EarthquakeGround motion, ground vibration, yesCrustCaused by the rapid release of energyVibration, will be generated duringseismic waveOne ofnatural phenomena。The main reason for earthquakes is the mutual compression and collision between plates on the earth, which causes the dislocation and rupture of plate edges and plate interior.The place where the earthquake starts is calledSourceThe ground directly above the source is called the epicenter.The place with the strongest ground vibration of destructive earthquake is called the meizoseismal area, which is often the area where the epicenter is located.Earthquakes often cause serious casualties, which canfire、flood、Toxic gasLeakage, spread of bacteria and radioactive substances may also causetsunami、landslide、collapse、Ground fissureAnd other secondary disasters.
According to statistics,earthAbout 5 million earthquakes occur every year in the last year, that is, tens of thousands of earthquakes occur every day.Most of them are too small or too far away for people to feel;Really canhuman beingsThere were more than ten or twenty earthquakes that caused serious harm;There are about one or two earthquakes that can cause particularly serious disasters.Earthquakes that people cannot feel must be recorded with seismometers;Different types of seismometers can record earthquakes of different intensities and different distances.Thousands of various seismographs are running around the world to monitor earthquake trends day and night.The current level of science and technology cannot predict the arrival of earthquakes, and earthquakes cannot be predicted for a long time in the future.The so-called examples of successful earthquake prediction are basically coincidental.For earthquakes, what we should do more is to improve the seismic grade of buildings and make good defense, rather than predict earthquakes.
In the first quarter of 2023, 22 earthquakes of magnitude 4 or above occurred in China's mainland.[6]
Earthquakes are the seismic waves generated by the sharp rupture of the earth's interior, which cause the ground vibration within a certain range.Earthquake is the rapid vibration of the earth's surface, which is also called earthquake in ancient times.Seismicity has a certain periodicity in time.It is characterized by frequent and strong seismicity in a certain period of time, which is called the seismicity period;The seismic activity in another period is relatively low in frequency and intensity, which is called the quiet period of earthquakes.The geographical distribution of earthquakes is controlled by certain geological conditions and has certain laws.Earthquakes are mostly distributed in unstable parts of the crust, especially the extinction boundary between plates, forming an active seismic zone.
Seismic location
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The earth is divided into three layers: the central layer isearth 's core, in the middle ismantle, the outer layer isCrust。
Seismic analysis
The average radius of the earth is about 6370km, and the thickness of the crust is about 35km. Most destructive earthquakes occur in the crust.But earthquakes occur not only in the crust, but also in the asthenosphere.According to the determination of the seismological department, deep focus earthquakes generally occur 300-700 kilometers underground.So far, the deepest known source is 720km.From this point of view, the traditional theory of plate compression stratum fracture cannot reasonably explain deep focus earthquakes, because there is no solid matter at 720km depth.Scientists imagine drawing the earth's rocks, which will be very helpful for predicting earthquakes.
Seismogenesis
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Superficiallithosphere。Earthquake refers to the rapid rupture and dislocation of the earth's crust and rock stratum after being stressed, which causes surface vibration or destruction.
The earthquake caused by geological tectonic activity is called tectonic earthquake;
Earthquake caused by volcanic activity is called volcanic earthquake;
Solid rock stratum (especiallylimestone）The earthquake caused by collapse is called collapse earthquake.
Earthquake is an extremely common and common natural phenomenon. However, due to the complexity of the crustal structure and the non intuitiveness of the source area, there is still no complete answer to the question about how earthquakes, especially tectonic earthquakes, develop and occur, and what their causes and mechanisms are. However, at present, scientists generally accept that tectonic earthquakes are caused by the movement of the crustal plates.
As the earth rotates and revolves endlessly, its internal materials are also constantly differentiated, so the crust, or lithosphere, surrounding the earth's surface is also constantly generating, evolving and moving, which contributes to the global crustal tectonic movement.Scientists have gone through a long period of observation, description and analysis on crustal structure and sea land changes, and have formed different hypotheses, ideas and theories.
The plate tectonic theory, also known as the new global tectonic theory, is a theory of crustal tectonic movement that was formed late (1960s) and has been accepted by many geoscientists.
Seismic type
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Classification according to the location of occurrence
Plate edge earthquake(Plate boundary earthquakes): earthquakes that occur on the plate boundary, and most earthquakes on the Pacific Rim seismic belt belong to this category.
Intraplate earthquake: Earthquakes that occur in the interior of plates, such as those in Eurasia (including China), belong to this category.
The intraplate earthquake is not only related to the plate movement, but also affected by the local geological environment. The causes and laws of its occurrence are more complex than those of plate edge earthquakes.
Volcanic earthquakeIt refers to the crustal vibration caused by the energy shock caused by volcanic eruption.
Classification according to different vibration properties
Natural earthquake: refers to the earthquake phenomenon occurring in nature;
Artificial earthquake: Ground vibration caused by blasting, nuclear test and other factors;
Pulsation: frequent micro movements of the earth's surface caused by atmospheric activities, wave impact and other reasons.
Classification according to the causes of earthquake formation
Relationship between plate tectonics and earthquakes and volcanoes
Tectonic earthquake: It is an earthquake caused by the great changes in geological structure due to the fault of rock strata and the occurrence of deformation dislocation. Therefore, it is called tectonic earthquake, also called fault earthquake.
Volcanic earthquakeIt refers to the crustal vibration caused by the energy shock caused by volcanic eruption.Volcanic earthquakes are sometimes quite strong.However, the areas affected by such earthquakes are usually limited to tens of kilometers away from the volcano, and the number of occurrences is relatively small, accounting for only about 7% of the number of earthquakes, resulting in less harm.
Subsidence earthquake: Earthquake caused by stratum collapse.The number of such earthquakes is less, accounting for only about 3% of the total number of earthquakes. The magnitude is very small, the scope of influence is limited, and the damage is also small.
Induced earthquake: An earthquake in a specific area caused by some external crustal factors (such as meteorite falling, reservoir impoundment, deep well water injection).
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 impoundment of large reservoirs increase the pressure on the earth's crust, sometimes triggering earthquakes.
Classification according to source depth
Seismotectonics
Shallow focus earthquake: For earthquakes with focal depth less than 70km, most destructive earthquakes are shallow earthquakes.
Deep focus earthquake: For earthquakes with focal depth of more than 300 km, the focal depth of the deepest earthquake recorded in the world is 786 km so far.
In a year, about 85% of the energy released by all earthquakes in the world comes from shallow earthquakes, 12% from medium earthquakes, and 3% from deep earthquakes.
