Seismic waves occur underground and propagate through the elastic vibration of rock and soil particles. Local seismic waves propagate near the ground. When the objects affected by the earthquake have expansion space, group movement is formed, which distorts the shallow stratum and distorts the objects on the ground in three dimensions. The seismograph only records the intensity and periodic changes of the movement of the object group where the seismograph is located. Neither the three-dimensional distorted trajectory of group movement nor the real seismic wave can be measured, so the wrong theory is obtained. It is considered that the earthquake first jumps up and down (caused by longitudinal waves) and then swings horizontally in one direction (caused by transverse waves), and accordingly the earthquake simulation experiment and theoretical research are carried out by shaking table, which is contrary to the actual situation.
Before we can directly measure the three-dimensional trajectory of group torsion, we can only analyze the ruins destroyed by the earthquake and obtain their motion information before the destruction. 1After the Tangshan earthquake with M = 7.8 on July 28th, 976, we made a lot of on-the-spot observation on Tangshan earthquake damage, combined with the data of Haicheng earthquake, Liyang earthquake and Longling earthquake, summed up ten laws of earthquake damage, made clear the mechanism of earthquake damage, and found torsional waves through stress analysis on this basis, and then put forward eight anti-seismic measures against torsional waves. The first paper was published in 1979, entitled "Torsion Wave and Earthquake". Later, many papers were published and reported by dozens of newspapers and magazines. Due to well-known reasons, the theory of torsion wave has not been accepted by authoritative experts so far.
1995 65438+1October 17 An earthquake of magnitude 7.2 occurred in Kobe, Japan. A TV monitor recorded the whole process of the office being destroyed by the earthquake. The tables and chairs are all twisted, and the maximum rotation angle reaches 170 degrees. It proves that the torsional wave based on the analysis of crumbling city walls 20 years ago is correct. However, twenty years later, in front of the live video of the torsional motion of the earthquake-stricken object, seismologists in Japan and the United States still describe the earthquake damage as caused by longitudinal waves and shear waves. That's a real pity. In order to change this abnormal situation, a paper published in 1992, The Discovery of Torsional Waves and Seismic Fortification, is reprinted in order to set the record straight.
Earthquake mechanism-discovery and fortification of torsional waves. Originally published in Xinjiang Geological Supplement 1992.
catalogue
order
First, the law of earthquake damage and existing problems
1, some laws of earthquake damage
(1) Linear objects stand upright as before.
(2) The reinforced iron frame has the best seismic performance.
(3) Cracking, twisting and twisting coexist at the same time
(4) The fracture forms are aging.
(5) Severity zoning of interval fracture.
(6) The two broken discs are not damaged.
(7) The ground is flat with little fluctuation.
(8) Flowering on all sides and falling vertically
(9) Underground buildings are well preserved.
(10) The earthquake resistance of human body is better than that of factory building.
2. Existing problems
The law of (1) earthquake damage 10 is contrary to the theory of longitudinal wave, shear wave and surface wave.
(2) Five key issues
Second, the formation mechanism of elastic waves
1, elastic vibration
2. Elastic waves
Generation mechanism of (1) elastic wave
(2) the propagation of elastic waves
(3) Elastic field and elastic wave steering
(4) transverse and longitudinal torsional waves
Tri-torsional wave and earthquake damage
1 destruction of torsional waves
2 twist-open type
Transverse torsional cracking
Longitudinal torsional crack
3 earthquake damage zoning
4 Relationship between torsion wave failure and gravity
5. Ground rotation
Torsional wave fortification
1 clip
2 Live broadcast is over
3 small at the bottom
4 reinforcement
5 tie a knot firmly
6 Lightweight
7 rings
8 floors underground
refer to
order
China is the first country to record the largest number of earthquakes and paleoearthquakes, and also the first country to invent seismographs and predict earthquakes. In the third year of Jin Taikang (282 BC), a bamboo slip was unearthed in the tomb of Wei Xiangwang (3 18-296 BC). Bamboo slips are inscribed with the history from Emperor Wu of the Han Dynasty to the twenty-second year (297 BC), which is called "Bamboo Slip Chronology". There were four earthquakes, the earliest being the seventh year of the summer. Since then, earthquakes have been recorded in many ancient books. During the 3800 years of BC 183 1- 1955, about 9000 earthquakes were recorded, of which about 1000 were destructive earthquakes. After sorting out, not only the China earthquake area is delineated, but also the periodic variation law in the active and quiet periods of earthquakes is revealed.
