1. What is an earthquake?
Earthquake, in a broad sense, refers to the vibration of the earth's surface, which is a ground vibration phenomenon in a certain range caused by the sudden and violent movement of rocks in a certain part of the earth's crust under the action of internal and external stresses. According to statistics, there are millions of earthquakes in the world every year, and only about 1% can be felt. Catastrophic earthquakes of magnitude 7.0 or above occur more than 20 times a year and as few as 35 times.
A strong earthquake can not only make buildings in ruins instantly, but also cause great losses to human life and property, which is a devastating disaster. At the same time, earthquakes can also induce large-scale secondary geological disasters such as sand liquefaction, collapse, landslide and debris flow. Earthquakes in the deep sea sometimes cause tsunamis.
Schematic diagram of earthquake cause
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Myths and legends about earthquakes
In ancient times, people often used gods to explain the causes of earthquakes. China folklore "Aoyu Tuoshan" holds that the underground Aoyu overturned and caused the earthquake. Japanese legends about earthquakes are very similar to those in China, to the effect that catfish riots caused earthquakes. Coincidentally, there are similar legends in the west. Poseidon in ancient Greek mythology was the god in charge of earthquakes. Now, no one will believe these legends. Earthquakes are actually as normal natural phenomena as rain and wind.
Earthquake legend of Japanese catfish turning over
2. Where are the three major earthquake zones in the world?
Earthquakes, especially in shallow earthquakes, are mostly related to fault dislocation. Over the years, China, the United States, Japan, Russia and other countries have studied earthquake in a planned way, especially the development of plate tectonics theory in 1960s, which made people have a further understanding of the world's major earthquakes.
Early earthquake research in the three major earthquake zones in the world found that earthquakes are not evenly distributed in all corners of the earth, but concentrated in some areas, and these areas where earthquakes are concentrated are earthquake zones. The main earthquakes in the world are the Pacific Rim seismic belt, the Mediterranean-Himalayan seismic belt (or Eurasian seismic belt) and the mid-ocean ridge seismic belt. China lies just between the Pacific Rim and the Mediterranean-Himalayan seismic belt.
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The characteristics of the three major earthquake zones in the world are different.
The Pacific Rim seismic belt is the largest seismic belt in the world, where about 80% of shallow earthquakes, 90% of moderate earthquakes and almost 100% of deep earthquakes are distributed, and the energy released accounts for about 80% of the total energy released by global earthquakes. The Mediterranean-Himalayan seismic belt is the second largest seismic belt, and the distribution of epicentres is more dispersed than that of the Pacific Rim seismic belt, so the earthquakes are wide and branched, and the energy released accounts for about 15% of the total energy released by global earthquakes. Earthquakes in the mid-ocean ridge seismic belt are all generated in the lithosphere, and most of them are weak earthquakes.
China is a country with many earthquakes.
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earthquake belt of china
Because different scholars don't completely agree on the geological structure of China, they also have different divisions on the seismic belt of China. The 1303 Hongdong-Zhao Cheng earthquake that occurred in the seventh year of Yuan Dade was the earliest earthquake with magnitude of 8.0 confirmed by seismologists in China, which occurred in the strong earthquake belt of Wei Fen Graben in Weihe Plain. 1556 65438+1On October 23rd, an earthquake occurred in hua county, Shaanxi Province, which killed 830,000 people. The Tangshan earthquake, which killed the most people after the founding of New China, happened on the Tangshan fault in Hebei Plain.
Division methods of China's seismic zones by different research institutions
China is located at the intersection of two major earthquake zones in the world. It is a country with frequent earthquakes and one of the countries with the strongest intraplate earthquakes in the world. According to historical records, earthquakes of magnitude 5.0 or above have occurred in all provinces of China. According to the Seismic Intensity Zoning Map of China (130,000) issued by the State Seismological Bureau 1977, 32.5% of the country's total area is earthquake intensity above VII, and 60% is earthquake intensity above VI. In addition, China has a high frequency of seismic activity. According to statistics, during the 80 years from 1900 to 1980, there were 9 earthquakes of magnitude 8.0 and 66 earthquakes of magnitude 7.0 ~ 7.9, with an average of nearly 1 time per year. In addition, except for a few moderate-deep earthquakes in Northeast China and Taiwan Province Province, the focal depth of most earthquakes is less than 40 km, and the focal depth in the eastern region is mostly 10 ~ 20 km.