Classification by far and near earthquakes
Local earthquake: An earthquake whose epicenter is less than 100km away.
Teleseism: An earthquake whose epicenter is more than 1000 km away.
Classification by magnitude
Weak earthquake: earthquake with magnitude less than 3;
Felt earthquake: earthquakes with magnitude equal to or greater than 3 and less than or equal to 4.5;
Moderate strong earthquake: an earthquake with magnitude greater than 4.5 and less than 6;
Strong earthquake: An earthquake with magnitude equal to or greater than 6, of which magnitude greater than or equal to 8 is called a huge earthquake.
Classification by damage degree
commonlyDestructive earthquake：An earthquake that caused several to dozens of deaths, or a direct economic loss of less than 100 million yuan (including 100 million yuan);
Moderately destructive earthquake：The earthquake caused dozens to hundreds of deaths, or direct economic losses of more than 100 million yuan (excluding 100 million yuan) and less than 500 million yuan;
Severe destructive earthquake：An earthquake of magnitude 7 or above in densely populated areas, an earthquake of magnitude 6 or above in large and medium-sized cities, or an earthquake that has caused hundreds to thousands of deaths, or caused direct economic losses of more than 500 million yuan but less than 3 billion yuan;
Extremely destructive earthquake: an earthquake of magnitude 7 or above in large and medium-sized cities, or an earthquake that causes more than 10000 deaths, or a direct economic loss of more than 3 billion yuan.
Classification of tectonic earthquakes
Schematic diagram of drilling deflation to prevent earthquake
Isolated earthquake: There is a prominent main shock with few aftershocks and low intensity;The energy released by the main shock accounts for more than 99.9% of the whole sequence;The difference between the magnitude of the main shock and the maximum aftershock is more than 2.4.
Main shock - aftershock type earthquake: the main shock is very prominent, and aftershocks are very abundant;The energy released by the largest earthquake accounts for more than 90% of the whole sequence;The difference between the magnitude of the main shock and the maximum aftershock is 0.7~2.4.
Double earthquake: In a sequence of seismic activity, more than 90% of the energy is mainly released by two earthquakes with similar occurrence time, location and size.
Swarm earthquake: There are more than two main shocks of similar size, and the aftershocks are very abundant;The main energy is released through multiple earthquakes with similar magnitude, and the energy released by the largest earthquake accounts for less than 90% of the whole sequence;The difference between the magnitude of the main shock and the maximum aftershock is less than 0.7.
Earthquake distribution
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Time distribution
Global seismicity from 1901 to 2010
Through the statistics of a large amount of historical and current earthquake data, it is found that the distribution of seismic activity in time is uneven: there are many earthquakes with large magnitude in a period of time, which is calledSeismic active period；In another period, there were fewer earthquakes with smaller magnitude, which is called the quiet period of seismic activity;It shows the periodicity of seismic activity.Every active period may have multiple earthquakes of magnitude 7 or above, or even huge earthquakes of magnitude 8 or so.The seismicity period can be divided into a long period of several hundred years and a short period of several decades;The activity periods of different seismic zones are also different.Of course, some earthquakes have no period.This is related to the geological conditions. For example, Xingtai, Hebei Province, about 100 years is a cycle, because the crust of the fault zone moves regularly. When the underground energy accumulates to the point where the crust must move, earthquakes occur. This kind of earthquake has a cycle.However, not all movements are regular, and movements beyond the regular ones will lead to occasional earthquakes, which tend to have huge energy and are triggered instantaneously, not periodically.
The period of seismicity in eastern China is generally longer than that in western China.The activity cycle in the east is about 300 years, and that in the west is about 100 to 200 years.For example, there were no records of destructive earthquakes in the Weihe Plain seismic belt in Shaanxi Province from 881 AD (the end of Tang Dynasty) to 1486 AD (606 years).Decades after the 1556 Huaxian M8 earthquake, earthquakes were relatively active.After 1570, there were no earthquakes with magnitude above 6 in this area, and there were few earthquakes with magnitude around 5.
Statistics on active and quiet periods of earthquakes in China in the 20th century by the Institute of Earth Environment, Chinese Academy of Sciences
Active period
Quiet period
Earthquakes above magnitude 7 (times)
Mmax
Ⅰ
1897-1914 (18 years)
1915~1919 (5 years)
six
eight point two five
Ⅱ
1920-1934 (15 years)
1935-1945 (11 years)
eleven
eight
Ⅲ
1946-1957 (12 years)
1958~1965 (8 years)
fifteen
eight point six
Ⅳ
1966~1976 (11 years)
1977-1987 (11 years)
thirteen
seven point eight
Ⅴ
1988~1997 (10 years)
ten
seven point six
Statistics of China Earthquake Information Network on China's Earthquake Active Period in the 20th Century
Year of entry
Earthquake above magnitude 7
Death toll (10000)
remarks
Ⅰ
1895-1906
10 times
—
Incomplete data
Ⅱ
1920-1934
12 times
25-30
—
Ⅲ
1946-1955
14 times
1-2
Mainly in the Qinghai Tibet region
Ⅳ
1966-1976
14 times
twenty-one
—
Ⅴ
1988-
？
？
not finished
geographical distribution
World earthquake distribution
According to statistics, 85% of the earthquakes in the world occur on the plate boundary, and only 15% of the earthquakes have less obvious relationship with the plate boundary.The seismic zone is the zone where earthquakes are concentrated. The earthquakes are concentrated in the seismic zone, but scattered outside the seismic zone.
There are three major seismic zones in the world:
Circum Pacific seismic belt:
Earthquake distribution
It is distributed around the Pacific Ocean, including the Pacific coast of South and North America, from Aleutian Islands, Kamchatka Peninsula and Japanese Archipelago to Taiwan Province of China in the south, and then turned southeast through the Philippines to New Zealand.This is the most widely distributed and earthquake prone seismic zone in the world, which releases about three quarters of the global energy.
Eurasian seismic zone:
From the Mediterranean Sea to the east, one branch goes through Central Asia to the Himalayas, then goes south through China's Hengduan Mountains, crosses Myanmar, turns east in an arc, and arrives in Indonesia.The other branch extends from Central Asia to the northeast, to Kamchatka, and is scattered.
Seismicity zone of mid ocean ridge:
This seismic activity belt winds in the middle of the oceans, almost connected to each other.The total length is about 65000km, the width is about 1000~7000km, and the axial width is about 100km.The seismic activity of the mid ocean ridge seismic activity zone is much weaker than that of the previous two zones, and both are shallow focus earthquakes, and no major destructive earthquakes have occurred.