Abroad, the earliest earthquake record was in Japan, which recorded the five-year (4 16) earthquake of Emperor Yingwu in Hanoi, Japan, which was 2247 years later than that in China.
Zhang Heng (78- 139), a famous scientist in the Eastern Han Dynasty, created the world's first seismograph "Houfeng seismograph", which was placed in Luoyang City and correctly reported the earthquake that occurred in Longxi on the third day of February in the third year of Yonghe in the Eastern Han Dynasty (138). Longxi is located in Lanzhou and Lintao, Gansu Province, 700 kilometers west of Luoyang. Europe made the seismograph in 1880, and its principle is basically similar to that of Zhang Heng, which is later than that of China 1700 years.
Thousands of years ago, China tried to find the cause of the earthquake from nature itself. When describing the earthquake that occurred in the summer decade, "Zhu Shuji Nian" wrote that "the five stars are staggered, and the meteor rains at night, causing earthquakes." During the Spring and Autumn Period and the Warring States Period, Yang Shu believed that "the qi of heaven and earth, without losing its order ..... the yang lies but does not come out, and the yin is forced but does not steam, so there is an earthquake". "Zhuangzi" put forward the theory that the sea is thin, saying: "The sea is three years old and a week old, and it is thin, so it moves." Wang Yun, an outstanding thinker in the Eastern Han Dynasty, first proposed that earthquakes are an "automatic" phenomenon of the earth's crust itself. He said more than once that "the ground will automatically". Shen Kuo thinks that the change of heaven and earth has its own laws and physics has its own changes.
According to earthquake precursors, there are also many records in the history of our country, and a set of experience of earthquake prediction is summarized. According to the records of Yuxiang County, Shanxi Province, in the 20th year of Jiaqing in Qing Dynasty (18 15), the experience in Pinglu area of Shanxi Province predicted a strong earthquake at midnight on September 20th, and Xianfeng in Qing Dynasty lasted for four years (65438). Predicting the impending devastating earthquake: "Before the earthquake, thunder was heard first, so the flag people here had already taken precautions and fled outside, so that many people were not killed, only seven men, women and children were injured." Ningxia's "Longde County Records" once summarized the earthquake precursors as "six ends of the earthquake": First, the well water is quiet without waves, and suddenly it is like ink and mud floats up, which is bound to be an earthquake. Second, the water in the pond is blown into a mess by the wind, and the bubble rises for no reason. If tea is boiled, there will be an earthquake. Third, when it rains at sea, the waves are high and urgent, which is natural; If the weather is fine and the typhoon doesn't work, the sea will suddenly turn around and be unusually rough, and it must be an earthquake. It's dark in the middle of the night, and the sky is suddenly bright and radiant, so the earthquake is bound to happen in the dark. The weather is sunny and warm, and the blue sky is clean. Suddenly, I saw dark clouds like wisps, winding and growing into snakes. If they stay in the air for a long time, they will inevitably cause earthquakes. Six, in the midsummer, steaming, sweating like rain, suddenly feel cool as ice and snow, cold air attacks people, muscles are millet, and earthquakes are inevitable. These six articles have made an incisive summary of earthquake precursors such as abnormal weather, tsunami, terrestrial light and earthquake clouds before the earthquake. It is only in recent years that the Japanese began to study earthquake clouds, and the results basically conform to the above description.
