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
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
Volcanic earthquake refers to earthquakes that caused by volcanic action, such as magmatic activity, gas explosion and so on.
Collapse earthquake
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
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
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.
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