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.
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