Since 1996, China has registered 18 destructive earthquakes, such as 1975 Haicheng earthquake in southern Liaoning (M = 7.3), Longling and Luxi earthquakes in Yunnan (M = 7.5-7.6) and 1976 Songpan earthquake in Sichuan (M = 7.3). This is also one of the few in the world. From a global perspective, the problem of earthquake mechanism has not been completely solved. Earthquake prediction based on surface phenomena can only be effective for some earthquakes, and it is impossible to predict all earthquakes. 1976 Tangshan earthquake has not been reported, and there is still a long way to go in earthquake research.
There are three kinds of seismic waves, besides the well-known longitudinal waves and shear waves, there are also torsional waves. Torsion wave is the main driving force of earthquake damage. The discovery of torsional waves reveals the inside story of earthquake damage and provides a theoretical basis for formulating anti-seismic measures.
First, the law of earthquake damage and existing problems
In 1970s, there were more than a dozen strong earthquakes in China, which caused serious loss of life and property. Only 1976 Tangshan M7.8 earthquake killed 242,000 people, and164,000 people were seriously injured, which aroused people's strong concern about the earthquake. After the Tangshan earthquake, it was found that the template pattern (plate 1) attached to the seismic intensity table [1] was quite different from the facts. For example, on the model diagram, poles and buildings are gradually inclined with the increase of earthquake intensity, and all collapse when the inclination angle is gradually increased by Xi-XII degrees. In fact, the Tangshan earthquake was not like this. Lunan district in Tangshan was the most seriously damaged, and most houses collapsed. According to other indicators, the strength is set at XI degrees, but the telephone pole is still upright (plates 2, 4 and 5) [2]. In addition, whether it is a seven-story building or a flat-topped hut, the broken walls left after the collapse are basically straight (plate 3) [2], and they are not inclined first and then burst. This shows that there is still a certain gap in people's understanding of earthquake damage, which needs further study. Through on-the-spot investigation, combined with Liyang earthquake, Haicheng earthquake and Longling earthquake, some laws were initially found.
1, some laws of earthquake damage
(1) Linear objects stand upright as before.
The intensity of the earthquake in Nanxian County, Hefeng, Tangshan City was X-XI degrees. Almost all factories and houses were destroyed, but many linear objects were completely preserved. For example:
① All trees are safe and sound without direct damage (plates 2, 4 and 5).
(2) Most poles stand upright as before (plates 2 and 5; Plate 6, 7).
The sand turning workshop of Tangshan Hebei Institute of Mining Management is a reinforced concrete workshop. After the earthquake, the pillars were broken, the beams were bent and the roof collapsed, but the two telephone poles in front of the factory building were still standing as before, and the trees behind the house were still the same (Plate 2).
Tangshan Kailuan Hospital was originally a newly-built seven-story building. Most of it collapsed in the earthquake, but the trees and telephone poles near it are still intact. The remaining corners are also upright (panel 3).
The 40cm-wide reinforced concrete column was broken, while the poles and trees in the same area were intact (Table 7).
The pier of Shengli Bridge in Xidu area of Tangshan collapsed, while the trees and telephone poles beside the bridge were intact (Figure 5).
③ The 65-meter-high microwave broadcasting tower in Tangshan stands tall in the vast ruins (Plate 8); It can still be used after the earthquake (periscope requires high directivity and cannot be used every few seconds). This shows that the earthquake has little effect on it.
④ The 40-meter-high reinforced concrete Tangshan station water tower remains the same.
⑤ The factory building of Tianjin Chemical Plant collapsed, but the vertical reaction tower, tall chimney and pole were well preserved.
Linear objects are also seriously damaged, mainly because brick chimneys and brick water towers are not reinforced, and the whole section falls off after cracking along individual brick joints.
⑵ The reinforced iron frame has the best seismic performance.
