An earthquake is a natural phenomenon that causes vibrations
during the rapid release of energy from the crust.
Earthquake usually caused serious casualties and fire disaster,
flood, leaking of poisonous gas, proliferation of bacteria
and radioactive substances and even some secondary disasters,
such as tsunami, landslide, collapse, ground cracks and so on.
It is a natural phenomenon that often occurs on the earth.
The earthquake originated from a point under the ground,
which is called the source of earthquake. Vibrations spread
from it and spread through the earth. The closet point to
the source on the ground is calleed epicenter, which is
the site exposing to earliest vibration. The place that
suffered from greatest ground vibration of a destructive
earthquake is called meizoseismal area, which is often
in epicenter. Earth shaking is the most direct and universal
demonstration of earthquake. A strong earthquake that occurs
under the sea or in coastal area can cause huge waves,
which is called tsunami. Earthquake is really frequent
with about 5 million earthquakes in the world, which have a great
influence on the whole society. When earthquake occurs,
the most basic phenomenon is consist vibration of the ground,
which is mainly the obvious shaking. People in meizoseismal area
sometimes feel jump up and down because the longitudinal
waves first arrive when seismic waves are coming fron the ground.
And then the transverse wave cause great vibration horizontally,
which is the main reason of earthquake diasters.
The greatest shaking lasted three minutes during the earthquake
in Chile in 1960. The disasters caused by earthquake destroyed
residential buildings and structures and human and livestock
casualties first. For example, in the earthquake in Tangshan,
Hebei in 1976, 70%-80% buildings collapsed and a lot of people
lost their lives.
According to the causes of earthquake, it can be divided
into the following types:
Tectonic earthquake refers to dislocation and rupture of the
underground strata. This kind of earthquakes is the most frequent
and destructive, accounting for more than 90 percent of the world’s earthquake.
Volcanic earthquake refers to earthquakes that caused
by volcanic action, such as magmatic activity, gas explosion and so on.
Such earthquakes are smaller and less frequent and even
if they are, they happens in the the Karst areas of limestone
or large underground mining areas.
Induce earthquake refers to earthquake that caused
by the reservoir impoundment, oilfield water injection
and some other activities. This kind of earthquakes
often happens in certain reservoir areas or oil fields.
Artificial earthquake refers to earthquake that caused
by underground nuclear explosion and explosive blasting.
An artificial earthquake is often caused by human activity,
such as industrial blasting and underground nuclear explosion;
the flooding of high-pressure water water in deep wells
and increasing pressure of crust due to increasing water
storage will trigger earthquake.
The ground vibration caused by the earthquake is
a complex motion, which is the result of the combined
action of longitudinal and transverse waves. In the epicentral
area, longitudinal waves make the ground vibrate up and down
and transverse waves, horizontally. As the p-wave spreads faster
and decays faster while s-wave spreads slower
and decay slower, so people in place that is far away
from the epicenter, are often not aware of vibrating
up and down, but are able to feel shaking horizontally.