Earthquake knowledge urgently needed

The earth can be divided into three layers. The central layer is the core, which is mainly composed of iron; the middle is the mantle; and the outer layer is the crust. Earthquakes generally occur in the earth's crust. The interior of the earth's crust is constantly changing, which produces forces (i.e. internal forces), which deform, fracture and dislocate the crustal rock layers, causing earthquakes. A super earthquake refers to a large earthquake with extremely strong shock waves. However, its occurrence accounts for 7% to 21% of the total earthquakes, and the degree of damage is several times that of an atomic bomb. Therefore, the impact of super earthquakes is very widespread and very destructive.

An earthquake is a phenomenon in which a sudden rupture inside the earth produces shock waves that cause the ground to vibrate within a certain range. An earthquake (earthquake) is a rapid vibration of the earth's surface. In ancient times, it was also called an earthquake. It is a natural disaster that often occurs on the earth, just like tsunamis, tornadoes, and freezing disasters. Earth vibration is the most intuitive and common manifestation of earthquakes. Strong earthquakes that occur under the sea or in coastal areas can cause huge waves, called tsunamis. Earthquakes are extremely frequent, with approximately 5.5 million earthquakes occurring around the world every year.

Earthquakes often cause serious casualties, can cause fires, floods, toxic gas leaks, spread of bacteria and radioactive materials, and can also cause secondary disasters such as tsunamis, landslides, collapses, and ground fissures.

The place where seismic waves originate is called the focus. The vertical projection of the earthquake source on the ground. The point on the ground closest to the earthquake source is called the epicenter. It is the earliest part to receive vibration. The depth from the epicenter to the source is called the focal depth. Usually, earthquakes with a focal depth less than 60 kilometers are called shallow earthquakes, earthquakes with a depth between 60 and 300 kilometers are called intermediate earthquakes, and earthquakes with a depth greater than 300 kilometers are called deep earthquakes. For earthquakes of the same size, due to different focal depths, the degree of damage to the ground is also different. The shallower the earthquake source, the greater the damage, but the smaller the spread, and vice versa.

Destructive earthquakes are generally shallow earthquakes. For example, the focal depth of the 1976 Tangshan earthquake was 12 kilometers.

The place where the ground vibrations of a destructive earthquake is strongest is called the epicenter, and the epicenter is often the area where the epicenter is located.

The distance between the observation point and the epicenter is called the epicentral distance. Earthquakes with an epicenter distance less than 100 kilometers are called local earthquakes, earthquakes between 100 and 1,000 kilometers are called near earthquakes, and earthquakes with an epicenter distance greater than 1,000 kilometers are called telequakes. The longer the epicenter distance, the greater the impact and damage. Small.

The ground vibration caused by earthquakes is a complex movement, which is the result of the simultaneous action of longitudinal waves and transverse waves. In the epicenter, longitudinal waves cause the ground to move up and down. Transverse waves cause the ground to shake horizontally. Since longitudinal waves propagate faster and attenuate faster, transverse waves propagate slower and attenuate slower, so in places far away from the epicenter, you often cannot feel up and down beating, but you can feel horizontal shaking.

When a large earthquake occurs somewhere, a series of earthquakes often occur within a period of time. The largest earthquake is called a mainshock. The earthquake that occurs before the mainshock is called a foreshock. Earthquakes that occur after an earthquake are called aftershocks.

Earthquakes have certain spatiotemporal distribution patterns.

From a time perspective, earthquakes have a periodic phenomenon of alternating active periods and quiet periods.

From a spatial perspective, earthquakes are distributed in a certain belt shape, called seismic zones. As far as continental earthquakes are concerned, they are mainly concentrated in the two major seismic belts: the Pacific Rim seismic zone and the Mediterranean-Himalayan seismic zone. The Pacific seismic zone contains almost 80% of the world's shallow source earthquakes (0 km to 60 km), all intermediate source earthquakes (60 km to 300 km) and deep source earthquakes (>300 km). The seismic energy released accounts for about 80% of the total energy.

Scale of earthquakes

Currently, there are two main standards for measuring the scale of earthquakes: magnitude and intensity.

Earthquake Magnitude

Earthquake magnitude is determined by the amount of energy released during an earthquake. The more energy an earthquake releases, the greater its magnitude. The largest earthquake recorded by mankind is the 9.5-magnitude earthquake that occurred in Chile on May 22, 1960. The energy released is equivalent to a hydrogen bomb with an explosive capacity of 18 million tons, or equivalent to a 1 million kilowatt power plant40 annual power generation. The energy released by the Wenchuan earthquake is approximately equivalent to a hydrogen bomb with a capacity of 900,000 tons of explosives, or the power generation capacity of a 1 million kilowatt power plant for two years (my estimate, for reference only).