Continental rift seismic active zone:
Compared with the above three belts, this belt has the smallest scale and is distributed discontinuously in the interior of the continent.In terms of landform, it is often manifested as deep lakes, such as East Africa Rift Valley, Red Sea Rift Valley, Baikal Rift Valley, Gulf of Aden Rift Valley, etc.
Earthquake distribution in China
Earthquake distribution in China and surrounding areas
Seismicity in China is mainly distributed in five regions: Taiwan Province and its adjacent waters;Southwest China, including Tibet, central and western Sichuan and central and western Yunnan;The western region, mainly in Gansu Hexi Corridor, Qinghai, Ningxia and the southern and northern foot of Tianshan Mountains in Xinjiang;North China, mainly on both sides of Taihang Mountain, Fenwei River Valley, Yinshan Yanshan area, central Shandong and Bohai Bay;Southeast coastal areas, Guangdong, Fujian, etc.
From Ningxia, China, through eastern Gansu, central and western Sichuan, to Yunnan, there is a seismic intensive belt that runs through the mainland of China and is roughly north-south. Strong earthquakes have occurred many times in history, and it is called the north-south seismic belt of China.The Wenchuan 8.0 earthquake on May 12, 2008 occurred in the central and southern part of the belt.The belt extends to Mongolia in the north and Myanmar in the south.
According to geomechanics, China can be roughly divided into 20 seismic zones.
The wave whose vibration direction is consistent with the propagation direction is longitudinal wave (P wave).The longitudinal wave from underground causes the ground to vibrate up and down.
The wave whose vibration direction is perpendicular to the propagation direction is shear wave (S wave).The shear wave from underground can cause horizontal shaking of the ground.Because the propagation speed of P-wave in the earth's interior is faster than that of S-wave, the P-wave always reaches the surface first, while the S-wave always lags behind.In this way, when a large near earthquake occurs, people generally feel up and down turbulence first, and then feel strong horizontal shaking after a few seconds to ten seconds.Shear wave is the main cause of damage.
Longitudinal wave: a wave whose vibration direction is consistent with the propagation direction of the wave, and its propagation speed is fast. When reaching the ground, people feel a jolt and objects jump up and down.
Shear wave: the vibration direction is perpendicular to the propagation direction of the wave, and the propagation speed is slower than the longitudinal wave. When reaching the ground, people feel shaking, and objects will swing back and forth.
Surface wave: when the body wave reaches the rock stratum interface or the surface, it will produce a wave with large amplitude propagating along the interface or surface, which is called surface wave.The surface wave travels faster than the shear wave, so it follows the shear wave.
Schematic diagram of transmission mode
Epicenter
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Source
The place where the earth's interior breaks directly is called the source. It is a region, but it is often regarded as a point when studying earthquakes.The point on the ground directly facing the source is called the epicenter, which is actually a region.
epicenter
The epicenter measured according to seismograph records is called micro epicenter, which is expressed in longitude and latitude;The epicenter determined according to the macro seismic survey is called the macro epicenter, which is the geometric center of the meizoseismal area (the area near the epicenter with the most serious damage), also expressed in longitude and latitude.Due to different methods, the macro epicenters and micro epicenters often do not coincide.Before 1900, when there was no instrument record, the epicenter of earthquakes was determined according to the scope of damage.
Epicenter distance
The distance from the epicenter to any point on the ground is called epicenter distance.The same earthquake is observed at different distances. The distance is different and the name is different.
Source depth
The distance from the source to the ground is called the source depth.
Meizoseismal region
The area with the most serious damage after the earthquake, the meizoseismal area is often the area where the epicenter is located.
Magnitude intensity
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Earthquake magnitude
Magnitude is a measure of the size of an earthquake. It is divided according to the amount of energy released by the earthquake, and is represented by "magnitude".The scale of magnitude was originally proposed by C.F. Richter, an American seismologist, when he studied the California local earthquake in 1935. It is stipulated that the maximum horizontal amplitude (single amplitude, in μ m) recorded by the "standard seismograph" (or "Anderson seismograph", with a period of 0.8 s, an amplification factor of 2800, and a damping factor of 0.8) at a distance of 100 km from the epicenterThe common logarithm of is the magnitude of the earthquake.Later developed into remote and non-standard seismograph records, which can also be used to determine the magnitude after conversion.Different categories of magnitude, such as surface wave magnitude (MS), body wave magnitude (Mb), and local earthquake magnitude (ML), can also be converted to each other.According to Richter's calculation method, the largest known earthquake in 2000 did not exceed 8.9 magnitude;The smallest earthquake has been measured to magnitude - 3 with high magnification microseismometer.According to the magnitude, it can also be divided into ultra micro earthquakes, micro earthquakes, weak earthquakes (or small earthquakes), strong earthquakes (or medium earthquakes) and large earthquakes.[1]
magnitude
Energy in ergs
magnitude
energy
term
zero
6.3×10¹¹
five
2×10¹⁹
Erg: A unit of energy.The energy per kilowatt hour is 3.6 × 10 ¹ ³
one
2×10¹³
six
6.3×10²º
two
6.3×10¹⁴
seven
2×10²²
two point five
3.55×10¹⁵
eight
6.3×10²³
three
2×10¹⁶
eight point five
3.55×10²⁴
four
6.3×10¹⁷
eight point nine
1.4×10²⁵
According to magnitude, earthquakes can be divided into the following categories:
The magnitude of weak earthquake is less than 3.If the source is not very shallow, such an earthquake is generally not easy to detect.
The magnitude of the felt earthquake is equal to or greater than magnitude 3 and less than or equal to magnitude 4.5.This kind of earthquake can be felt by people, but generally does not cause damage.
The moderate and strong earthquakes have magnitudes greater than 4.5 and less than 6.It is an earthquake that can cause damage, but the degree of damage is also related to many factors such as focal depth, epicenter distance, etc.
The magnitude of strong earthquake is equal to or greater than 6.The earthquake with magnitude greater than or equal to 8 is also called a huge earthquake.
Earthquakes with a Richter scale of more than 4.5 can be monitored worldwide.
earthquake intensity
earthquake intensity
The damage caused by earthquakes of the same size is not necessarily the same;The same earthquake caused different damages in different places.In order to measure the extent of earthquake damage, scientists have "made" another "ruler" --earthquake intensity。On the China Earthquake Intensity Scale, the human feeling, the degree of earthquake damage to ordinary buildings and other phenomena are described, which can be used as the basic basis for determining the intensity.The factors affecting the intensity include magnitude, focal depth, distance from the source, ground conditions and stratigraphic structure.
