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.
Geological background of the Wenchuan earthquake: plate movement and
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
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)
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.
Wenchuan earthquake is thrust、 dextrorotation、compression - type fault earthquake
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
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.