An earthquake is the shaking of the surface of the Earth, resulting from the sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes occur anywhere in the earth where there is sufficient stored elastic strain energy to drive fracture propagation along a fault plane.
The place that earthquake occurs called earthquake focus. The surface above the focus is called epicenter. In destructive earthquake, the area of highest degree of the ground tremble is called meizoseismal area which is also the epicenter. Earthquake may cause serious casualties, fire hazard, flood, leakage of toxic gas, spreading of bacteria and radioactive material. It will also induce serious secondary disasters, such as tsunami, landslide, collapse and ground fracture.
According to statistics, there are more than five million earthquakes occur each year, it means thousands of earthquakes occurs everyday. Most of them are too small or too far, people can not feel it. About ten to twenty earthquakes could cause serious hazard and one or two may cause severely damage to human. The earthquakes that human could not feel can be recorded by seismograph. There are thousands of seismographs to monitor the tendency of earthquake day and night.