Generally speaking, the larger the magnitude, the shallower the source and the greater the intensity.Generally, the damage in the epicenter area is the heaviest and the intensity is the highest. This intensity is calledEpicenter intensity。From the epicenter to the surrounding area, the seismic intensity gradually decreases.Therefore, an earthquake has only one magnitude, but its damage is different in different regions.An earthquake can be divided into several regions with different intensities.This is the same reason that after a bomb explodes, the damage degree is different between near and far.BombexplosiveQuantity, like magnitude;The destruction degree of the bomb to different places is like the intensity.
Intensity is not only related to magnitude, but also to focal depth, surface geological characteristics, etc.Generally speaking, earthquakes with shallow focus and large magnitude have small damage area, but the damage degree in the epicenter area is relatively heavy;Earthquakes with deeper hypocenters and larger magnitudes have larger impact areas, while the intensity in the epicenter area is lighter.
In order to evaluate the intensity in practical work, it is necessary to formulate a unified evaluation standard.This standard is called seismic intensity scale.There are several different intensity scales used in countries around the world.In western countries, the improved Magari intensity scale, or MM. The intensity table is divided into 12 intensity levels from I to I.In Japan, the non sense is rated as 0 degree, and the sense is divided into I to VII degrees, a total of 8 grades.Both the former Soviet Union and China classified the intensity scale according to 12 intensity levels.In 1980, China revised the earthquake intensity scale.
Earthquake sequence
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A series of earthquakes with common seismogenic structures that occur continuously in a certain place and within a certain period of time are called earthquake sequences.
The magnitude of the main shock is very prominent, and the released seismic wave energy accounts for more than 90% of the total energy of the whole sequence, or the difference between the maximum magnitude and the minor magnitude is 0.8-2.4, which is called the main shock type sequence;
There is not a prominent main earthquake in the earthquake sequence, but consists of two or more earthquakes with similar magnitude. The energy released by the largest earthquake generally accounts for less than 80% of the total energy of the whole sequence, or the difference between the largest magnitude and the second largest magnitude is less than 0.7, which is called earthquake swarm or multi earthquake sequence;
The magnitude of the main shock is particularly prominent. There are few foreshocks and aftershocks, and the magnitude is also very small. Large and small earthquakes are disproportionate. The maximum magnitude and sub major magnitude are greater than 2.5, which are called isolated or monogenic sequences.
Disaster damage
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The earth vibration is the most intuitive and common manifestation of earthquakes.A strong earthquake that occurs at the bottom of the sea or in coastal areas can cause huge waves, calledtsunami。A strong earthquake in the mainland will cause landslidescollapse, ground fissures and other secondary disasters.
Destructive earthquakes are generally shallow earthquakes.For earthquakes of the same size, the damage to the ground is different due to different focal depths.The shallower the source, the greater the damage, but the smaller the coverage, and vice versa.Destructive earthquakeAs in 1976tangshan earthquake The focal depth of is 12 km.
Earthquakes can be caused byseismographAs measured, the magnitude of the earthquake is used to represent the energy released by the source, expressed in terms of "Richter scale", and the intensity is expressed in terms of "revised Magari seismic intensity table".The energy released by an earthquake determines the magnitude of the earthquake. The greater the energy released, the greater the magnitude. The difference between earthquakes is one magnitude, and the energy difference is about 30 times.The magnitude difference is 0.1, and the average difference of energy released is 1.4 times.
Destructive phenomenon
Direct disaster damage
Post Tangshan Earthquake Site
Direct earthquake disasters are the primary phenomena of earthquakes, such as earthquake fault dislocation, and the disasters caused by ground vibration caused by seismic waves.It mainly includes: ground damage, damage to buildings and structures, damage to mountains and other natural objects (such as landslides, debris flows, etc.), tsunami, ground light burns, etc.
During an earthquake, the most basic phenomenon is the continuous vibration of the ground, which is characterized by obvious shaking.People in the meizoseismal area sometimes feel the jumping up and down first before they feel the big shaking.Because seismic waves come from the earth to the ground, longitudinal waves arrive first.The shear wave then produces horizontal shaking with large amplitude, which is the main cause of earthquake disaster.[2]When the Chile earthquake occurred in 1960, the maximum shaking lasted for 3 minutes.The first disaster caused by the earthquake is to destroy houses and structures, causing casualties of people and animals. For example, in the 1976 Tangshan earthquake in Hebei Province, China, 70%~80% of buildings collapsed, causing heavy casualties.
earthquake
Earthquakes also have a great impact on the natural landscape.The main consequence is that faults and ground fissures appear on the ground.The surface faults of large earthquakes often extend for tens to hundreds of kilometers, often with obvious vertical and horizontal offsets, which can reflect the characteristics of tectonic changes at the source (see Nongwei earthquake, San Francisco earthquake).However, not all surface faults are directly related to the movement of the source, and they may also be secondary effects caused by seismic waves.Especially in areas with thick surface sedimentary layer, ground fissures often appear on the slope edge, river bank and both sides of the road. This is often due to topographic factors, and shaking on one side without support causes the topsoil to collapse and collapse.The shaking of the earthquake makes the topsoil sink, and the shallow groundwater will rise to the surface along the ground fissures when squeezed, forming the phenomenon of sand blasting and water bursting.Large earthquakes can improve local topography, or uplift, or subsidence.Urban and rural roads are cracked, railway tracks are twisted, and bridges are broken.In modern cities, water supply, power supply and communication are blocked due to underground pipeline rupture and cable cutting.The leakage of gas, toxic gas and radioactive substances can lead to fire, toxic substances, radioactive pollution and other secondary disasters.In mountainous areas, earthquakes can also cause landslides and landslides, often resulting in the tragedy of burying villages and towns.Collapsed rocks block rivers, forming an earthquake lake in the upstream.
Secondary disaster
Earthquake secondary disaster is a disaster caused by the direct disaster that destroys the original balance or stability of nature or society.Mainly: fire, flood, poison gas leakage, plague, etc.Fire is the most common and serious secondary disaster.
Fire: earthquake fires are mostly caused by uncontrolled fire sources after the collapse of buildings.Because the fire fighting system is damaged after the earthquake, and the social order is chaotic, the fire is not easy to be effectively controlled, which often leads to disasters.
tsunami
Tsunami: When an earthquake occurs, the seabed strata break, and some strata rise or sink violently, causing the entire water layer from the seabed to the sea surface to shake violently, which is called an earthquake tsunami.