Schematic diagram of earthquake zone and distribution of major earthquakes in China
4. What are the types of earthquakes?
According to different classification methods, earthquakes can be divided into many types. Of course, several classification methods may cross each other. For example, the "5. 12" Wenchuan earthquake is a tectonic earthquake; At the same time, the focal depth 10 ~ 20km belongs to shallow earthquakes. According to the characteristics of its earthquake sequence, it belongs to the main earthquake type; The magnitude reaches 8.0, which is a major earthquake; The epicenter intensity reached ⅶ degree, which was a felt earthquake; The epicenter is located in the mountainous area of Sichuan, which belongs to continental earthquake and is prone to secondary geological disasters such as collapse, landslide and debris flow. The earthquake occurred inside the plate, belonging to intraplate earthquake. Therefore, we can discuss an earthquake from different angles in order to make preparations for further judging earthquake disasters and taking preventive measures in advance. It should be noted that among various classification methods, classification by cause is the most commonly used, among which tectonic earthquakes account for the largest proportion, accounting for more than 90% of the global earthquakes, and strong earthquakes are almost all tectonic earthquakes, which are destructive.
Statement of earthquake type
sequential
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Can reservoirs also induce earthquakes?
Induced earthquake refers to the earthquake caused by human engineering activities, and reservoir-induced earthquake is one of them. In 1950s and 1960s, the number of large and medium-sized reservoirs increased sharply, especially after 1960s, many large reservoirs experienced earthquakes of magnitude 6.0 or above. The earliest reservoir-induced earthquake of magnitude 6.0 or above in China was the Xinfengjiang Reservoir Earthquake in Guangdong Province (1962, 6. 1 magnitude). Thousands of houses were seriously damaged and several people were killed or injured.
5. What caused the earthquake?
Since ancient times, people always want to know the essence of earthquakes, among which the most widely accepted theory is elastic rebound, that is, fault theory, which was originally put forward by German (American) seismologists to explain the horizontal movement of the San Andreas fault in the 1906 San Francisco earthquake. 1906 On April 18, an earthquake of magnitude 7.8 occurred in San Francisco, USA. The fault along the San Andreas fault is 400 kilometers long. Marin County, north of San Francisco, has a horizontal displacement of 6 meters and a vertical displacement of nearly 1 meter. Why is this?
As we know, the earth's lithosphere is not a whole, but divided into six plates by some fault tectonic belts. No matter the collision at the plate edge or the movement of the fault zone inside the plate, stress will be generated in the rock stratum, which will make the energy inside the earth accumulate slowly. When the stress accumulates to a certain extent, the rock will break and form faults.
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Are there any other theories about the cause of the earthquake?
193 1 year, Japanese scholars put forward the theory of magma impact caused by earthquake. According to this theory, due to the impact of magma on the weak parts of the crust, the crust broke and moved, which led to the earthquake. Some people think that the earthquake activity in Tengchong, China may be related to underground magmatism. 1963, New Zealand scholars put forward the theory of phase transition, which holds that deep substances, under high temperature and high pressure, cause the surrounding rock mass to be compressed or stretched rapidly in the process of changing its crystalline state, thus causing earthquakes. However, these two theories have not been further demonstrated and applied.
We all have this experience in our daily life: when we bend the spring hard and suddenly let it go, it will return to its original state and release energy at the same time. Similarly, the elastic rebound theory holds that in-situ stress makes the rocks on both sides of the fault elastically deform and store energy. When the energy exceeds the friction resistance between the two plates of the fault, the energy is suddenly released in the form of an earthquake, and the deformed rock returns to its original shape, thus causing an earthquake.
Schematic diagram of seismic elastic rebound theory
Stress between plates 6. How to describe earthquakes in seismological terms?
