Earth

Earthquakes

When a rock mass moves and breaks near a fault, it generates intense vibrations that spread towards the rocks and the subsoil in all directions. These vibrations are called seismic waves and the tectonic phenomenon that provokes them is called earthquake. The origin of the rock mass movement is called hypocentre and seismic waves can move in the subsoil or the Earth’s surface in different ways: with horizontal movements (P waves), with vertical movements (S waves), or with movements that are similar to sea waves (L waves). L waves are responsible for the major damages on the Earth’s surface, for example damages to houses. If an earthquake originates in the sea, vibrations spread in the water and sea waves of up to 20 m height can be created, destroying costal areas (tsunami). The effects of an earthquake are classified and described on a table that includes 12 degrees of intensity. The first degree is an earthquake that men do not feel, and can be registered only by appropriate tools that are called seismographs. The 12th degree is an earthquake that destroys any human creation, moves rock masses, breaks the Earth’s surface into big crevices, provokes many landslides and victims. This scale is called “Mercalli”, that is M.C.S.: Mercalli-Cancalli-Sieberg, after the names of the volcanologists that studied it. Where do earthquakes occur? Earthquakes can hit the surface of continents or ocean floors, and they originate in quite well-defined seismic areas. Earthquakes are associated to volcanic activity, tectonic movements that lead to the creation of mountains, ocean ridges and ocean trenches. If you want to understand this phenomenon better, learn more about what happens on ocean floors and how mountain chains are formed, refer to the “Landscapes” section. The countries that are more hit by earthquakes are Japan, Chile, Peru, California, Greece and Italy. The study of earthquakes Seismology, the science that studies earthquakes or seismic movements, and in particular the way seismic waves move in the subsoil, studies the internal part of the Earth. In fact, the speed and direction of waves change according to the solid or melted rocks they spread through. Chemical analyses and the study of minerals allowed us to understand the composition of earth rocks only up to 100 km depth. In fact all transformation processes of magmatic, volcanic and metamorphic rocks, that make rocks go deep down or up to the surface, hardly occur at major depth. Mines allow to explore some kilometres of subsoil, but drillings do not exceed 10 kilometres of depth. If we consider that the earth radius is 6347 km, we immediately understand that most of our planet cannot be studied directly, but by analysing wave behaviour.