Plague: After a strong earthquake, the water source and water supply system in the disaster area are damaged or polluted, and the living environment in the disaster area is seriously deteriorated, so it is very easy to cause epidemics.The quality of social conditions is closely related to the prevalence of post disaster diseases.
Landslide and collapse: the secondary disasters of this kind of earthquake mainly occur in mountain areas and tableland areas. Due to the strong vibration of the earthquake, the unstable cliff or tableland slope will collapse or slide.Although this kind of secondary disaster is local, it is often destructive, and the whole village and family are buried.
Flood: earthquake may cause reservoir and river levee breach, or water overflow due to mountain collapse blocking river channel.
In addition, the development of social economy and technology also brings new secondary disasters, such as communication accidents, computer accidents, etc.The occurrence or magnitude of these disasters is often more closely related to social conditions.
Long term response characteristics of landslides to earthquakes
Professor Li Zhenhong's team from Chang'an University, together with the research team from Newcastle University in the United Kingdom and the University of Milan Bicocca in Italy, has carried out years of in-depth research on earthquake accelerated landslides, realized the first large-scale detection of earthquake accelerated landslides, and revealed the long-term response characteristics of landslides to earthquakes,The relevant scientific research achievements are entitled "Satellite radar observation reveals the triggering and recovery of earthquake accelerated landslides in central Italy", which was published in the international academic journal Nature Communication on November 29, 2022.This study revealed three post earthquake velocity evolution stages of earthquake accelerated landslides: acceleration, stability and recovery stages. This phased evolution from activation to recovery may be controlled by the gradual attenuation of earthquake energy or the closure of micro cracks generated by earthquakes.[5]
Extent of damage
In addition to the magnitude, the damage degree of earthquake disasters is also related to the depth of the source, the distance from the epicenter, the geological conditions of the epicenter area, the seismic performance of buildings, people's awareness of earthquake prevention, emergency measures and the degree of prediction and prevention.
Destructive characteristics
Earthquake disasters are instantaneous.The earthquake happens in a flash, and the time of earthquake action is very short, the shortest time is more than 10 seconds, and the longest time is two or three minutes, which causes landslides, house collapses, and makes people unprepared and unprepared.The civilization that human beings have worked hard to build is destroyed in an instant. When the earthquake broke out, people could not organize effective resistance actions in a short time.
The earthquake caused heavy casualties.The earthquake caused a large number of houses to collapse, which is the main culprit of casualties, especially some earthquakes occurred at night when people were sleeping soundly.According to the incomplete statistics of the expert group of the "International Decade for Natural Disaster Mitigation" in 1988, the total number of deaths caused by earthquake disasters in the world in the 20th century exceeded 1.2 million, of which the largest number was the 7.8 magnitude Tangshan earthquake in China on July 28, 1976, with more than 242 thousand deaths and more than 164 thousand serious injuries.From 1900 to 1986, the number of earthquake deaths accounted for 58% of all natural disaster deaths, of which China had the largest number of earthquake deaths, accounting for 42%. This was mainly due to the poor earthquake resistance capacity of buildings and dense population in China before.Statistics show that about 60% of the deaths are caused by the collapse of masonry buildings with poor seismic capacity.
Earthquake prediction
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earthquake precursor
All kinds of natural signs that may be related to earthquake preparation and occurrence before an earthquake are called earthquake precursors.There are generally two types:
Micro precursors:The changes before earthquakes that are not easily perceived by human senses and can only be measured by instruments.For example, ground deformation, changes in the earth's magnetic field and gravity field, changes in the chemical composition of groundwater, small earthquake activities, etc.
Macro precursor:Earthquake precursors that can be perceived by human senses.Most of them appear near earthquakes.Such as the rise and fall of well water, turbidity, abnormal animal behavior, ground sound, ground light, etc.
Groundwater anomaly
① Abnormal changes of water level and quantity.For example, during drought, the water level of wells rises, and the water volume of springs increases;In wet season, the water level drops or the spring water stops flowing.Sometimes, well water flows and spurts.
② Changes in water quality.For example, well water and spring water change color, taste (such as becoming bitter and sweet), become muddy, and have peculiar smell.
③ Changes in water temperature.The water temperature exceeds the normal variation range.
④ Others.For example, turning flowers and bubbling, air jet noise, shaft wall deformation, etc.
Biological anomaly
Abnormal animal behavior of earthquake precursors
Animals are "living instruments" for observing earthquake precursors. They often show various abnormal behaviors before earthquakes, which indicates the approaching of disasters to people.It has been found that hundreds of animals have some abnormal behaviors before the earthquake, of which more than 20 have common abnormal reactions. The most common animal abnormalities are:
Panic reaction: such as large livestock not entering the circle, dogs barking, birds or insects flying, abnormal group migration, etc.
Inhibitive abnormality: such as behavior becomes sluggish, or stupefied, at a loss;Or refuse to eat, etc.
Changes in life habits: for example, hibernating snakes come out of their holes, mice are not afraid of people during the day, and a large number of frogs go ashore.
Electromagnetic anomaly
electromagnetismAnomaly refers to the failure of household appliances such as radios, televisions, fluorescent lamps, etc. before the earthquake.The most common phenomena are radio failure, cell phone signal weakening or disappearing, and electronic alarm clock failure.
Ground sound
Near the earthquake, there is often a sound coming from deep underground, which is called "ground sound".The ground sound usually occurs minutes, hours, days or earlier before an earthquake;Most of them appeared a few minutes before the earthquake.
The sound of the ground voice is different and diverse from that familiar to ordinary people.Such as: "like a train coming from the ground", "like the sound of stone quarrying and firing cannons", "similar to the roar of machines", "howling wind", "the sound of stones rubbing against each other", and so on.However, sometimes it is difficult to distinguish the ground sound from the wind, thunder, and machine roar from afar.
Ground light
The ground light is also a macroscopic phenomenon before an earthquake. It has been observed before many earthquakes in China. They generally appear before or during an earthquake, but also a few hours or earlier before an earthquake.
