WHY EARTHQUAKES OCCUR
ON EARTH
Although earthquakes to human scale phenomena seem rude, aggressive and sometimes a major marked character of uncertainty, the scale of these phenomena that take place on the planet, corresponding to soft and slow processes that respond to internal dynamics which for centuries has attracted the curiosity of men. It is perhaps due to the unexpectedness of its occurrence, or perhaps to its unusual destructive power which we are not confronted daily experience, that earthquakes correspond to one of the most striking natural phenomena to human beings, both in its social and personally.
Commonly the term is often associated with earthquake "shake the surface of the Earth" or "vibrations due to the passage of elastic waves caused by sudden movements inside the earth" (definition found in dictionaries .) However, earthquake science uses the concept applied to the phenomenon that takes place at source or where there is radiation energy.
Earthquakes can be of different types: there are those who may be accompanied by eruptions as a result of rapid volcanic magma movements, collapse of magma chambers and fissures of the same during the ascent of magma through a dyke or chimney of a volcano, and also those produced by major landslides, also there are raves rock during mining work, but by far the most important, both in terms of size (magnitude) and in number, are tectonic earthquakes. The latter are caused by a rapid slide that occurs in geological faults or by a sudden slip in the contact zone between two tectonic plates.
Due to its cooling process, the outer layers of Earth are brittle or brittle behavior and address the tectonic forces responded by fracturing. Faults are fractures in shear (cut) in which the slip occurs in a direction parallel to the fracture surface. This sliding is resisted by friction due to the fault walls are glued, welded against each other, as a result of compressive stress that exists within the Earth at depths greater than 1 to 2 km layer is brittle about 10-50 km thick, and the deformation it undergoes in response to tectonic stress field is mainly expressed located on the slip-faults. This slippage occurs almost entirely by rapid and abrupt movements, irregular in nature, constituting essentially the earthquake phenomenon. The underlying cause that can explain this behavior lies in the friction properties of many types of rocks. These, under the conditions of pressure and temperature that the Earth has in depth are such that friction has an unstable behavior known as stick-slip. This is characterized by long periods in which the rough surfaces friction remain supportive, but when the slide starts, there is a dynamic instability accompanied by a rapid and major landslide that initiates the whole process of earthquake. This instability does occur because the dynamic friction, the charge in resisting the movement once the slide has begun, is less than static friction.
Once the instability is manifested in a certain domain of the fault plane (nucleation of the earthquake), it dynamically spreads over the surface of the fault rupture speeds close to the speed of the waves shear (S waves) of the medium, the order of 3.2 km / s, and stops only where the rupture front can not overcome the static friction dynamically. Within this domain that is being created behind the front of rupture and break in front of it, is where the landslide materializes seismic (earthquake process.) The sliding speed between two points located respectively on both sides of the fault plane is typically about 1 m / s and can reach even higher values \u200b\u200bin the very front of the rupture. It is in the place of rupture where the radiation is mainly generated elastic waves of high frequencies, the overall responsibility of the high level of aggressiveness that can reach senses movements in the Earth's surface.
Looking at a global scale, earthquakes are the main agent of tectonics, the process which makes the landscape of the earth's surface. This is clearly evident when we look at a map of global seismicity distribution. Here we can clearly identify the regions most active tectonic structures of the earth. The most notable relate to plate boundaries, where is concentrated most of the deformation of the earth's surface. These limits are classified as:
• Divergent boundaries, along which the plates move apart. These include major mountain systems existing in the middle of the ocean at depths as evidenced by a narrow band of epicentres of the earthquakes. There are also opening systems (divergent) continental and one of the most notable is the one found in East Africa. The earthquakes that occur in these areas are not the greatest in the world, as the thickness of the brittle layer in these regions is rather thin and hot. The main activity in these regions is in the process of creating new ocean floor is controlled by an undersea volcanic activity.
• The second type of plate boundary type corresponds to the slip, where the best known exponent is the system of the San Andreas fault in California, USA. There attached two plates move relative to each other in a direction parallel to limit contact.
• A third type are the convergent plate boundary, which in turn are divided into two sub-classes:
• When convergence is between an oceanic plate with respect to either a continental plate (case in Chile where the Nazca oceanic plate "subducted" under the South American continental plate) or another oceanic (case Marianas where the oceanic Pacific Plate "subducted" under the Philippine oceanic plate). The subduction of one plate corresponds to a penetration of it in Earth's mantle. Area contact between two converging plates focus the bulk of the strain involved and the presence of an oceanic trench characterizes the process. Behind these graves are the island arcs formed by volcanic processes as a result of a phenomenon of progressive dehydration and partial melting of oceanic crust plate carried by the ocean in the process of penetration into the mantle.
• The other class corresponds to a convergence between two continental plates in which there is no subduction. In this case we have a huge area of \u200b\u200bcollision orogenic giving rise to phenomena such as the Himalayas.