Seismic terms describing earthquakes
Earthquakes can be described by focus, epicenter, epicentral distance, focal depth, magnitude and intensity. The earthquake source is the birthplace of the earth's vibration when an earthquake occurs, and it is the first place where underground rocks break. The projection point of the source on the ground or the corresponding position of the source on the ground is the epicenter. The ground distance from a designated point to the epicenter of an earthquake is called epicentral distance, sometimes expressed by length, such as kilometers, and sometimes by the degree of its corresponding geocentric angle (central angle). The central angle 1 degree is equal to11/km. According to the magnitude of epicentral distance, earthquakes can be divided into near earthquakes, near earthquakes and far earthquakes. The focal depth is the distance from the vertical projection of the focal point to the surface, usually expressed in kilometers. Among them, shallow earthquakes accounts for more than 90% of the global earthquakes, with the highest frequency and the greatest impact on human beings. Magnitude and intensity are measures of earthquake strength.
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How to indicate the epicenter of an earthquake?
After an earthquake, the location of the epicenter is usually indicated by the place name where the epicenter is located. For example, we can say that the epicenter of the Wenchuan earthquake is located in Yingxiu. But the scientific method is expressed by the latitude and longitude of the earth. There are two ways to determine the epicenter. One is to determine the epicenter location according to the degree of earthquake damage, that is, to locate the epicenter in the most severely damaged extreme earthquake area, which is called macro epicenter. Another method is the vertical projection of the source on the ground measured by the instrument, which is called microseismic. Due to the influence of the physical state and geological conditions of the earthquake area, the place with the greatest ground damage is not necessarily directly above the earthquake source, so the macro epicenter does not necessarily coincide with the micro epicenter.
7. What is the order of magnitude?
At present, the Richter scale, which is widely used in the world, was first put forward by the seismologist Kurt (USA), and its range is 1 ~ 10, which is obtained by the common logarithmic calculus of the maximum amplitude of seismic waves recorded by seismographs at observation points.
The magnitude is calculated by the seismic spectrum recorded by seismograph.
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What is the relationship between earthquake magnitude and energy?
Magnitude is a "scale" to measure the magnitude and intensity of earthquakes, and it is a relative scale to measure the magnitude of earthquakes. The higher the earthquake magnitude, the greater the energy released. Generally speaking, there is a general rule that the energy released by earthquakes with different magnitudes through seismic waves is about 1.4 14 times for every magnitude difference of 0. 1; If the magnitude difference is 0.2, the energy difference is (1.4 14)2≈2 times, and so on. If the magnitude difference is 1.0, the energy difference is about (1.4 14) 10, which is 32 times.
Comparison table of magnitude and corresponding energy
When richter applied this achievement to all parts of the world, it was found that it also had defects, that is, it could not accurately reflect the magnitude of earthquakes. So, on the basis of the original, he developed two orders of magnitude. One is to calculate the magnitude of earthquake body wave to measure the earthquake in the deep crust (body wave magnitude MB); The other is to use the surface wave of the earthquake to calculate the magnitude to measure the farther and stronger earthquake (surface wave magnitude Ms).
However, in the 1960s and 1970s, scientists found that when the magnitude exceeds 8.6, even if a larger scale is displayed, it is difficult to increase the measured value, which is the problem of magnitude saturation. Therefore, seismologists turn to a representation method with richer physical meaning and more direct reflection of the physical essence of the earthquake process, namely moment magnitude (Mw), which was put forward by Dickinson Xiong Bo (USA, Japan) in 1977. Moment magnitude can better describe the physical characteristics of earthquakes, such as the magnitude of stratum dislocation and the energy of earthquakes.
8. Why was the magnitude corrected after the earthquake?
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The magnitude of the 2004 Sumatra earthquake in India has been revised many times.
The magnitude of the Sumatra earthquake in the Indian Ocean in 2004 was 9. 1, which was the third largest earthquake after the Chile earthquake with magnitude 9.5 in 1960 and the Alaska earthquake with magnitude 9.2 in 1964. This result comes from many studies. After the earthquake, the magnitude measured by the Meteorological and Geographical Bureau of Medan, Indonesia was 8. 1. China Seismological Network measured 8.7; The data of the American Earthquake Monitoring Network was 8.9, and later the US Geological Survey finally revised the result to 9. 1.