There are many colors of ground lights, including red, yellow, blue, white, purple, and some are like electric lights.They have different shapes, such as strip light, sheet light, spherical light, cylindrical light, fire like light, etc.The appearance time of ground light is generally very short, so it is difficult to observe.It is also difficult to identify the ground light, because its shape and color are sometimes similar to welding light and lightning.
earthquake monitoring
Means and methods
(1) Earthquake measurement: record the temporal and spatial distribution and characteristics of large and small earthquakes in a region, so as to predict large earthquakes.It is often said that "a small earthquake will make a big earthquake", which is a special case of earthquake prediction.Of course, it should be noted that "minor earthquakes" do not necessarily lead to "major earthquakes".
Time and earthquake measuring instruments
(2) Crustal deformation observation: Before many earthquakes, the crustal deformation in the earthquake area increases, which can be several to dozens of times as much as usual.For example, measuring the parameters of relative vertical rise and fall or horizontal displacement on both sides of the fault is an important basis for earthquake prediction.
(3) Geomagnetic measurement: the basic magnetic field of the earth can directly reflect the physical processes of various depths of the earth and even the core of the earth. The geomagnetic field and its changes are one of the important sources of information on the physical processes in the deep earth.The study of seismomagnetic benefits has its theoretical basis and experimental basis, as well as the fact of earthquake cases.
(4) Geoelectric observation: during earthquake preparation, there will be underground media (mainly rocks)resistivityBecause these changes are related to the deformation and fracture process of rocks under stress, extracting this information can predict earthquakes.
(5) Gravity observation: the earth's gravity field is one of the relatively stable geophysical fields, which is related to the location of observation points and the density of the earth's internal media.Therefore, the crustal deformation and rock density changes can be understood through the change of gravity field, so as to predict earthquakes.
(6) In situ stress observation: regardless of the mechanism of earthquake preparation, its essence is a mechanical process, which is the result of the stress action in the crust under certain tectonic background conditions.Observing the change of crustal stress can capture the information of earthquake precursors.
(7) Dynamic observation of groundwater physics and chemistry: abnormal phenomena of groundwater dynamics before earthquakes, macro phenomena such as well water level rising, water turning and bubbling, well water changing color and taste, etc;Micro phenomena such as changes in chemical composition of water (such as changes in dissolved radon in water), changes in solid tide (fluctuations in groundwater level caused by celestial tide generating force), etc.Through the observation of groundwater dynamics, we can directly understand the influence and stress of the aquifer by the surrounding, so as to carry out earthquake prediction.
There are many regular monitoring means and forecasting methods like this.Seismologists put forward cautious analysis and prediction opinions based on the results of observation by various means and comprehensive consideration of environmental factors, tectonic conditions and geodynamic factors.
Monitoring facilities
Including monitoring instruments, equipment and facilities in the seismic station;Observation caves, instrument rooms, observation wells (water points), well rooms, observation lines, communication facilities, power supply facilities, water supply facilities, special dikes, special roads, lightning protection devices and their ancillary facilities outside the seismic station;Observation equipment, relay stations, telemetry point rooms, etc. of the receiving center of the seismic telemetry network;Special seismic survey signs, survey sites, etc.There are about 1400 earthquake monitoring stations in China, including about 700 professional stations.Of the 1400 stations, about 40% were interfered by the surrounding environment, and the observation effect was extremely unsatisfactory. Another 20% had suffered considerable damage, so they must be re selected and relocated.
Monitoring environment
The first paragraph of Article 8 of the Regulations on the Protection of Seismic Observation Facilities and the Environment for Seismic Observation stipulates that "the scope of protection of the seismic observation environment refers to the smallest area around the seismic monitoring facilities that cannot have interference sources that affect their working efficiency." Three tables with "minimum distances" are given.The Law of the People's Republic of China on Protection against and Mitigation of Earthquake Disasters stipulates that "the protection area of the earthquake observation environment shall be delimited in accordance with the requirement that there should be no interference sources around the earthquake monitoring facilities that affect their work efficiency." Usually, the minimum distance between the interference sources and the earthquake monitoring facilities is used to delimit the earthquake observation environment protection zone,For some interference sources, such as railways, electrified railways, high voltage transmission lines, power plants, building clusters, radio transmitting devices, etc., for which the minimum distance from the relevant seismic monitoring facilities is not clearly specified in the regulations or specifications, they shall be determined by the administrative department or institution for seismic work of the people's government at or above the county level in conjunction with the relevant departments through field measurement.
observed data
Data meaning
(1) In Earth ScienceBasic theoretical researchRole of
Earth science is a science based on observation. The basic theoretical research of Earth science cannot be separated from a large number of Earth observation data information.For example, deep structure of the earthGeodynamicsThe study of current crustal movement requires a large number of seismic, geomagnetic, gravity and crustal deformation data.Gu Dengbao, a famous geophysicist, said that earthquakes are the bright lights illuminating the interior of the earth.Just modernSeismic observationIn particular, digital seismic observation enables geophysicists to reveal the internal structure of the earth and the changes in the earth's internal media.Continental drift andplate tectonicsIts formation is closely associated with seismic and geomagnetic observations.Geophysicist studying in the United StatesSong XiaodongThrough the research on a large number of continuously observed seismic data, Dr. Li and the American geophysicist found the important phenomenon that the rotation speed of the earth's core is different from that of the earth's outside, which was listed as one of the major discoveries of geoscience in the twentieth century.Therefore, the basic data of geophysics and geochemistry is an important basis for human understanding of the earth and its formation, and the basis for geoscience innovation and discoveryChina Seismological BureauThere is great interest in opening up geophysical and geochemical observation data.
(2) It is widely used in national economic construction and national major engineering project decision-making
China is in the period of large-scale economic construction, and seismic scientific data is of great significance to the national economic construction and the decision-making of major national engineering projects.Large industrial and mining enterprisesnuclear power plant, reservoir, railway and expressway construction shall be subject to earthquake andgeologic hazard Safety assessment and related research work.As determined by Chinathe Project of the Three Gorges of Changjiang River、south-to-north water diversionQinghai Tibet Railway, West East Gas Transmission, etcMajor construction projectsAs well as the construction of various important facilities in the Western Development, seismic hazard zoning and earthquake prediction results of various scales, multiple geophysical observation data and active geological structure data are required as the scientific basis for solving relevant problems in the project decision-making and implementation process.
data classification
Seismic scientific data can be divided into five categories according to their access ways:
Observation data: including earthquake, geomagnetism, gravityTopographic deformation、GeoelectricityUnderground fluid, strong vibration, currentcrustal movementAnd other observation data.This is the largest category of seismic scientific data.
Detection data: including artificial earthquake, magnetotelluric, seismic mobile array and other data.
Survey data: including seismogeologyearthquake disaster , seismic field scientific research, engineering earthquake damage, earthquake damage prediction, seismic remote sensing and other data.