① Two railway steel frame overpasses in Tangshan and more than a dozen pairs of steel frame wells in Kailuan Coal Mine are intact (plate 9).
(2) The 65-meter-high TV relay tower in lunan district, Tangshan remains in place (Plate 8).
③ The brick window columns of the office building of Tangshan Qixin Cement Factory are broken and scattered, but the steel bars in the columns are intact, supporting the superstructure (plate 10).
④ Some early poles in Tangshan were damaged by cement after the earthquake, and the steel bars were intact, and the poles were still upright (plate 1 1).
(3) Splitting, twisting and twisting coexist at the same time
No matter wall cracks, ground cracks or chimney cracks, all cracks have torsion, especially the fracture and torsion of columns. For example:
(1) A three-story warehouse in Qixin Cement Plant in Tangshan, the first and second floors are basically intact, and the three-story window columns are all broken, with different rotation directions and angles. Turn right 40o (disk12) with the largest existing rotation angle; The one with a large rotation angle had fallen off at that time and could not be inspected.
② In the sand-turning workshop of Tangshan Hebei Institute of Mining and Metallurgy, all the wall columns are broken and rotated, with different rotation directions and angles (Plate 2).
③ The gatepost of Tangshan Cultural Palace breaks and rotates (plate 13).
④ The ground fissure in Tangshan No.10 Middle School rotates to the right 1.2m (plate 14).
⑤ The chimney of Tangshan Cultural Palace rotates horizontally (plate 15).
⑥ The chimney of Haicheng County Committee in Liaoning Province is divided into 4 sections, and the top section is 40o right (disk 16).
⑦ The brick chimney in Yingkou City, Liaoning Province is horizontally broken into five sections, all of which are twisted and dislocated (plate 17).
⑧ Horizontal torsion occurred in some tracks of Hebei Ninghe Railway Station, and no damage was found in other tracks and poles (plate 18)[2].
Pet-name ruby Fengnan Railway Station in Hebei Province, one rail was twisted repeatedly, and the right rail and platform were also deformed, while other rails, trees and telephone poles were well preserved (plate 19)[2].
Take part in the horizontal rotation of two basketball stands after the earthquake of Zhen 'an Middle School 1976 M7.3 in Longling, Yunnan. One left hand 14o, one left hand 18o, maximum translation 1.22m (plate 20)[2].
(1 1) The deck of Luanhe Bridge on Jingyu Highway collapsed and twisted at the same time in Tangshan earthquake (plate 2 1)[2].
(4) The fracture forms are aging.
The damage caused by the earthquake mainly develops along the fragile zone of the object itself, whether it is buildings on the ground, roads on the surface or underground strata. For example:
① Fracture along the four corners of the building (plate12; License plate 13).
(2) crack outward along the four corners of doors and windows (plate 22; License plate 12).
③ Cut and twist along the top and bottom of the pillar (plate 13).
④ Fracture of parallel brick joints (plates 15, 16, 17).
⑤ Fracture parallel to or perpendicular to the road extension direction (Plate 23).
⑥ Vertical cement field boundary line fracture.
⑦ Parallel rivers and ponds break.
The form of fracture caused by earthquake is basically the same as that caused by non-earthquake "natural aging" and man-made destruction. The earthquake only accelerates the development of aging fracture and is an acute attack. The failure mode is not directly related to the source orientation. Like building cracks, the development of ground fissures is only controlled by local basement structures near the surface. Ground fissures are generated along its fragile zone, and their distribution and extension direction are not directly related to earthquake sources and seismogenic structures. Ground fissures are relatively shallow, generally only a few meters, and those more than ten meters deep have not been reported.
5] Severe and severe divisions of nasal septum rupture.
① Horizontal zoning
Tangshan Public Security School has three three-story buildings. Same shape, parallel arrangement, spacing 10m. A building in the south completely collapsed, and a building in the middle kept part of the skeleton in the middle, while a building in the north was only scattered in the front.
