Questions about earthquakes and volcanoes

Earthquake is the rapid vibration of the earth's surface, which was also called earthquake in ancient times. Just like wind, rain, lightning, landslides and volcanic eruptions, they are natural phenomena that often occur on the earth. It originates from a point underground, which is called the focus. Vibration comes from the source and propagates in the earth. The closest point on the ground to the earthquake source is called the epicenter, which is the place where the vibration was first received. 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. There are about 5 million earthquakes around the world every year, which have a great impact on the whole society.

The occurrence of earthquakes

Earthquake is a phenomenon in which the local medium in the earth ruptures rapidly, producing seismic waves, thus causing ground vibration in a certain range. The place where the earthquake started is called the source, and the ground directly above the source is called the epicenter. The strongest ground motion of a destructive earthquake is called the extreme earthquake zone, which is often the area where the epicenter is located.

seismism

When an earthquake occurs, the most basic phenomenon is the continuous vibration of the ground, mainly the obvious shaking. People in the earthquake zone sometimes feel jumping up and down before they feel a big earthquake. This is because seismic waves travel from underground to the ground, and longitudinal waves arrive first. Shear waves then produce a large horizontal vibration, which is the main cause of earthquake disasters. 1960 during the Chile earthquake, the biggest shaking lasted for 3 minutes. The first disaster caused by the earthquake was the destruction of houses and structures, causing human and animal casualties. For example, in the 1976 Tangshan earthquake in China, 70% ~ 80% of buildings collapsed, causing heavy casualties. Earthquakes also have a great impact on the natural landscape. The main consequence is that there are faults and ground fissures on the ground. The surface faults of large earthquakes often extend from tens to hundreds of kilometers, and often have obvious vertical and horizontal offsets, which can reflect the characteristics of structural changes at the source (see the Houwei earthquake and the San Francisco earthquake). However, not all surface faults are directly related to the motion of the source, and may also be caused by the secondary influence of seismic waves. Especially in areas with thick surface sediments, ground fissures often appear at the edge of hillsides, banks of rivers and both sides of roads. This is often due to topographical factors. Without support on one side, the topsoil is loose and cracked due to shaking. The shaking of the earthquake makes the topsoil sink, and the shallow groundwater will rise to the surface along the ground fissure, forming the phenomenon of sand blasting and water inrush. A big earthquake will change the local topography, or uplift or sink. Urban and rural roads are cracked, tracks are twisted and bridges are broken. In modern cities, water, electricity and communication are blocked due to the rupture of underground pipelines and the cutting of cables. The leakage of gases, toxic gases and radioactive substances will lead to secondary disasters such as fire, poisoning and radioactive pollution. In mountainous areas, earthquakes can also cause landslides and landslides, which often lead to the tragedy of burying villages and towns. The collapsed rocks blocked the river and formed an earthquake lake upstream. 1923 During the Great Kanto Earthquake in Japan, a mudslide occurred in Kanagawa Prefecture, which went down the valley as far as 5 kilometers.

Terminology and related knowledge of earthquakes

The structure of the earth is like an egg, which can be divided into three layers. The middle layer is the "yolk"-the core; In the middle is "egg white"-mantle; The outer layer is the "eggshell"-the shell. Earthquakes usually occur in the earth's crust. The earth is constantly rotating, and the interior of the crust is constantly changing. The resulting force led to the deformation, fracture and dislocation of the crust and strata, so an earthquake occurred. The place where an earthquake occurs underground is called the source. The place from the source vertically upward to the surface is called the epicenter. The distance from the epicenter to the source is called the focal depth. Earthquakes with a focal concentration of less than 70km are shallow earthquakes, earthquakes with a focal concentration of 70km to 300km are moderate earthquakes, and earthquakes with a focal concentration of more than 300km are deep earthquakes. The earthquake with the deepest focal depth was 1963, which occurred in the northern part of Irian Jaya province, Indonesia, with a focal depth of 786 kilometers. Earthquakes of the same size have different damage to the ground because of different focal depths. The shallower the source, the greater the damage, but the smaller the spread, and vice versa.

