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Earthquakes

 

Earthquakes , the earthquakes seismós .

 

With the general name of earthquakes or earthquakes all these convulsive movements of the earth's crust is designated, that are classified as microearthquakes, when they are imperceptible; macrosismos, when they are noticed by humans causing damage to goods and houses, and megasismos, when they are so violent that can cause the destruction of buildings, the ruin of entire cities and mass casualties. The megasismos macrosismos and are known by the name of earthquakes or earthquakes. The study of seismic events is the subject of seismology.

 

The origin of 90 % of earthquakes is tectonic, fractured zones related to the failures, that its effects are felt in large parts. Other types are caused by volcanic eruptions and there is a third group of earthquakes, local calls, affecting a very small region. These are due to subsidence cave, the cavities underground mine galleries; disorders caused by layers of plaster solutions, salt or other substances, landslides or resting on clay layers.

 

The waters of the seas are agitated by the earthquakes when they occur in their substantive or costs. Sometimes it just receives a jolt, which is noted in the vessels; but how often this cause a tidal wave that propagates along the surface with the same speed as the wave of the crashing tide and the coast can cause major disasters. These large seismic waves called tsunamis and translational, name by which they are designated in Japan and tsunami.

 

An earthquake is caused by the energy liberated by the rapid movement of two crustal blocks, relative to each other. This movement originates theoretically spherical waves seismic waves, propagating in all directions from the point of maximum movement, hypocenter called the focus, and the point of the earth's surface located directly above the hypocenter where waves arrive first, the epicenter.

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HISTORY OF SEISMOLOGY

 

Those who live in earthquake zones have wondered since antiquity about the nature of this phenomenon. Some philosophers of ancient Greece attributed to subterranean winds, while others assumed they were fireworks in the depths of the Earth. By the year 130 d.C. el erudito chino Chang Heng, thinking that the waves were rushing to propagate from the source, ordered a bronze vessel to record the passage of these waves so that eight balls swaying gently in the mouths of eight dragons placed at the circumference of the vessel; a seismic wave cause the fall of one or more of these.

This and other seismic waves have been observed for centuries, but no scientific theories about the causes of earthquakes were proposed to the modern age. One of them was made by the Irish engineer Robert Mallet in 1859. Perhaps drawing on his knowledge of the strength and behavior of building materials, Mallet proposed that earthquakes were produced either by flexion and containment of elastic materials that are part of the earth's crust, well its collapse and fracture.

Later, in the early 1870, English geologist John Milne invented the predecessor of today's earthquake recording devices, the seismographs (Greek, seismos, `Agitation '). It was a suspended pendulum with a needle on a smoked glass plate; It was the first instrument used in seismology allowing discern between primary and secondary waves. The modern seismograph was invented in the early twentieth century by Russian seismologist Boris Golitzyn. Your device, equipped with a magnetic pendulum suspended between the poles of an electromagnet, initiated the modern era of seismic research.

 

The vibrations are detected by instruments called seismographs. Some are vertical pendulums great weight, that register movement through a needle or stylet, on a smoked paper. Others are horizontal and oscillate seismic shaking plotted a graph with a needle on a smoked paper rolled into a drum or cylinder which rotates uniformly.

The graph can also be indicated by a light beam incident on a photo paper, which are marked on the time intervals for hours, minutes and seconds. Others are called inverted pendulums astatic, consisting of a large mass, that remains stationary, resting on a stem. At present are electromagnetic seismographs, collecting the recording movements on magnetic tape that can be processed and digitized by computer. Through various observations and comparison of data from different observatories, can be plotted on a map the isoseismal lines, connecting points where the phenomenon is registered with the same intensity and homosistas, linking all points where vibration is seen at the same time.

Each observatory should be different types of seismographs: two horizontal, oriented along the meridian and parallel of the place and a vertical; possible to appreciate all the particulars of any earthquake.

The seismograms are graphs labeled by Stiletto seismograph, or the light beam, the role of the rotating drum. In a seismogram can differentiate several parts based on proximity or distance from the epicenter to the observatory respect.

A seismograph detects and records seismic waves an earthquake or explosion generates on earth. The illustration shows a seismograph to record vertical ground movements.

 

TYPES AND LOCATIONS OF EARTHQUAKES

 

At present, three general classes of earthquakes are recognized: tectonic, Volcanic and artificial. Earthquakes of the first are, far, the most devastating besides that pose special difficulties for scientists trying to predict.

Recent earthquakes cause plate tectonics are the tensions created by the

movements of about 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 one of them is slid in parallel to another, as in the San Andreas fault in California and Mexico, or is subducted (slides under another). Earthquakes in subduction zones are almost half of destructive seismic events and free 75% of seismic energy. They are concentrated in the so-called Ring of Fire, a narrow band of about 38.600 km in length that matches the shores of the Pacific Ocean. In these earthquakes the points where the Earth's crust is broken usually deep, up 645 km underground. I do not Alaska, the disastrous earthquake on Good Friday 1964 is an example of this case.

 

Tectonic earthquakes located outside the Ring of Fire are produced in various media. Ocean ridges (centers of seafloor spreading) They are the backdrop for many of the moderate intensity occurring at depths relatively small. Hardly anyone feels these earthquakes representing only 5% of the earth's seismic energy, but are recorded every day in the global network of seismic stations. Another scenario of tectonic earthquakes is an area extending from the Mediterranean and the Caspian Sea, through the Himalayas, ending in the Bay of Bengal. In this region, where free 15% of seismic energy, continental masses of the Eurasian plates, African and Australian meet high mountain ranges forming young. The resulting earthquakes, produced depths

between small and intermediate, have frequently devastated regions of Portugal, Algeria, Morocco, Italy, Greece, Turkey, Former Yugoslav Republic of Macedonia and other areas of the Balkan Peninsula, Iran and India.