The five buildings newly built by Tangshan Cement Design Institute are all brick-concrete structures. The same design and construction, the basic conditions are similar. However, interval collapse and damage are very different (plate 24)[3].
Earthquake damage classification of Xinhua West Road, lubei district, Tangshan City (Plate 25)[2].
1975 Haicheng m 7.3 earthquake has a similar situation.
② Vertical zoning
The destruction of each floor of a building is often inconsistent. In the Tangshan earthquake, some buildings were seriously damaged only on the first floor (plate 22), and some buildings were seriously damaged on the second floor (plate 10). Some of them were only seriously damaged on the third floor (plate 12).
A brick chimney in Tangshan Huaxin Textile Factory, about 40 meters high, was broken into seven sections at roughly equal intervals, each section was twisted and deformed, but it was still in use and did not fall down.
There was a chimney in Haicheng County Committee, which was broken into four sections after the earthquake, and the last section turned right at 40o[4] (disk 16).
It is no coincidence that the three chimneys of Jiangsu Liyang Shangxing Agricultural Machinery Factory are not connected with each other, but the same fracture (plate 26) appears at the same height.
License plate number 26. The three chimneys of Shangxing Agricultural Machinery Factory in Liyang, Jiangsu Province are not connected with each other. After the earthquake, the same faults occurred at the same height, and they all burst in all directions.
[6] Two broken discs are not damaged.
No matter how long the fracture is and how big the crack is, the two objects in the fault block are still as strong as ever, and the influence range of the fracture is generally small, such as:
① Tangshan-Laoting Highway is staggered, with the right side dislocated 1.2m, and the trees on both sides stand as before (plate 27)[2].
(2) During the Haicheng earthquake, the frozen soil of Baqian Commune in Jinxian County cracked into a ground fissure with a width of about 20cm and a length of 2km, splitting 100 trees. The lower part of a tree trunk is split in two, and the upper part is left-handed by 8cm, but other trees nearby are still intact (Plate 28)[2].
③ Tangshan No.10 Middle School cut off the walls, toilets and roads through a ground fissure, and shifted to the right1.2m. However, the walls, toilets, roads and trees on both sides of the fissure were intact. Underground pipelines are also staggered, but both ends are not damaged (disk 14).
Once the ground is leveled, there are few ups and downs.
After the Tangshan earthquake, except for some local ups and downs caused by the collapse of mined-out areas or other influences, most of the ground and pavement remained intact. There are few waves, and both sides of the ground fissure are mostly flat (plates 23, 27 and 28).
Flowering in all directions.
The destruction of buildings, especially masonry structures and cement parts, is generally split in sections, blooming in all directions, collapsing and falling vertically. Just like folding lanterns together, few of them fall down as a whole.
① The chimney of Tangshan Locomotive and Rolling Stock Factory burst on all sides (plate 29).
② The water tower blocks of Hebei Institute of Mining and Metallurgy are scattered in all directions (plate 30).
③ The four corners of the brick chimney in Tangshan Workers' Hospital burst outward.
④ 1979 Jiangsu Liyang M6 earthquake, there were two brick kilns and eight chimneys in Shangpei Brick Factory near the epicenter, all of which collapsed in all directions (plate 3 1).
⑤ In the agricultural machinery factory of Shangxing commune in Liyang, Jiangsu Province, three chimneys stand side by side and crack in all directions at the same height (plate 26).
⑥ The 7th floor of Tangshan Kailuan Hospital collapsed vertically, the upper and lower floors almost overlapped, and the remaining corner remained upright (plate 3).
⑦ The collapsed floors in the inpatient department of Tangshan Railway Hospital overlap (plate 32).
⑧ The western section of the third floor of the reading room of Tangshan Hebei Institute of Mining and Metallurgy all fell vertically (plate 22), and the eastern section remained upright.
Levies The underground buildings are well preserved.