After the earthquake, the magnitude may be revised many times. For example, on March 1 1, a major earthquake occurred in the sea near the east coast of Honshu, Japan. The magnitude was initially announced as 8.6, and later revised to 9.0. This is mainly because: ① People use different magnitude scales. For example, China is used to using "surface wave magnitude", while Europe and America mostly use body wave magnitude or moment magnitude; ② The seismic instruments used by seismic stations around the world are different, and the calculation formulas are also inconsistent, which leads to differences in magnitude determination; ③ The earth is uneven, and the attenuation degree of seismic wave energy propagating in different paths is different; ④ In a short time after the earthquake, due to the lack of data, the magnitude error measured quickly is large. Since then, with more and more detailed information and data, it is possible to correct the magnitude more than once and finally get more reliable results.
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China current general earthquake intensity table.
9. What is the concept of strength?
Intensity is another "ruler" to measure the strength of an earthquake, which refers to the degree to which the ground and buildings are affected and damaged by the earthquake. Intensity is different from magnitude, which reflects the size of the earthquake itself and is only related to the energy released; Intensity reflects the damage degree of the reflected earthquake itself, and factors such as magnitude, focal depth, epicentral distance and geological conditions will affect the intensity. Therefore, an earthquake has only one magnitude, and the intensity is different in different regions. For example, the magnitude of an earthquake is like the wattage of a fluorescent lamp. The greater the wattage, the brighter the electric light. For the same fluorescent lamp, due to various factors, such as the distance from the light source, the brightness of each place is different, and the intensity of different regions is the same.
Schematic diagram of seismic intensity
Seismic intensity table is a table that classifies and arranges people's feelings about earthquakes and various phenomena that the ground and buildings are damaged by earthquakes according to different degrees. At present, there are many kinds of earthquake intensity tables in the world. At present, the earthquake intensity table commonly used in China is compiled by Xie Yushou (China) in 1957, which is based on Siebel intensity table and combined with the characteristics of China buildings.
10. Main shock and aftershock
The occurrence of an earthquake is not isolated, but always connected in series, that is, earthquake time series, including main earthquake type, earthquake swarm type and isolated earthquake. Main earthquake refers to the earthquake sequence with prominent magnitude and many aftershocks, which is the most common type of earthquake sequence. The energy released by the largest earthquake accounts for more than 90% of the whole sequence. This largest earthquake is called the main earthquake, and other smaller earthquakes after the main earthquake are called aftershocks.
For example, the Wenchuan earthquake in 2008 was the main shock, with frequent aftershocks. According to China Seismological Network, as of June 17, 2009, there were 297 aftershocks of magnitude 4.0 or above in Wenchuan area, including 254 aftershocks of magnitude 4.0-4.9, 35 aftershocks of magnitude 5.0-5.9 and 8 aftershocks of magnitude 6.0 or above. The largest aftershock magnitude is 6.4. In addition, in some cases, large earthquakes will trigger faults far from the original epicenter and cause aftershocks. For example, the Landers earthquake with magnitude 7.3 near Los Angeles in 1992 triggered secondary events in 14 places, including aftershocks 1250 kilometers away.
After the Wenchuan earthquake, aftershocks occurred frequently.
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Is there a connection between the main earthquake and the aftershock?
Seismologists have studied 20 earthquakes with magnitude above 7.5 that occurred in China from 1900 to 2003, and found that the largest aftershocks of most sequences occurred within 200 days after the earthquake, 68% within 10 days after the earthquake, 77% within 30 days after the earthquake and 95% within 120 days after the earthquake. Moreover, the magnitude of the largest aftershock is also related to the interaction between the two plates of the fault: when the two plates of the fault are pulled apart by the Coriolis force effect, the aftershock intensity is small, and the average magnitude difference between the main shock of the left-lateral strike-slip inverse earthquake and the largest aftershock is about 2.0; When the two plates of the fault are squeezed by the Coriolis force effect, the aftershock is strong, and the average magnitude difference between the main shock and the largest aftershock of the right-lateral strike-slip earthquake is about 0.6. These findings have an important reference function for predicting the occurrence of strong aftershocks in the future.
