What is an earthquake?

　　Earthquake is a natural phenomenon in which the rapid release of energy from the Earth's crust causes vibrations and generates seismic waves. The mutual compression and collision between plates on Earth, causing displacement and rupture of plate edges and interiors, is the main cause of ground vibrations (i.e. earthquakes).

　　The location where an earthquake begins is called the epicenter, and the ground directly above the epicenter is called the epicenter. The place where the ground vibrations of destructive earthquakes are most intense is called the epicenter, which is often the area where the epicenter is located. Earthquakes often cause serious casualties, including fires, floods, toxic gas leaks, the spread of bacteria and radioactive substances, as well as secondary disasters such as tsunamis, landslides, collapses, and ground fissures.

　　According to statistics, there are over 5 million earthquakes on Earth every year, which means there are tens of thousands of earthquakes occurring every day. The vast majority of them are too small or too far away for people to feel; There are about ten to twenty earthquakes that can truly cause serious harm to humans; There are about one or two earthquakes that can cause particularly serious disasters. Earthquakes that people cannot feel must be recorded using seismometers; Different types of seismometers can record earthquakes of different intensities and distances. Thousands of various seismic instruments operate around the world day and night to monitor the movement of earthquakes.

　　What is earthquake magnitude?

　　Earthquake magnitude is a measure of the size of an earthquake given by instruments. Considering the attenuation of seismic waves during propagation, the determination of earthquake magnitude needs to take into account the depth of the earthquake and the distance from the epicenter. Nowadays, the determination of earthquakes relies on the seismic waves recorded by instruments. Different earthquake magnitudes can be obtained by taking different seismic wave phases. The commonly referred to Richter magnitude is a type of near earthquake magnitude. In addition, there are also surface wave magnitude, body wave magnitude, magnitude determined by P-phase, magnitude determined by PP phase, magnitude determined by duration, etc. There are currently more than 20 methods for determining magnitude, and the magnitude determined by different methods can be converted through certain formulas. The commonly used magnitude now is the Ms magnitude, also known as the unified magnitude, which can be directly represented by M.

　　Our country currently uses the unified magnitude Ms, and the final result is to take the average magnitude of multiple stations.

　　The magnitude of earthquakes is divided into nine levels. Generally, earthquakes below 2.5 levels are not felt by people, earthquakes above 2.5 levels are felt by people, and earthquakes above 5 levels can cause damage. Abbreviated as magnitude.

　　1. Generally, earthquakes smaller than magnitude 1 are referred to as super microseismics

　　2. Earthquakes with M ≥ 1 and less than 3 are called weak or microseismic

　　If the epicenter is not very shallow, this type of earthquake is generally not easy for people to detect.

　　3. Earthquakes with M ≥ 3 and less than 4.5 are called felt earthquakes

　　People can feel this kind of earthquake, but it generally does not cause damage.

　　4. Strong earthquakes with M ≥ 4.5 and less than 6 are called moderate earthquakes

　　It belongs to earthquakes that can cause damage, but the severity of the damage is also related to various factors such as the depth of the epicenter and the distance from the epicenter.

　　5. Earthquakes with M ≥ 6 and less than 7 are called strong earthquakes

　　6. Earthquakes with M ≥ 7 and less than 8 are called major earthquakes

　　Earthquakes with a magnitude of 7.8 or above are called massive earthquakes.

　　What is seismic intensity?

　　Seismic intensity refers to the strength of the impact of an earthquake on the earth's surface and engineering structures. (Or interpreted as the degree of earthquake impact and damage). It is not positively correlated with earthquake severity and is a macro scale determined by people's feelings during an earthquake or the degree of object response after the earthquake, the degree of damage or destruction to engineering buildings, and the changes in the ground surface without instrument records.

　　China divides the intensity into 12 degrees, and the impact and damage of earthquakes of different intensities are generally as follows:

　　Grade I; No sensation, only the instrument can record it;

　　Grade II; Individual sensitive individuals feel sensations in complete stillness;

　　Grade III; A few people indoors feel it while still, and the hanging objects sway slightly;

　　Grade IV; Most people indoors and a few people outdoors feel it, with hanging objects swaying and unstable vessels making noise;

　　V degree; Most people feel it outdoors, livestock are restless, doors and windows make noise, and cracks appear on the surface of the walls

　　VI degree; Unstable standing of people, escape of livestock, overturning of utensils, damage to rudimentary sheds, steep slopes and landslides;

　　VII degree; The house is slightly damaged, memorial archway and chimney are damaged, and cracks and sand blasting and water gushing appear on the surface;

　　VIII degree; Many houses are damaged, with a few experiencing roadbed collapse and underground pipeline rupture;

　　IX degree; Most of the houses were damaged, a few collapsed, memorial archway, chimneys, etc. collapsed, and rails were bent;

　　X degree; Houses topple, roads are damaged, mountains and rocks collapse in large numbers, and large waves hit the shore on the water surface;

　　Ⅺ degree; A large number of houses have collapsed, the roadbed and embankment have collapsed in large sections, and the surface has undergone significant changes;

　　XII degree; All buildings are generally destroyed, and animals and plants are destroyed due to drastic changes in terrain;

　　For example, the 1976 Tangshan earthquake had a magnitude of 7.8 and an epicenter intensity of 11 degrees; Affected by the Tangshan earthquake, the earthquake intensity in Tianjin was 8 degrees, the intensity in Beijing was 6 degrees, and as far as Shijiazhuang, Taiyuan, etc., it was only 4-5 degrees.

　　What are the characteristics of earthquakes in our country?

　　China is a country prone to earthquakes, with shallow earthquake sources and significant hazards. About half of the cities are located in areas with a basic intensity of 7 degrees or above; Among the 118 large cities with a population of over one million, 85.7% are located in earthquake prone areas; Nearly two-thirds of the area is located in areas with a basic intensity of 7 degrees or above.

　　There are two major seismic zones in the interior of China. One is the central seismic zone, which mainly includes Xinjiang, Qinghai, Ningxia, Gansu, Xizang, Sichuan, Yunnan and other western provinces and regions. This seismic zone is characterized by high frequency of destructive earthquakes and large magnitude, especially in Yunnan and Sichuan. It is not only dangerous for earthquakes, but also relatively dense in population;

　　The second is the Great North China Seismic Belt, which mainly includes provinces and regions such as Shaanxi, Shanxi, Hebei, Tianjin, Beijing, Liaoning, Shandong, Henan, and Inner Mongolia. Although the frequency of earthquakes in this seismic belt is lower than that in the central seismic belt, the earthquake losses are much greater in densely populated provinces than in the western region. In the last century, starting from the Xingtai earthquake, three strong destructive earthquakes occurred successively in Xingtai, Haicheng, and Tangshan within a decade.

　　The number of casualties in developing countries during earthquakes of the same intensity is tens of times that of developed countries.

　　In the past century, China has had the highest number of deaths from earthquakes in the world. The casualties in China are about ten times that of Japan.

　　Review of World Earthquake Deaths