The earthquake magnitude classification method jointly proposed by American seismologists Charles Francis Richter and Beno Gutenberg in 1935 is generally used internationally, which is now commonly used. Said earthquake scale on the Richter scale. The Richter scale is the base-10 logarithm of the maximum amplitude of seismic waves, and a distance of 100 kilometers from the epicenter is chosen as the standard. The energy released increases approximately 32 times for each level of enhancement on the Richter scale, and the energy difference for earthquakes of two levels is 1000 (~ 32 x 32) times.

Earthquakes smaller than 2.5 on the Richter scale are generally not easily felt by people and are called small earthquakes or microearthquakes; earthquakes with a Richter scale of 2.5-5.0 will be felt by people near the epicenter to varying degrees. About hundreds of thousands of felt earthquakes occur around the world every year; earthquakes greater than 5.0 on the Richter scale can cause varying degrees of damage to buildings and are called destructive earthquakes. Earthquakes measuring above 4.5 on the Richter scale can be detected around the world. The largest earthquake in recorded history occurred in Chile, South America, at 19:11 on May 22, 1960. According to the U.S. Geological Survey, the Richter scale reached 9.5.

Earthquakes of the same magnitude may not necessarily cause the same damage; the same earthquake may cause different damage in different places. In order to measure the extent of earthquake damage, scientists have "produced" another "ruler" - earthquake intensity. On the China Earthquake Intensity Scale, people's feelings, general house earthquake damage levels and other phenomena are described, which can be used as the basic basis for determining the intensity. Factors that affect the intensity include magnitude, focal depth, distance from the earthquake source, ground conditions and stratigraphic structure.

Generally speaking, only in terms of the relationship between intensity, source, and magnitude, the larger the magnitude, the shallower the source and the greater the intensity. Generally speaking, after an earthquake occurs, the epicenter has the heaviest damage and the highest intensity; this intensity is called the epicenter intensity. The intensity of the earthquake gradually decreases as it spreads from the epicenter to the surrounding areas. Therefore, an earthquake has only one magnitude, but the damage it causes is different in different areas. In other words, an earthquake can be divided into several areas with different intensities. This is the same as when a bomb explodes, the degree of damage near and far away is different. The amount of explosive in a bomb is like the magnitude of the earthquake; the degree of damage caused by the bomb to different locations is like the intensity. For example, on February 10, 1990, a magnitude 5.1 earthquake occurred in Changshu-Taicang. Some people said that it was magnitude 4 in Suzhou and magnitude 3 in Wuxi. This is wrong. No matter where it is, it can only be said that a 5.1-magnitude earthquake occurred in Changshu-Taicang. However, the earthquake intensity in Shaxi Town of Taicang was 6 degrees, in Suzhou it was 4 degrees, and in Wuxi it was 3 degrees. There is also a magnitude 8 earthquake that occurred in Wenchuan, Sichuan on May 12, 2008, causing great losses.

There are several different intensity scales used in countries around the world. What is more popular in Western countries is the improved Mercalli intensity scale, referred to as the M.M. intensity scale, which is divided into 12 intensity levels from 1 to 12 degrees. In Japan, no sensitivity is rated as 0 degrees, and sensitivity is divided into degrees I to VII, with a maximum of 8 levels. The former Soviet Union and China both divided intensity scales into 12 intensity levels. China re-edited the earthquake intensity scale in 1980

China Earthquake Intensity Scale

1 degree: no sense - only instruments can record;

2 degrees: slight Feeling - particularly sensitive people feel it when they are completely still;

3 degrees: Rare feeling - a few people in the room feel it when they are still, and the hanging objects swing slightly;

4 Degree: Many people feel it - most people indoors, a few people outside feel it, hanging objects swing, unstable utensils make noise;

5 degrees: awakening - most people outside feel it, domestic animals are restless, Doors and windows rattle, cracks appear on the wall surface;

6 degrees: Panic - people stand unsteadily, livestock flee, utensils fall, simple shacks are damaged, steep slopes slide;

7 Degree: House damage - minor damage to the house, archway, chimney damage, cracks on the ground surface and sand blasting and water leakage;

8 Degree: Building damage - many houses were damaged, a few damaged the roadbed, collapsed, and underground pipes ruptured ;

9 degrees: widespread destruction of buildings - most houses destroyed, a few collapsed, archways, chimneys, etc. collapsed, rails bent;

10 degrees: widespread destruction of buildings - houses collapsed , roads were destroyed, rocks collapsed in large quantities, and large waves crashed onto the shore;

11 degrees: Destruction - a large number of houses collapsed, large sections of roadbeds and embankments collapsed, and the ground surface changed greatly;

12 Degree: mountains and rivers changed scenery - all buildings were generally destroyed, the terrain changed drastically, and animals and plants were destroyed;

For example, the 1976 Tangshan earthquake had a magnitude of 7.8 and an epicenter intensity of 11 degrees; those affected by the Tangshan earthquake The earthquake intensity in Tianjin was 8 degrees, in Beijing it was 6 degrees, and in places as far away as Shijiazhuang and Taiyuan it was only 4 to 5 degrees.