SEISMOLOGY AND STRUCTURE OF THE EARTH

We're going to try to explain the mechanism that represents the main cause of earthquake; this is the movement of the surface of the ground, known as tectonic of plates.

The earth can be seen as a 6350 km radius sphere. To understand the causes of earthquake, we have to understand the interior structure of the earth, this structure has been revealed studying the behavior of seismic waves, but to understand the generation of seismic waves, we have to understand the mechanism of earthquakes, and to understand the mechanism of earthquakes we have to know the structure of the earth, so this can be difficult to study, because this turns in a circle, where what we want to know becomes exactly in the research base, so all the research has to be made in an iterative procedure.

For example, the careful study of the changes experienced by seismic waves when traveling through the crust has revealed its interior structure.

The outermost layer is the crust; there are two types of crust: the oceanic crust, with thickness between 5 and 15 km; and the continental crust, with thickness between 30 and 80 km, under the crust lies the mantle subdivided in concentric layers. The exterior part of the mantle is rigid and with the crust conform what we know as lithosphere, the lithosphere has an average thickness of 60km under the oceans and 100km under the continents. Under the lithosphere is the asthenosphere that extends for about 200km.

Asthenosphere has plastic characteristics, it forms a soft plate in partial melting probably and the lithosphere is floating on the asthenosphere.

The rest of the mantle continues about 2900 km, and then appears a discontinuity that marks the beginning of the inner core.