Another category of tectonic earthquakes includes infrequent but large destructive earthquakes

produced in areas far from any other form of tectonic activity. Prime examples of this are the three massive tremors that shook the region of Missouri, in 1811 and 1812; had enough power to be felt 1.600 miles away and displacement occurred that diverted the Mississippi River. Geologists believe that these tremors were symptomatic of the forces that tear the crust, as they created the Rift Valley in Africa.

From them the lessons in tectonic earthquakes, rarely those of volcanic origin are very large or

destructive. His main interest is that usually advertise volcanic eruptions. These earthquakes are caused when magma rises filling the lower chambers of a volcano. While the slopes and summit dilate and bow, breaking rocks in voltage can be detected by a multitude of small quakes. On the island of Hawaii, seismographs can record up to one thousand small daily earthquakes before an eruption.

Humans can induce the occurrence of earthquakes when performing certain activities, for example in the filling of new dams (dams), in the underground detonation of atomic explosives or pumping fluids deep terrestrial. You can even produce sporadic tremors caused by the collapse of old underground mines.

 

EFFECTS OF EARTHQUAKES

 

Earthquakes produce different consequences that affect the inhabitants of the active seismic regions.

They can cause great loss of life to demolish structures such as buildings, bridges and dams. They also cause landslides.

Another destructive effect of earthquakes, especially submarine, They are called tidal waves. Since these waves are not related to the tides is more appropriate to call seismic waves or tsunamis, its Japanese name. These high walls of water have struck populated coastlines with enough force to destroy entire cities. In 1896, Sunriku, Japan, with a population of 20.000 people, suffered this devastating fate.

Soil liquefaction is another seismic hazard, especially where there are buildings constructed on land that has been filled. The soil used as fill may lose all consistency and behave like quicksand when subjected to shock waves of an earthquake; buildings resting on this material are swallowed underground, as occurred in 1906 in the San Francisco earthquake.

 

SCALES OF INTENSITY

 

Seismologists have two scales designed to measure quantitatively describe earthquakes.

One is the Richter scale American seismologist name Charles Francis Richter that measures the energy released at the focus of an earthquake. It is a logarithmic scale with measurable values ​​between 1 and 10; a magnitude 7 is ten times stronger than a magnitude 6, hundred times more than another of magnitude 5, thousand times more than a magnitude 4 and thus in such cases. Is estimated to occur annually in the world around 800 earthquakes with magnitudes between 5 and 6, about 50.000 with magnitudes between 3 and 4, only 1 with magnitude between 8 and 9. In theory, the Richter scale has no peak, but until 1979 believed to be the most powerful earthquake magnitude would 8,5. However, since then, progress measures seismic techniques have enabled seismologists to redefine the scale; now considered 9,5 the practical limit.

The other scale, introduced at the beginning of the twentieth century by the Italian seismologist Giuseppe Mercalli, measures the intensity of an earthquake with gradations between I and XII. Since seismic surface effects decrease with distance from the focus, Mercalli measurement depends on the position of the seismograph. A current I is defined as a sensed event by few, while intensity XII to the catastrophic events that cause total destruction is assigned. The tremor intensities between II and III are almost equivalent to the magnitude between 3 and 4 on the Richter scale, while XI and XII levels in the Mercalli scale can be associated with the magnitudes 8 and 9 on the Richter scale.

 

DEVASTATING EARTHQUAKES

 

Historical records of past earthquakes in the mid eighteenth century are almost nonexistent or unreliable. Among the ancient earthquakes for which there is reliable records which occurred in Greece in the 425 a.C., that turned on an island Euboea; which destroyed the city of Ephesus in Asia Minor in the 17 d.C.; which swept in Pompeii 63 d.C., and Rome that destroyed part of the 476 and Constantinople (now Istanbul) at 557 and in the 936. Strong earthquakes occurred in England in the Middle Ages in 1318, in Naples in 1456 and Lisbon 1531.

The earthquake 1556 that cottage cheese 800.000 people in Shaanxi (Shensi), province of China, was one of the worst natural disasters in history. In 1693 an earthquake in Sicily was a 60.000 lives; the early eighteenth century, the Japanese city of Edo (on the site of modern Tokyo) was destroyed and killed a 200.000 people. In 1755 Lisbon was devastated by an earthquake and about 60.000 people died in this disaster appears Candido, novel of French writer Voltaire. The shock was so strong that it was felt as far into the interior regions of England.

Quito, the capital of Ecuador, suffered an earthquake in 1797 in which they killed more than 40.000 people. One of the most famous earthquake was the San Francisco area of 1906 which caused extensive damage and claimed approximately 700 lives. In Latin America, August of the same year in Valparaíso, Chile, an earthquake killed a few 20.000 people; January 1939 in the city of Chillán, also in Chile, died 28.000 people. In 1970, in northern Peru died 66.794 people. The Managua earthquake, Nicaragua, the 23 December 1972 completely destroyed the city and killed more than 5.000 people. The 19 September 1985, an earthquake in Mexico City killed thousands of people. In 1988 a strong earthquake hit northern Armenia causing the death of a 25.000 people. The

magnitude earthquake 7,2 on the Richter scale occurred on 17 January 1995 in the area of ​​Hanshin-Awaji Earthquake in Japan, had a destructive effect on Kbe city where some 100.000 buildings were destroyed and perished over 6.000 people. Northeastern Turkey was shaken 1999 by an earthquake, magnitude 7,4 on the Richter scale, that killed tens of thousands of people. The 26 January 2001 an earthquake (of 7,9 degrees on the Richter scale) ravaged state Gujart in India.

Earthquake San Francisco 1906

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

Graphic Natural Disasters

catastrofes

 

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