500 events of a magnitude equal to or over 2.5). This is why it is practically impossible to distinguish a foreshock from shocks that are part of the normal seismic activity of the earth’s crust.