Seismic Movements


Seismic movements are the vibrations of the Earth's crust, of short duration, but of variable intensity In this planisphere we observe the two most active seismic zones of the Earth: the Circumpacific seismic zone and the Mediterranean seismic zone. The first one affects the whole western part of America and the eastern part of Asia and Australia. This zone coincides with the tertiary orogenic zones and the two volcanic zones of the Earth.


Causes that originate the seismic movements


The seismic movements are produced by two main causes: the volcanic activity and the diastrophism.


Volcanic activity


Volcanic activity consists of the displacement of magma, or rocks in a state of fusion, from the deep layers towards the exterior, through conduits and fissures of the earth's crust .The magma moves with great force to the impulse of the gases it contains. When it rubs against the earth's crust, it makes it vibrate, producing in these cases tremors of volcanic origin. These tremors are frequent in areas where there are active volcanoes.


The diastrophism or movement of the plates


It is the movement that is produced in the terrestrial crust to the impulse of the geologic forces that come from the interior The terrestrial crust, formed by the continents and the oceanic bottoms, is fractioned in twelve tectonic plates, that we can observe them in the planisphere that is next. Each plate moves in different directions and with its displacements give rise to earthquakes, volcanism and the formation of mountain ranges or orogenic movements.


The tectonic plates and the origin of the earthquakes


In front of the coast of Ecuador, Peru and Chile is the Nazca plate, which moves slowly from west to east. Its eastern front is located in the sea trench that runs parallel to the coast, between Ecuador and Chile. In front of the Nazca plate is the South American plate, which is very large and extends from the central area of the Atlantic Ocean and much of South America, to the sea trench already mentioned. This plate moves in the opposite direction to the Nazca plate, that is from east to west. Both plates, the Nazca and the South American ones, are located in the aforementioned sea trenches . Both submerge in the interior of the earth's crust through these pits, at different speeds. As both plates rub together at great depths, they produce vibrations in the earth's crust which give rise to seismic movements, the epicentres of which are located at the bottom of the Pacific Ocean. These geological or tectonic faults are the result of seismic movements whose epicentres are not found in the marine trenches. It is worth remembering that the entire western part of South America, occupied by the Andes Mountains, is a seismic zone.


The tectonic plates and the origin of the mountain ranges


The above-mentioned tectonic plates move in the opposite direction at different speeds. The American plate , in its advance towards the sea pits, found resistance in the Nazca plate and, therefore, it folded in the western part of America, forming the Andes mountain range. The sedimentary materials accumulated in the interior of that gigantic sea (geosynelinear) folded successively during the 60 million years and it has not yet reached its end Also on the bottom of the Atlantic Ocean a gigantic mountain range is being formed, known as the MesoAtlantic. This mountain range extends from north to south, forming a gigantic S . The MesoAtlantic range is formed by the outcrop of magma or rock in a state of fusion, from the interior of the earth's crust. The magma ascends between the fronts of two tectonic plates that move, from this mountain range, in opposite directions.the udamerican plate moves in a western direction, to submerge through the marine graves located in front of the Peruvian coast, together with the Nazca plate. As a result of this displacement the Atlantic floor is expanding and Africa is moving further and further away from America.


Hypocenter and Epicenter


When a seismic movement takes place, we speak of the hypocentre and the epicentre . The hypocentre is the place where the movement originates, that is to say, where the friction of the materials that descend through it takes place, or the sliding of the rocks that cause the vibrations. The epicenter is the point located on the surface of the earth, but in a vertical line of the hypocenter, where the seismic movement is registered with greater intensity and violence. When the epicenter of a seismic movement is located at the bottom of the sea, it takes the name of a tidal wave, whose manifestations are the tsunamis or gigantic waves that invade the coasts of the continents and islands, causing great destruction. However, in many earthquakes there is an absence of such noise, because the shaking is violent. These movements are called aftershocks and are produced by the rearrangement of the rocks inside the crust.



Measurement of the Intensity of Seismic Movements


Seismic movements are measured in their intensity by seismographs, devices that are based on the principle of inertia. The scale used is the modified Mercali scale .


The Seismic Zones of the Globe


The seismic movements can be registered in any place of the Earth, but most of the epicenters are grouped in two zones that coincide with the two volcanic zones of the Globe, which are, at the same time, the two tertiary and recent orogenic zones.


The Circum-Pacific Seismic Zone


This zone coincides with the Pacific Circle of Fire and the Tertiary folds of the Andes; all of Central and Western North America; and the West Indies. Finally, on the eastern coasts of Asia and Oceania, 68% of the total number of earthquakes are recorded in this area.


The Mediterranean Seismic Zone


This zone coincides with the quasi-Asian tertiary folds: Alps, Balkans, Caucasus and the Himalayas. It extends from the southern coast of Portugal and North Africa, through the Mediterranean Sea area, Asia Minor and the Himalayan mountain range, with branches to the Insulindia, Tibet and China. In this area 21% of the seismic movements are registered. Outside these two areas, only 11% of the remaining seismic movements take place.






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