In geophysics earthquakes (from the Latin "terrae motus", than "movement of the earth"), also known as earthquakes or tremors (Latin Tellus, the Roman goddess of the Earth), are vibrations or sudden fluctuations, rapid and more or less powerful of Earth's crust caused by the sudden displacement of an underground rock mass .

Such a displacement is generated by the forces of tectonic nature which act constantly within the earth's crust causing the release of energy in an internal area of ​​the Earth said "hypocenter", typically located on the top of pre-existing fractures of the crust said faults. The fracture generates series of elastic waves known as "seismic waves" propagate in all directions from the hypocenter, giving rise to the phenomenon observed on the surface. The place of the earth's surface located on the vertical line of hypocenter is called the "epicenter" and is generally the most affected. The branch of Geophysics which studies these phenomena is seismology.

Almost all earthquakes that occur on Earth's surface are concentrated in particular places (near the border between two tectonic plates) where contact and consists of faults: these are indeed the tectonically active areas, ie where the plates are moving more or less slowly "rubbing" or "bumping" with respect to each other generating earthquakes of intraplate. More rarely earthquakes occur away from border areas between plates, for tectonic readjustments in volcanic areas, results of deep undergrund movement of magma masses.

According to the model of plate tectonics, the movement of the plates is slow, constant and imperceptible (except for some devices), but it distorts the rocks on the surface and underground. However, sometimes and somewhere, because of the internal forces (pressures, tensions and friction) between the rock masses, such distortions stop and the involved surface accumulate tension and energy for tens or hundreds of years until, on reaching of the breaking load, the stored energy is sufficient to overcome the resisting forces causing the sudden and abrupt displacement of the rock mass involved. This sudden movement (which in a few seconds releases stored energy to tens or hundreds of years) thus generates seismic waves and the associated earthquake.

There are three types of seismic waves:

Compression or longitudinal waves (P)

The longitudinal waves force the oscillating of rock's particles in the same direction of wave propagation. They generate so "compression" and subsequently "rarefaction" in the  material in which they propagate. The propagation speed depends on the elastic characteristics of the material and its density. In general, however, travel at a speed between 4-8 m/s. Since the P waves are propagated more quickly, they are also the first (P = Primaries) to reach the seismometers, and then to be recorded by seismographs. These seismic waves longitudinally through all the Tupi of matter: solid, liquid and gas.

Shear waves or transverse (S)

S waves, or waves "secondary", propagate only in solids perpendicularly to their direction of propagation (shear waves). They are slower than P waves, traveling in the crust with a speed between 2-4 m / s. S waves can not spread through the fluids and gases because they do not resist shear. A difference of P-waves S-waves do not cause changes in volume.

Surface waves (R and L)

Surface waves, unlike what one might think, do not manifest themselves in the epicenter, but only at a certain distance from this. These waves are the result of the combination of P-waves and S-waves, and are therefore very complex. Surface waves are the ones that cause the most damage. The waves of the Rayleigh Rayleighonde, also known as R-waves, move the particles according elliptical orbits in a vertical plane along the direction of propagation, as is the case for the waves in the water. Love waves, also called L waves, moving instead the particles transversely to the direction of propagation (such as S-waves), but only on the horizontal plane. All seismic waves are subject to attenuation and with the distance depending on the characteristics of the propagation medium.