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Earthquakes

 

Earthquakes , earthquakes or tremors .

 

All these seizure-like movements of the Earth's crust is designated with the general name of earthquakes or tremors, you are on microsismos, When are imperceptible; macrosismos, When they are noticed by the man and cause damage to belongings and houses, and megasismos, When they are so violent to cause the destruction of buildings, the ruin of entire cities and large numbers of victims. The macrosismos and megasismos are known with the name of Earth tremors or earthquakes. The study of seismic phenomena is the subject of seismology.

 

The origin of the 90 % the earthquake is tectonic, related to fractured zones or failure, you leave feeling its effects in large areas. Other types are caused by volcanic eruptions, and there is a third group of seismic movements, local calls, It affects a very small region. These are due to subsidence of Caverns, underground cavities or mine galleries; disorders caused by layers of plaster solutions, salt or other substances, or landslides of land that is resting on clayey layers.

 

The waters of the seas are eventful by seismic movements when these occur at the bottom or on the coasts. Sometimes only feels a Jolt, It is noticed in the boats; but frequently formed by this cause a huge wave that propagates along the surface with the same speed as the tidal wave and crashing on the shores can cause major disasters. These seismic waves are called translation and also tsunami, name that designates in Japan or tidal waves.

 

An earthquake is caused by the energy released by the quick two blocks of the Earth's crust movement, one with respect to the other. This movement originates theoretically spherical waves seismic waves, It is spread in all directions from the point of maximum movement, called hypocenter or focus, and the Earth's surface point located on the vertical of the hypocenter where waves arrive first, the epicenter.

             

 

HISTORY OF SEISMOLOGY

 

Those living in earthquake zones have asked since ancient times about the nature of this phenomenon. Some philosophers of ancient Greece attributed them to underground winds, while others believed that they were fire in the depths of the Earth. By the year 130 d.C. the Chinese scholar Chang Heng, thinking that the waves should propagate through the Earth from the origin, It had before it a bronze vessel to register the passage of these waves in such a way that eight balls swung gently in the mouths of eight dragons located in the circumference of the pot; a seismic wave would lead to the collapse of one or more of them.

This and other forms have been observed seismic waves for centuries, but most scientific theories about the causes of earthquakes were not proposed until the modern age. One of them was made by the Irish engineer Robert Mallet in 1859. Perhaps inspired by his knowledge of the force and the behavior of building materials, Mallet proposed that earthquakes occurred well by the bending and containment of elastic materials forming part of the Earth's crust, by its collapse and fracture.

Later, in the Decade of 1870, the English geologist John Milne devised the predecessor of the current earthquake recording devices, or seismographs (from the Greek, earthquakes, 'shaking'). It was a pendulum with a needle suspended over a plate of smoked glass; It was the first instrument used in seismology allowing to distinguish between primary and secondary waves. The modern seismograph was invented in the early 20th century by the Russian seismologist Borís Golitzyn. Your device, equipped with a magnetic pendulum suspended between the poles of an electromagnet, He began the modern era of the seismic research.

 

The vibrations are detected by instruments called seismographs. Some are pendulums vertical great weight, It entered the movement through a needle or stylet, on smoked paper. Others are horizontal and to oscillate by the seismic shock plotted a graph with a needle on rolled smoked paper to a drum or cylinder rotates evenly.

The graph can also be designated by a ray of light falling on a photographic paper, which are marked time intervals for hours, minutes and seconds. Others are pendulums invested astatic called, constituted by a mass, It remains stationary, supported on a stem. At present seismographs are electromagnetic, collecting registration movements in magnetic tapes that can be processed and scanned through computers. Through various comments and comparison of data from different observatories, the isoseismal lines can be drawn on a map, you join the points in which there has been the phenomenon with the same intensity and the homosistas, that it unites all points in that vibration can be seen at the same time.

Each Observatory should have different types of seismographs: two horizontal, oriented according to the Meridian and parallel of the place and vertical one; to make it possible to appreciate all the particularities of any seismic movement.

The seismograms are the graphics marked by the stylet of the seismograph, or the bright Ray, on the role of the drum. Several parts according to the proximity or remoteness of the epicenter on the Observatory can be differentiated in a seismogram.