1 1. What is a seismic wave?
The huge energy released during an earthquake stimulates elastic waves that spread around, which is called seismic waves and is also the carrier of seismic energy. Seismic waves are divided into body waves and surface waves. The waves propagating through the crust and rock mass in the medium are called body waves, including longitudinal waves and shear waves. Waves propagating along the ground through refraction and reflection are called surface waves.
earthquake wave
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"domino effect" caused by deep earthquake
The fall of dominoes will cause a chain reaction, which is the "domino effect". In earthquake research, scientists found that shallow earthquakes would also trigger this effect. Because of the propagation of seismic waves, shallow earthquakes will trigger another earthquake hundreds of kilometers away, which may be stronger. For example, after an earthquake of magnitude 8.8 occurred in Chile on February 27th, 20 10, Beijing time, a series of earthquakes were triggered: on March 4th, five days later, a strong earthquake of magnitude 6.7 occurred in Kaohsiung, Taiwan Province Province, China; Two days later, on March 6th, an earthquake of magnitude 7. 1 and magnitude 4.2 occurred in Sumatra Island, Hebei Province and the far southwest of Tangshan City, China. Studying this effect of earthquake is of great significance to understand the law of earthquake.
P wave is faster than S wave (P wave speed is 5 ~ 6 km/s, S wave speed is 3 ~ 4 km/s), so when seismic instruments record seismic waves, P wave arrives first, so it is called primary wave. The shear wave arrives late, so it is also called the second wave. Surface wave is a secondary wave excited after body wave reaches the ground, which is limited to ground motion. This kind of wave can be divided into two types, one is Loew wave which moves in a snake shape on the ground, and the other is Rayleigh wave which rolls on the ground.
12. How to determine the epicenter by using seismic waves?
The epicenter of an earthquake is determined by seismic waves. During an earthquake, longitudinal waves first reach the surface, then shear waves, and there is a time interval between them. People can preliminarily judge the distance of the epicenter according to the perceived time interval: the shorter the interval, the closer the epicenter is. Similarly, there is a time difference between the arrival of P wave and S wave at the same seismic station, which is proportional to the distance between the epicenter and the seismic station, and the epicentral distance can be obtained through it. According to the epicentral distances obtained by three seismic stations that are not in a straight line, the location of the epicenter of the earthquake can be roughly calculated by the three-point intersection method, so as to prepare for further understanding of the earthquake and earthquake relief. It can be seen that seismic waves are not heinous "bad guys". Seismic waves can serve us as long as they are used reasonably.
Determination of the epicenter location of Haiti earthquake by three-point intersection method
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An earthquake is a lamp leading to the center of the earth.
Earthquakes instantly kill people and destroy objects, but this is only one aspect. In fact, people began to understand the internal structure of the earth from the earthquake. 1906, oldham (Britain) first tried to infer the internal structure of the whole earth from the time when seismic waves passed through the earth. 1909, Mohorovicic (former Yugoslavia) calculated that there was a discontinuity 56 kilometers underground, which was later called Moho surface. 19 14, Gutenberg (a Jew, born in Germany) confirmed the existence of the core according to the earthquake body wave, and measured the discontinuity between the mantle and the core, with a depth of 2900 kilometers, which is quite accurate and has not been greatly improved so far.
Earthquakes and volcanoes.
What kind of pictures will come to your mind when you mention volcanoes? Lava spewed out, hot air filled, and volcanic ash filled the sky? In many people's impression, the volcano is like a violent person, hopping and panting. Where is the volcano? What does it have to do with the earthquake?