The distance from a place to 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 farther 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 the jump up and down, but you can feel the horizontal shaking.

The magnitude of the earthquake itself is expressed by magnitude, and the magnitude is determined according to the energy of elastic waves released during the earthquake. China generally uses the Richter scale. Earthquakes with magnitude less than 2.5 are usually called small earthquakes, earthquakes with magnitude 2.5-4.7 are called inductive earthquakes, and earthquakes with magnitude greater than 4.7 are called destructive earthquakes. For every magnitude difference of 1, the energy released by the earthquake is about 30 times different. For example, an earthquake of magnitude 7 is equivalent to 30 earthquakes of magnitude 6 or 900 earthquakes with magnitude difference of 0. 1, and the average difference of energy released is 1.4 times.

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 has certain zones, called seismic zones, which are mainly concentrated in the Pacific Rim and Mediterranean-Himalayan seismic zones. The Pacific seismic belt almost concentrates more than 80% of shallow earthquakes (0 km ~ 70 km) in the world, and all moderate earthquakes (70 km ~ 300 km) and deep earthquakes release about 80% of the total energy.

The degree of ground vibration at a certain point during an earthquake is called seismic intensity. China divides the earthquake intensity into 12 degrees.

Although both magnitude and intensity can reflect the strength of an earthquake, their significance is the same. The same earthquake has only one magnitude, but the intensity varies from place to place, and the intensity value varies from place to place. For example, on1February 1990 10, an earthquake of magnitude 5. 1 occurred in Changshu-Taicang. Some people say that Suzhou is level 4 and Wuxi is level 3, which is wrong. No matter where it is, it can only be said that an earthquake of magnitude 5. 1 occurred in Changshu-Taicang, but this time, the earthquake intensities in shaxi town, Taicang, Suzhou and Wuxi were 6 degrees, 4 degrees and 3 degrees respectively.

Seismic intensity is a frequently used term. There are qualitative and quantitative standards for dividing strength. On the China Earthquake Intensity Table, people's feelings and the damage degree of ordinary houses are described, which can be used as the basic basis for determining the intensity.

Cause of earthquake

There are many reasons for the vibration of the earth's surface. According to the causes of earthquakes, earthquakes can be divided into the following categories:

1. Crustal earthquake

Earthquakes caused by dislocation and rupture of deep underground strata are called tectonic earthquakes. This kind of earthquake has the highest frequency and the greatest destructive power, accounting for more than 90% of the global earthquakes.

2. Volcanic earthquake

Earthquakes caused by volcanism, such as magmatism and gas explosion, are called volcanic earthquakes. Volcanic earthquakes can only occur in volcanic active areas, and earthquakes in volcanic active areas only account for about 7% of global earthquakes.

3. Collapse earthquake

An earthquake caused by the collapse of an underground cave or mine top is called a collapse earthquake. Such earthquakes are relatively small in scale and few in frequency. Even if it exists, it often occurs in limestone areas with dense caves or large underground mining areas.

4. Induced earthquake

Earthquakes caused by reservoir impoundment and oil field water injection are called induced earthquakes. This kind of earthquake only occurs in some specific reservoir areas or oil fields.

5. Artificial earthquake

The ground vibration caused by underground nuclear explosion and explosive blasting is called artificial earthquake. Artificial earthquakes are earthquakes caused by human activities. Such as vibration caused by industrial blasting and underground nuclear explosion; High-pressure water injection in deep wells and water storage in large reservoirs increase the pressure on the earth's crust and sometimes induce earthquakes.

The place where seismic waves are generated is called the source. The vertical projection of the source on the ground is called the epicenter. The depth from the epicenter to the source is called the focal depth. Generally, the focal depth less than 70km is called shallow earthquakes, the depth of 70-300km is called Zhongyuan earthquake, and the depth greater than 300km is called deep earthquake. Destructive earthquakes usually occur in shallow earthquakes. For example, the focal depth of the Tangshan earthquake in 1976 was 12km.

Los Angeles earthquake

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Los Angeles Earthquake-Google Pictures

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Los Angeles earthquake-Baidu pictures

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