A seismograph detects and records the seismic waves generated by an earthquake or an explosion in the Earth. The illustration shows a seismograph to record vertical land movements.

 

TYPES AND LOCATIONS OF EARTHQUAKES

 

Are currently recognized three general classes of earthquakes: tectonic, volcanic and artificial. The first of these earthquakes are, with difference, the most devastating and pose special difficulties to scientists trying to predict them.

The recent earthquakes from plate tectonics cause are the tensions created by the

movements of the around twelve plates, major and minor, that form the Earth's crust. The majority of tectonic earthquakes occur at the boundaries between these plates, in areas where any one of them slips in parallel to another, as in the failure of San Andres in California and Mexico, or is subducted (slides under another). Earthquakes in subduction zones are almost half of the destructive seismic events and release the 75% the seismic energy. They are concentrated in the so-called ring of fire, a narrow band of some 38.600 km in length which coincides with the shores of the Pacific Ocean. The points where the Earth's crust is broken in these earthquakes tend to be deep, up to 645 miles under ground. In Alaska, the disastrous earthquake of Friday's 1964 an example of this case is.

 

Tectonic earthquakes located outside the ring of fire are produced in different media. The mid-oceanic ridges (expansion of the seabed centers) they are the scene of many of the moderate intensity which occur at relatively small depths. Almost no one feels these earthquakes which represent only a 5% land seismic energy, but are registered every day in the global network of seismological stations. Another scenario of tectonic earthquakes is an area that stretches from the Mediterranean Sea and the Caspian Sea, through the Himalayas, ending at the Bay of Bengal. In this region, where is released the 15% the seismic energy, the continental masses of Eurasian plates, African and Australian together forming mountain ranges high and young. The resulting earthquakes, produced at depths

between small and intermediate, they have frequently devastated regions of Portugal, Algeria, Morocco, Italy, Greece, Turkey, Ex−Republica Yugoslav Republic of Macedonia and other parts of the Balkan peninsula, Iran and the India.

Another category of tectonic earthquakes include the infrequent but large destructive earthquakes

produced in areas far from any other form of tectonic activity. The main examples of such situations are three massive tremors that shook the region of Missouri, in 1811 and 1812; they were powerful enough to be felt to 1.600 km of distance and produced displacement that diverted the river Mississippi. The geologists believe that these tremors were a symptom of the forces that tear the crust, such as those that created the Rift Valley in Africa.

Two kinds of non-tectonic earthquakes, of volcanic origin are rarely very large or

destructive. His main interest lies in that tend to announce volcanic eruptions. These quakes originate when magma rises to fill the lower chambers of a volcano. While the slopes and the top are dilated and lean, the breakdown of rocks in voltage can be detected thanks to a multitude of small tremors. On the island of Hawaii, seismographs can register up to one thousand small earthquakes per day before an eruption.

Humans can induce the occurrence of earthquakes when they perform certain activities, for example in new reservoirs filling (dams), in the underground detonation of Atomic explosives or pumping of liquids from the terrestrial depths. Even sporadic tremors due to underground collapse of old mines can be produced.

 

EFFECTS OF EARTHQUAKES

 

Earthquakes produce different consequences affecting the inhabitants of active seismic regions.

They can cause many losses of lives to demolish structures such as buildings, bridges and dams. They also cause landslides.

Another destructive effect of earthquakes, especially the submarines, they are the so-called tidal waves. Since these waves are not related to the tides it is more appropriate to call them seismic waves or tsunamis, its Japanese name. These high walls of water hit the shores populated with such force to destroy entire cities. In 1896, Sunriku, in Japan, with a population of 20.000 people, suffered this devastating fate.

The liquefaction of the soil is another seismic hazard, especially where there are buildings constructed on ground that has been filled. The Earth used as fill can lose all its consistency and behave like quicksand when subjected to the shock of an earthquake waves; structures that rest on this material are engulfed in the ground, as occurred in 1906 in the San Francisco earthquake.

             

INTENSITY SCALES

 

Seismologists have designed two scales of measurement to describe earthquakes quantitatively.

