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Seismic process

2018-10-29  |   Editor : houguangbing  

Although people feel that earthquakes often occur in the blink of an eye, but in fact, from gestation, occurrence to adjustment after earthquake, usually takes a certain time course. They can be divided into four stages: seismogenic, impending, seismogenic and aftershocks according to their physical properties.

Seismogenic stage -- the accumulation stage of ground stress. At this stage, there will be a series of abnormal changes in the geophysical properties of rocks in the seismogenic area, weak deformation and displacement will occur in rocks.

Adjoining seismic stage-- the stress accumulation in the source area is very close to the state of instability. At this time, there will be abnormal terrain changes、seismic wave velocities are abnormal, and various phenomena such as groundwater and animal activities.

Earthquake stage -- large scale rupture of rocks in the source area, or the closure of earthquake fault suddenly disconnects and bounces back, at the same time, release a large amount of strain energy, cause a strong tremor in the earth's crust.

Aftershocks phase-- in the early stages, the remaining strain energy will continue to be released, rock breaks further and produces small earthquakes: To its late stage, seismic activity will gradually calm down, geomagnetic, earth electricity, groundwater and other activities will also return to normal.

Further reading

Geological background of the Wenchuan earthquake: plate movement and Wenchuan earthquake

Experts believe that the Indian Ocean plate subducts toward the Asian plate, causing rapid uplift of the Tibetan plateau. The plateau material flows slowly to the east, extruded to eastward at the eastern edge of the plateau along the Longmen mountain tectonic belt, the stubborn block of rigid blocks encountered under the Sichuan basin, the long - term accumulation of energy resulting in tectonic stress, it was suddenly released in Yingxiu area, Beichuan, Longmen Mountain.

The disaster of Yingxiu town in the central area of Wenchuan earthquake is shown in figure 2-5a, 2/3 of the houses collapsed, 6,566 people died and 4,432 were missing from the town's 16,000 permanent residents (Source: http://sll.sinaimg.cn)

Longmen Mountain is located at the edge of Sichuan basin. It can be seen from the structural location map of Longmen Mountain fault zone (figure 2-5b),Longmen Mountain is in this place, form a "Y" glyph. Wenchuan earthquake occurred at the intersection of the "Y" glyph. As the Indian Ocean plate continues to push westward,material from the Tibetan plateau is forced to move east and southeast, Sichuan basin is a rigid body, blocking the east stream of matter. Wenchuan earthquake is in the "Y" glyph hinterland. This dragon gate fault in history is also the place where earthquakes frequently occur,Longmen Mountain earthquake just the belt of strong changes in the earth's crust, here are the intersections of several fault lines, so it's very complicated.

Fig 2-5:Yingxiu town in Wenchuan earthquake center (a); Wenchuan earthquake is located in the Longmen Mountain tectonic belt (b) (Source of information: www.chla.com.cn)

Seismogenesis

The India Ocean plate subducted to the Asian plate, resulting in the rapid uplift of the Qinghai Tibet Plateau, then causing the earthquake. Plateau material flow slowly to the east, extrude eastward along the Longmen mountain tectonic belt at the eastern margin of the plateau, meeting the stubborn block of rigid block under Sichuan basin, leading to Long-term accumulation of tectonic stress energy, finally released in the Longmen Mountain Beichuan - Yingxiu suddenly. Thrust, dextrorotation, compression - type fault earthquake. Sichuan earthquake occurred in the brittle crust-- primary transition zone, shallow focal depth, longer duration, so destructive greatly.

Earthquake type

Wenchuan earthquake is thrust、 dextrorotation、compression - type fault earthquake

Focal depth

Southern deeper, up to20km, north shallower, is10km

Sphere of influence

Including the epicenter within 50km of the county, and large and medium-sized cities within 200km range. The quake was felt obviously in Beijing, Shanghai, Tianjin, Ningxia, Gansu, Qinghai, Shaanxi, Shanxi, Shandong, Hebei, Henan, Anhui, Hubei, Hunan, Chongqing, Guizhou, Yunnan, Inner Mongolia, Guangxi, Hainan, Hong Kong, Macau, Tibet, Jiangsu, Zhejiang, Liaoning, Fujian ,Taiwan and nationwide multiple provinces (districts and cities). Earthquake of Shaanxi, Gansu and Sichuan provinces are most serious. The earthquake was felt even in Thai capital Bangkok, Hanoi, Vietnam, Philippines、Japan and other places.

Specialist detailed analysis: The Wenchuan earthquake was destructive

The Wenchuan earthquake is the most destructive and widespread earthquake since the founding of new China. The intension and intensity of the earthquake exceeded the 1976 Tangshan earthquake. Earthquake research and geological disaster research experts in China have analyzed the main reasons why the Wenchuan earthquake was more destructive than the Tangshan earthquake.

Firstly, from the magnitude, Tangshan earthquake is 7.8, Wenchuan earthquake is 8.0, Wenchuan slightly stronger.

Secondly, from the geo-mechanism of the fault dislocation, Tangshan earthquake is tensile, hanging wall fell down. Hanging wall of Wenchuan earthquake raised up, greater influence than Tangshan earthquake.

Thirdly, the fault dislocation time of the Tangshan earthquake was 12.9s, Wenchuan earthquake was 22.2s, the longer of the fault dislocation time, the longer time of people felt the strong shock, that is to say, duration of building swing in Wenchuan earthquake was longer than that in Tangshan earthquake.

Fourthly, the area affected by the Wenchuan earthquake、caused by the sudden area was larger than Tangshan earthquake.

Fifth, geological disasters and secondary disasters induced by Wenchuan earthquake were much larger than Tangshan earthquake. Because the Tangshan earthquake occurred mainly in the plain area, Wenchuan earthquake occurred mainly in mountainous areas, the types of secondary and geological disasters were different, Wenchuan earthquake triggered collapse, landslides, debris flow and other disasters, it is much more serious than the secondary geological disaster of Tangshan earthquake.

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