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The Earthquake Overview

2018-09-27  |   Editor : houxue2018  

The earth has underwent earthquake throughout its geological period, Written records dated back thousands of years. In China, Researchers can get seismic data from old dynastic documents,Literature and other sources.《Bamboo annals》is the earliest chronicle book in our country, one earthquake was recorded in 1831 BC in Taishan, Shandong province. However, earlier seismic records, including hieroglyphs, were in the Middle East and the Arab region.Some of the greatest earthquakes in history lead to some of the greatest disasters. It causes massive damage in an instant.The seven wonders of the ancient world were all destroyed by earthquake.The Great Pyramid of Giza, Hanging Gardens of Babylon, The Lighthouse of Alexandria, The Temple of Artemis at Ephesus,The Mausoleum at Halicamas-sus,The Colossus of Rhodes,The Status of Zeus at Olympia(《Natural disasters》by Chen ke)

According to statistics,More than five million earthquakes occur on the earth every year,about 50,000 earthquakes can be felt among them,about 800 earthquakes have been disrupted, about 10 serious disasters created by magnitude 7 or above.

China is an earthquake-prone country,.It is also one of the countries which most affected by the earthquake,10 years from 1966 to 1976,14 earthquakes of magnitude 7 or above have occurred in mainland China. Twelve of them occurred in the Sichuan and Yunnan region of northern north china and southwest of china.The massive earthquake has inflicted serious losses to our people's life and property and social development. For example,7.8 magnitude earthquake, in July 28, 1976 in Tangshan, Hebei province,the death toll reached by 242,000; Wenchuan 8.0 magnitude earthquake in Sichuan ,in May 12, 2008,has inflicted heavy casualties and property losses. Up to 25 September 2008,The Wenchuan earthquake has killed 69,229 people, 17,923 people were missing and 374,645 were injured,direct economic losses exceeded 845.1 billion yuan.

Occurrence reason

What is earthquake? This is a question that people have been thinking and exploring.Now,Most people believe that earthquakes are caused by sudden breaks in underground rock.

The constant motion of the earth's interior inflicted massive deformation of the crust, it is the earthquake source.The mass and rapid dislocation of the surface rock is the reason of the strong ground movement. As the earth’s moving and changing,huge amounts of energy were accumulated gradually,in some vulnerable areas of the earth's crust, rock breaks suddenly,or the dislocation of the original fault,there was an earthquake,most earthquakes occur in the earth's crust.

Scientists have been unable to come up with a definitive answer up to now, but after more than 100 years of unremitting efforts, some famous seismic models have also been produced. Elastic rebound model is one of them. In 1911, Reid analyzed geodetic data on the basis of before and after the 1906 San Francisco earthquake,presented the theory of seismic elastic rebound.It is considered that when the stress on both sides of faults is greater than the rock can bear, that is to produce rupture, and earthquake occurs.

The first scientific understanding of earthquake began in China in the eastern han dynasty in 132 AD ,the appearance of Zhang heng seismograph(Fig 2-1).Seismographs are based on such a scientific understanding of the nature of earthquakes, it means that the earthquake is a distant ground shock, moreover this concept establishes a direct connection between earthquakes and seismic waves.The concept was not reconfirmed by western scientists until the 18th century,The appearance of seismograph and based on the scientific idea ,which actually represented the beginning of earthquake science.Modern seismology begins with the emergency of the precision seismographs in the late 19th century.The great San Francisco earthquake of 1906,provided the direct observational data for understanding ,what an earthquake is.The San Francisco earthquake occurred in the SAN andres fault in California, USA.During the earthquake , 3 ~ 4m right - rotation dislocation occurred in the two plates of the fault(stand on a dish of faults,watch the motion of the other plate, going to the right which be called right-handed movement,to the left which be called the sinistral movement),The fence along the farm perpendicular to the fault was clearly staggered by a distance of 3 to 4m.

Earthquakes are related to the deep structure of the earth,Continental Cenozoic compression zone、Tensile crack zone and Stable continental have different crustal and upper mantle structures,the corresponding areas have different characteristics of seismic activity. People assume that there is an anomalous mantle beneath the crust in some seismically active areas. In the upper crust of the tensile basin, listric faults are widely distributed and the large-scale ground collapsed after the earthquake, It indicates that vertical force source exists in the crust and upper mantle .Therefore, the distribution and occurrence of earthquakes in the basin, it is difficult to explain with fault elastic rebound theory.

The great earthquake occurred in the deep earth's crust below 10km, how can rocks move quickly? The earth's crust is cracked, but the friction is so great that the crack doesn't expand. There's only fluid, there's less friction, to be approximately free surface, therefore the fracture occurs. Therefore, the study of the source fluid mechanism, the source and distribution of fluid is the key to the study of earthquake genesis.

It has been the last decade that the study of the inner fluids of the earth, which presented as a major scientific issue. Researches show that , fluids play an important role in the evolution of the earth, seismic fluid mainly comes from the crust and upper mantle below the hypocenter. The inhomogeneity of underground medium is an important condition for earthquake. There are water-bearing rocks and fluid inclusions in the crust and upper mantle, high concentration of brine containing various heavy metals: There are also large amounts of hot fluid and non-biological natural gas, in addition, volatile fluids in the evolution of icebergs and magmas are also seismic fluids.

Fig 2-1 :Zhang heng seismograph

According to the study of deep crust structure and deep fluid, scientists have proposed a new model of earthquake genesis, rock mass potential transition model, the earthquake fluid origin theory for short. It is considered that there is uneven distribution of fluid in the crust, elastic strain energy or the sudden release of gravitational potential energy, large acceleration and kinetic energy are obtained for the staggered rock mass, then earthquake occurs, the model can be used to explain the migration of foreshocks, aftershocks and earthquakes, phenomena such as the precursors of fluid activity in large earthquakes, and the regularity of earthquake distribution in different structural areas

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