Earthquakes and volcanoes
There are about 455 known active volcanoes in the world, but they are not evenly distributed all over the world, but mainly distributed in the Pacific Rim and the Mediterranean. There are also volcanic eruptions in the ocean, and some islands are formed by volcanic eruptions. Sometimes, earthquakes can also lead to volcanic eruptions. For example, the 1960 Chile earthquake triggered a volcanic eruption; 20 1 1 Japan's 9.0 earthquake also triggered a volcanic eruption. In addition, through the collection of historical volcanic activity data, it is found that volcanoes are closely related to strong earthquake activity: at the peak of seismic activity, volcanic activity is frequent, and about 0.4% explosive volcanoes erupt within a few days after the earthquake. For example, Kilauea Volcano1975165438+10 in Hawaii erupted within one and a half hours after the earthquake of magnitude 7.2 in June.
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The north and south poles of the earth are "the pure land of earthquakes"
If we look back at the history of earthquakes, we will find that there have been no earthquakes in the north and south poles of the earth. First of all, the Antarctic continent and the Arctic continent are surrounded by the mid-ocean ridge and the mainland respectively, and there is no prerequisite for earthquakes. Secondly, the ice layer in this area can reach 80% to 90% of its total area, which has exerted great pressure on the bottom of the rock stratum, basically balanced with the extrusion pressure of the stratum structure, and dispersed and weakened the deformation of the crust. Therefore, the north and south poles of the earth are a "pure land of earthquakes".
Earthquakes have never visited the north and south poles of the earth.
14. Is the earthquake related to the moon?
In addition to the causes of earthquakes generally accepted by the scientific community, some scholars have suggested that the occurrence of earthquakes may be related to the moon. 65438+February 26th, 2004, the Indian Ocean earthquake occurred on the 5th day of the lunar calendar 1 1 month/kloc-0. Coincidentally, the 1960 Chile 9.5 earthquake also coincided with the first day of the fifth lunar month.
Earthquakes may be related to the gravity of the moon.
Some scholars believe that earthquakes often occur in the "new moon" (the first day), the first quarter moon (the eighth day), the second quarter moon (the twenty-four solar terms) and the fifteenth solar terms, and the day before and after. We know that the "new moon" is on the first day of every month in the lunar calendar. At this time, the earth, the sun and the moon are in a straight line, and the gravity on the earth reaches the maximum. When the moon circles behind the earth and the hemisphere illuminated by the sun faces the earth, it is called "looking", usually on the fifteenth day of each month in the lunar calendar, when the sun and the moon are on one side and the gravity is opposite. During the first quarter (day 8) and the last quarter (day 24), the sun, the earth and the moon are arranged in a right triangle. There are many earthquakes at this time, and some scholars think it is related to the magnetic field. Of course, this is just a hypothesis. Whether there is such a law is still waiting for us to study.
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Will the sun quake?
Do you know that the sun will have an earthquake like the earth? In recent years, scientists in the United States and Britain have captured an amazing message by studying the data sent back by meteorological satellites:1July 9, 996, there was a strong vibration inside the sun. According to experts' estimation, the energy released by this "solar earthquake" is almost equal to 4000 times that released by the San Francisco earthquake in 1906. if
15. Has the world entered an active period of earthquakes?
We have just experienced the pain of Wenchuan earthquake and the sadness of Yushu earthquake. 20 1 1 year, the Yingjiang earthquake in Yunnan and the 9.0 earthquake in Japan occurred one after another. People can't help asking, has the world entered an active period of earthquakes?
A large number of seismic records reflect that seismic activity is indeed periodic: in a long time scale, seismic activity sometimes increases and sometimes decreases, the former is active and the latter is calm. For example, during the period from 1950 to 1965, there were 46 earthquakes of magnitude 7.0 or above. Since the Indian Ocean earthquake in 2004, there have been more than 8.0 earthquakes 1 1 times in the world, slightly higher than the average, but still within the normal range.
Whether an earthquake has entered an active period cannot be judged simply by the number of earthquakes in a certain period. Nowadays, people feel that there are more earthquakes, not only because the density of seismic network has increased, the instruments and equipment are accurate, and the number of earthquakes recorded has increased, but also because the population of all parts of the world has increased, the economy has developed, the casualties and property losses caused by earthquakes of the same level are greater than before, the media information and communication means are developed, and earthquake news spreads faster and wider, so the impression of earthquake harm has deepened.