Global earthquake distribution area,1963–1998.

The earth can be divided into three layers. The middle layer is the core, mainly composed of iron; The middle is the mantle; The outer layer is the crust. Earthquakes usually occur in the earth's crust. The interior of the crust is constantly changing, and the resulting force (that is, internal force) makes the crust strata deform, fracture and move, so an earthquake happened. Superearthquakes refer to large earthquakes with extremely strong seismic waves. However, its occurrence accounts for 7%~2 1% of the total earthquake, and the damage degree is several times that of the atomic bomb, so the influence of the super earthquake is very extensive and destructive. Earthquake is a phenomenon that the ground vibrates in a certain range by the shock wave produced by the rapid rupture in the earth. Earthquakes are the speed of the earth's surface.

Global plate tectonic movement

Rapid vibration, also known as ground motion in ancient times. Like tsunami, tornado and freezing disaster, it is a natural disaster that often happens on the earth. Ground vibration is the most intuitive and common manifestation of earthquakes. Strong earthquakes at the bottom of the sea or in coastal areas will cause huge waves, which is called tsunami. Earthquakes are extremely frequent, and there are about 5.5 million earthquakes every year in the world. Earthquakes often cause serious casualties, which can cause fires, floods, toxic gas leakage, the spread of bacteria and radioactive substances, and may also cause secondary disasters such as tsunami, landslides, collapses and ground fissures. The place where seismic waves are emitted is called the source. The vertical projection of the source on the ground, and the point closest to the source on the ground is called an earthquake.

Volcanic distribution zone of China earthquake

Medium. It was the first place to receive vibration. The depth from the epicenter to the source is called the focal depth. Usually, the focal depth is less than 60 kilometers, which is called shallow earthquakes, the depth is 60-300 kilometers, and the depth is more than 300 kilometers, which is called deep earthquake. Earthquakes of the same size have different damage degrees to the ground due to different focal depths. The shallower the source, the greater the damage, but the smaller the spread, and vice versa. Destructive earthquakes usually occur in shallow earthquakes. For example, the focal depth of the Tangshan earthquake in 1976 was 12km. The strongest ground motion of a destructive earthquake is called the extreme earthquake zone, which is often the area where the epicenter is located. The distance between the observation point and the epicenter is called epicentral distance. Earthquakes with epicentral distance less than 1000 km are called near earthquakes, earthquakes with epicentral distance between 100- 1000 km are called near earthquakes, and earthquakes with epicentral distance greater than1000 km are called teleseisms. Among them, the longer the epicentral distance, the smaller the impact and damage. The ground vibration caused by earthquake is a complex movement, which is the result of the joint action of longitudinal wave and shear wave. In the epicenter, longitudinal waves made the ground jump up and down. Shear waves make the ground shake horizontally. Because longitudinal waves travel faster and decay faster, while shear waves travel slower and decay slower, they are far from the epicenter, so you often can't feel jumping up and down, but you can feel horizontal shaking. When a large earthquake occurs in a certain place, a series of earthquakes often occur within a period of time, the largest of which is called the main earthquake, the earthquake before the main earthquake is called the foreshock, and the earthquake after the main earthquake is called the aftershock. Earthquakes have a certain temporal and spatial distribution law. In terms of time, earthquakes have periodic phenomena of alternating active periods and quiet periods. From a spatial point of view, the distribution of earthquakes is a definite zone, which is called earthquake zone. As far as continental earthquakes are concerned, they are mainly concentrated in the Pacific Rim seismic belt and the Mediterranean-Himalayan seismic belt. The Pacific seismic belt concentrates almost 80% of shallow earthquakes (0 km ~ 60 km), all moderate earthquakes (60 km ~ 300 km) and deep earthquakes (>: 300 km), and the released seismic energy accounts for about 80% of the total energy.