Experimental data: including: structural physics experiment, neotectonic age test, building structure seismic experimentGeotechnicalSeismic engineeringExperimental data.
Thematic data: This kind of data is comprehensive data, which is mainly established to serve the work objectives of an important research topic, major engineering projects, and comprehensive research in a specific region.For example: exploration and research of geoscience large cross-sectionVolcanic monitoringResearch, reservoirearthquake monitoringResearch, mine earthquake monitoring research, typical earthquake damage, crustal stress environment data of mainland China, Three Gorges Project, Qinghai Tibet Railway, seismic safety evaluation of buildings and other data.
Seismograph
Seismograph restoration model
In 132 AD, Zhang Heng, a scientist in the Eastern Han Dynasty, invented the world's first seismograph——SeismographAnd has been verified in practical application.Unfortunately, the object and pattern of the seismograph were lost, leaving only written records, and the object gradually became a mystery for thousands of years.
The records of Zhang Heng's seismograph can be found in three historical books, namely, the History of the Continued Han Dynasty (Sima Biao), the History of the Later Han Dynasty (Yuan Hong), and the History of the Later Han Dynasty (Fan Ye).These historical materials describe the appearance, internal structure, working process and seismic observation of seismograph.In the long years that followed, many people at all times and in all over the world tried to restore the seismograph, but no successful restoration model appeared. Most of them were in the conceptual model stage, or inconsistent with the historical records, or the restored physical model could not work normally.
After 2002, with the support of the China Seismological Bureau and the State Administration of Cultural Heritage, the research group of "Zhang Heng Seismometer Scientific Restoration" was established, which is composed of experts from China Seismological Network Center, Academy of Fine Arts of Tsinghua University, National Museum, Beijing Institute of Mechanical Industry Automation, Henan Museum and other disciplines.The research group has established a new seismograph restoration model, realizing the leap from conceptual model to scientific model.In 2005, it passed the expert appraisal and national acceptance.In August 2008, the small casting of the finalized model was completed.
earthquake prediction
The medium and long-term earthquake prediction refers to the medium and long-term earthquake prediction.It is called long-term earthquake prediction to predict the earthquakes that may occur in a certain place in a few years to decades, or even hundreds of years.The prediction of earthquakes that may occur in a certain place in a few months to years is called medium-term earthquake prediction.It is called short-term earthquake prediction to predict the earthquake that may occur in a certain place in a few days to dozens of days, or even months.It is called impending earthquake prediction to make predictions of earthquakes that may occur in a certain place within a few hours to a few days.
The main purpose of medium - and long-term earthquake prediction, especially long-term earthquake prediction, is to predict the area, time range and maximum earthquake intensity that may occur, and to analyze the earthquake trend in a certain area.
Short term prediction, especially imminent earthquake prediction, requires the location, time and magnitude of the earthquake to be determined quickly, on the spot and accurately so that necessary and resolute preventive measures can be taken before the arrival of a strong earthquake.
Short term prediction should be based on medium and long term prediction, and imminent earthquake prediction is based on short term prediction.However, it is impossible to strictly distinguish the work of earthquake prediction from one another.
scientific research institution
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Some well-known international scientific research institutions in the field of seismology include:
(1) Determination of seismic fortification requirements: formulate zoning map, carry out seismic sub zoning, and carry out seismic safety assessment
(2) Seismic design: design in accordance with seismic fortification requirements and seismic design specifications
(3) Seismic construction: construction shall be carried out according to the seismic design
In short, it is to set up measures to prevent earthquake disasters during project construction, which involves the whole process of project planning, site selection, engineering design and construction, and completion acceptance.
Choose a good construction site. Never build a house on a site that is not conducive to earthquake resistance. The sites that are not conducive to earthquake resistance are:
(1) Active fault and its vicinity;
(2) Water saturated loose sand layer, soft silt layer, soft artificial fill layer;
(3) Ancient rivers, old ponds and river beaches;
(4) Steep slopes and river banks prone to cracking, subsidence and sliding;
(5) Slender and protruding mountain mouth, towering hill or terrace facing paddy fields on three sides.
Housing environment
(1) Under tall and large buildings (structures) or other high suspended objects: tall buildings, tall chimneys, water towers, tall billboards, etc., which are easy to collapse and threaten the safety of houses in an earthquake;
(2) Under dangerous objects such as high-voltage lines and transformers: electrical appliances are prone to fire due to short circuit during earthquakes, which often endangers housing and personal safety;
(3) Near the production place or warehouse of dangerous goods: if the factory is damaged during the earthquake, resulting in toxic gas leakage, gas explosion and other accidents, housing will be endangered.
Note: The atlas data is from China Earthquake Information Network
Building reinforcement
In order to resist the sudden attack of earthquake, we should always pay attention to the maintenance of old houses.If there are cracks or slanting flashes on the wall, it should be repaired in time;The soil wall that is easy to be weathered, crisp and alkaline shall be plastered regularly;The roof leakage shall be repaired quickly;After the heavy rain, the ponding around the house shall be removed immediately to avoid long-term soaking of the wall foundation.Wooden beams and columns shall be prevented from decay and moth eaten. If they are damaged, they shall be repaired in time.
If necessary, the house shall be simply strengthened by strengthening the wall.There are two kinds of walls, one is load-bearing wall, the other is non load-bearing wall.The reinforcement methods include brick removal and joint filling, reinforcement reinforcement, wall attachment reinforcement, etc.
Reinforcement of buildings and roof.Generally, the method of cement mortar refilling and reinforcement thickening is adopted.
Reinforcement of protruding parts of buildings.For example, for chimney, parapet, water tank room out of the roof, staircase and other parts, appropriate measures shall be taken to set vertical braces and remove unnecessary accessories.
Earthquake emergency
[Possible damage]
Direct injury:Injured by falling objects or collapsing houses indoors;Injured by collapsed buildings outside;Injured by rolling stones on the mountain in the wild;Burned by the ground light.
Indirect injury:Fire caused by earthquake;Flood caused by earthquake;Poison gas leakage caused by earthquake;Explosion of dangerous goods caused by earthquake.
[Shock absorption principle]
First, we should adjust measures to local conditions, not set rules;Second, act decisively and do not hesitate;III. Obey the command in public places and do not act without authorization.