One is the Richter scale name the American seismologist Charles Francis Richter that measures the energy released in an earthquake focus. It is a logarithmic scale with measurable values between 1 y 10; a tremor of magnitude 7 It is ten times stronger than one of magnitude 6, a hundred times more than another of magnitude 5, thousand times than one of magnitude 4 and so in similar cases. It is estimated that annually occur in the world around 800 earthquakes with magnitudes between 5 y 6, a few 50.000 with magnitudes between 3 y 4, and only 1 with andagnitude between 8 y 9. In theory, the Richter scale has no peak, but until 1979 It was believed that possible stronger quake had magnitude 8,5. However, Since then, progress in the techniques of seismic measures have enabled seismologists redefine the scale; today is considered 9,5 the practical limit.

The other scale, introduced at the beginning of the 20th century by the Italian seismologist Giuseppe Mercalli, It measures the intensity of a quake with gradations between I and XII. Since the seismic surface effects diminish with distance from the focus, the Mercalli measure depends on the position of the seismograph. An intensity I is defined as an event seen by few, Whereas intensity XII is assigned to the catastrophic events that cause total destruction. Tremors with intensities between II and III are almost equivalent to the magnitude between 3 y 4 en la escala de Richter, mientras que los niveles XI y XII en la escala de Mercalli se pueden asociar a las magnitudes 8 y 9 en la escala de Richter.

 

DEVASTATING EARTHQUAKES

 

Historical records of previous earthquakes in the middle of the 18th century are almost non-existent or little reliable. Among the ancient earthquakes for which there are reliable records is that occurred in Greece in the 425 BC, It became to Evia island; that destroyed the city of Ephesus in Asia minor in the 17 d.C.; which destroyed Pompeii in the 6d.C.C., and those who destroyed part of Rome in the 476 and Constantinople (now Istanbul) in the 557 and in the 936. In the middle ages strong earthquakes occurred in England in 1318, in Naples in 1456 and in Lisbon in 1531.

The earthquake of 1556 who killed to 800.000 people in Shaanxi (Shensi), province of China, It was one of the greatest natural disasters of history. En 1693 an earthquake in Sicily was a 60.000 lives; at the beginning of the 18th century, the Japanese city of Edo (on the site of the modern Tokyo) It was destroyed and died a few 200.000 people. En 1755 Insbon was devastated by an earthquake and around 60.000 people died this disaster appears in Candide, novel by the French writer Voltaire. The Jolt was so strong that it felt even in the inner regions of England.

Quito, the capital of Ecuador, suffered an earthquake in 1797 that killed more than 40.000 personas. One of the most famous earthquakes was that of the San Francisco area of 1906 that caused extensive damage and killed approximately 700 lives. In Latin America, in August of that same year in Valparaiso, Chile, an earthquake killed a 20.000 personas; in January of 1939 in the city of Chillán, also in Chile, died 28.000 personas. En 1970, in the North of Peru died 66.794 personas. The earthquake of Managua, Nicaragua, the 23 December of 1972 it completely destroyed the city and killed more than 5.000 personas. El 19 September of 1985, un terremoto en la ciudaThede México provpeoplemuerte de miles de personas. En 1988 a strong earthquake shook the North of Armenia, resulting in the death of a 25.000 personas. El

earthquake of magnitude 7,2 on the scale of Richter occurred the 17 de enero de 1995 in the area of Hanshin−Awaji in Japan, It had a destructive effect on the January must where a few 100.000 buildings were destroyed and more than perished 6.000 people. The North-East of Turkey was shaken in 1999 by an earthquake, of magnitude 7,4 in the scale of Richtermientras levels 11th and 12th on the Mercalli scale can be associatede enero deantities, that caused the death of tens of thousands of people. The 26 de enero de 2001 an earthquake (of 7,9 on the scale of Richter) It struck the Gujart State in the India.

San Francisco earthquake of 1906

The San Francisco earthquake (USA) in 1906 caused the death of more of 3.000 people and affected some 28.000 buildings. With an approximate intensity of 7,9 on the Richter scale, the earthquake is still one of the largest in the history of the world. After this earthquake, the residents worked together to rebuild the city.

Graphic natural disasters

 catastrofes 257x300 Terremotos

 

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April 2014
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