[Key points of shock absorption]
Schematic diagram of earthquake caused by wall rock springing of high-pressure reservoir
1. It is a better way to take shelter nearby during the earthquake and quickly evacuate to a safe place after the earthquake.This is because the early warning time is very short when an earthquake occurs, and people are often unable to act on their own, coupled with the deformation of doors and windows, it is very difficult to run out of the room;If it is in the building, it is almost impossible to run out.However, if the early warning phenomenon is found in the bungalow and the outdoor space is relatively open, you can try to run out of the bungalow for shock absorption.
2. Hide under or next to objects that are solid, hard to dump, and can cover the body indoors, and where there is small space and support;Open and safe place away from buildings outdoors.
3. Lie down, keep the body's center of gravity to the lowest, face down, and do not press the mouth and nose to facilitate breathing;Try to curl up when squatting or sitting down;Grab the solid objects around to prevent injury from falling or being exposed to the solid objects due to body displacement.
4. Lower your head, protect your head and back neck with your hands, and if possible, put objects beside you, such as pillows and bedding, on your head to protect your head and neck;Lower your head and close your eyes to prevent foreign objects from hurting your eyes;When possible, wet towels can be used to cover the mouth and nose to prevent dust and toxic gas.
5. Do not light an open fire casually, because there may be flammable and explosive gas in the air;Avoid crowds and crowd.This is true on all occasions, including streets, apartments, schools, shops, entertainment places, etc.Because, instead of being able to escape from the danger, people may be injured due to falls, stampedes, collisions, etc.
[Post disaster life]
Pay attention to diet and personal hygiene.
Fire prevention shall be paid attention to when building and living shockproof shed
We will actively engage in recovery and reconstruction.
Take preventive drugs as required to strengthen the body's resistance and prevent and kill diseases.
Earthquake
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The largest number of earthquake deaths at all times and in all over the world: about July 1201, all cities in the Near East and the eastern Mediterranean region were damaged by the earthquake, with the largest number of deaths, estimated to be about 1.1 million.On January 23, 1556, the 8.0 earthquake in Huaxian County, Shaanxi Province, China, caused more deaths than the former. The vast number of victims died of disease and starvation. Hundreds of miles of mountain villages were cut off, with an estimated death toll of more than 830000.
The first earthquake in the world that successfully predicted and achieved obvious results in disaster reduction: February 4, 1975,haicheng earthquake (The successful prediction by Chinese seismologists has been called "a miracle in the history of seismology" by the world scientific and technological circles.).
The largest seismic zone in the world: the circum Pacific seismic zone (including the Pacific coast of North and South America, and from Aleutian Islands, Kamchatka Peninsula, and Japanese Archipelago to Taiwan Province of China in the south, and then to the southeast through the Philippines, and to New Zealand).
The world's largest earthquake tsunami: the Babushan Tsunami, which occurred in Ishigaki Island in the Ryukyu Islands, Japan, on April 24, 1771 (it is estimated that the peak of the huge waves is 84.7 meters high, and the overwhelming waves throw the entire coral reef weighing 850 tons more than 2.092 kilometers. The waves hit by this earthquake are measured to travel at a speed of 788.557 kilometers per hour).
The largest earthquake recorded by instruments in the world: at 19:11 on May 22, 1960, the Chilean earthquake (magnitude 8.9, also known as magnitude 9.5 due to different calculation methods).
The most typical cities in the world are "straight down earthquakes": the Tangshan earthquake in China in 1976 and the Hanshin earthquake in Japan in 1995.
The earliest record of earthquakes in China: The earliest record of earthquakes in Chinese history can be found in the "Year of the Secretary of Bamboo", in which it is mentioned that "in the 35th year, the emperor ordered Xia to march after Miao", and "in the 35th year, the emperor ordered Xia to march after Miao and Miao came to court".The notes in the external notes of Tongjian refer to the Jizhong Chronicle along with the Nest, which says that the three seedlings will die in the sky, rain, blood, ice, earth, crack and spring in summer.The Taiping Emperor cited this saying: When the three seedlings were about to perish, the earthquake collapsed and the spring gushed.Emperor Shun was about in the 23rd century BC, and it has a history of more than 4000 years.
The earliest earthquake disaster recorded in the West: the Lisbon earthquake in Portugal in 1755.
The earthquake with the highest magnitude since the global scientific record: it occurred in the northern part of Chile about 3800 years ago, with a magnitude of 9.5. The resulting tsunami affected New Zealand more than 8000 kilometers away, severely destroying the coastline and making the coastal areas uninhabitable for 1000 years.[4]
statistical data
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In the first quarter of 2023, there will be 22 earthquakes of magnitude 4 or above in China's mainland, including one of magnitude 6.0 – 6.9 and two of magnitude 5.0 – 5.9, mainly in Xinjiang, Sichuan, Tibet and other places.Among them, the Shaya, Xinjiang, earthquake of magnitude 6.1 on January 30 and Wensu, Xinjiang, earthquake of magnitude 5.1 on February 27 were both located in sparsely populated areas and did not cause major disaster losses.On January 26, the Sichuan Luding M5.6 earthquake did not cause casualties.[6]
In 2023, there will be 18 earthquakes with magnitude above 5 in China.Among them, 11 earthquakes with magnitude above 5 occurred in the mainland, including 2 earthquakes with magnitude above 6, namely, the Shaya earthquake with magnitude of 6.1 in Xinjiang on January 30 and the Jishishan earthquake with magnitude of 6.2 in Gansu on December 18;There were four earthquakes with magnitude above 5 in the sea area, which were 6.1 in the South China Sea on April 3, 5.0 in the Beibu Gulf on June 24, 6.4 in the East China Sea on September 18, and 5.0 in the Nan'ao County sea area of Shantou City, Guangdong Province on October 23;Three earthquakes with a magnitude of more than 5 occurred in Taiwan, the largest being the 5.9 earthquake in the sea area of Hualian County, Taiwan, on October 24.On December 18, 2023, a magnitude 6.2 earthquake occurred in Jishishan, Gansu Province, which is the largest earthquake in the mainland of that year.Since 2023, there have been 11 earthquakes with a magnitude of more than 5 in the mainland, the largest being the Jishishan 6.2 earthquake in Gansu on December 18, and the activity level is significantly weaker than that in 2022 (27, the largest being the Mengyuan 6.9 earthquake in Qinghai on January 8, 2022) and the historical average (20).In 2023, the time distribution of earthquakes with magnitude above 5 will be uneven. There will be 3 earthquakes from January to February, only 2 earthquakes from March to September, and 6 earthquakes from October to December. The activity characteristics of earthquakes with magnitude above 5 will change from weak to strong.[7]