The civil air defense tunnel in Tangshan has not been damaged except for some small cracks in some places. Compared with the ground, the damage of underground roadway in Kailuan Coal Mine is extremely serious. There were more than 600 workers in Lujiatuo underground during the earthquake, and all of them returned to the ground safely after the earthquake. Majiagou No.3 well resumed coal production 10 day after the earthquake. The inpatient department of the railway hospital in Xidu area was originally a three-story building. The two floors above the ground completely collapsed, and the semi-underground was basically intact, with only a few small cracks (No.32 slab). Two railway tunnels in Tangshan were not damaged.
⑽ The human body is more earthquake resistant than the factory building.
Most of the people killed and injured in the Tangshan earthquake were indirectly injured because of the collapse of buildings. No one died directly from vibration, and people are more earthquake-resistant than factories built of reinforced concrete. People have strong earthquake resistance.
These earthquake damage laws are also common in major epicenters at home and abroad, such as 1970 1.5 Yunnan Tonghai earthquake with M = 7.7; September 1923, Kanto 8.3 earthquake in Japan; 1February 29, 960, adil, Morocco, M5.8 earthquake, etc.
2. Existing problems
In addition to the developed brain, the human body's ability to resist natural mutations is generally poor. Afraid of cold in winter and heat in summer; When the wind blows, the station is unstable and the water can't stand it; Only the earthquake resistance is extremely strong, even better than reinforced concrete and tall buildings. It is incredibly wonderful and simple, and needs no proof. Why can people resist earthquakes than factories? This is a very interesting question. Whether this phenomenon can be correctly explained is the touchstone to test all earthquake theories. To solve this problem, we must first understand the destructive effect of earthquakes. In the past, it was thought that the damage of earthquakes was caused by longitudinal waves, shear waves and surface waves, but in the future, the two were the main ones, but they were not. Most of the above-mentioned 10 earthquake damage laws cannot be explained by longitudinal waves and shear waves, nor can they be explained by so-called surface waves.
(1) The law of earthquake damage 10 is contrary to the theory of longitudinal wave, shear wave and surface wave.
(1) If earthquake damage is really caused by longitudinal waves and shear waves, the vibration of ground objects should be shaking up and down (longitudinal waves) and horizontal shaking (shear waves), and its destructive effect should be similar to the shock waves produced by strong winds, waves and atomic bombs. First of all, the top-heavy and upright linear objects, such as people, trees, telephone poles, water towers and iron towers, should be damaged seriously. It should not be those reinforced concrete workshops with low center of gravity, firm structure, large load and high strength, but the opposite. It can be seen that the main force of earthquake damage cannot be longitudinal wave and shear wave;
② Seismic faults are all rotating, but the vibration of longitudinal wave and shear wave is not.
(3) Under the action of longitudinal and transverse waves, the building should tilt first, then break and collapse to one side. In fact, it collapsed in all directions and fell almost vertically, which once again proved that the damage of the earthquake was not caused by longitudinal waves and shear waves;
(4) Whether it is shear wave or longitudinal wave, their propagation is continuous, the intensity is gradual, and it gradually attenuates from the epicenter. In the range of tens to hundreds of square kilometers in the same seismic intensity zone, the damage to the same building should be roughly the same, but this is not the case. The earthquake damage is banded and varies greatly in a short distance. Half of a building collapsed and the other half was intact; Two side-by-side rails, one twisted and the other intact. These are not explained by longitudinal waves and shear waves.
⑤ Earthquake damage is not the function of "surface wave" (it is generally believed that surface wave plays a major role). According to the present explanation, the so-called surface wave is a secondary wave that propagates along the surface after the body wave reaches the surface. Some of its vibration directions are perpendicular to the propagation direction, which is the same as the nature of shear waves, including two types. One vibrates back and forth on the ground (Ralph wave), and the other rolls on the ground (Rayleigh wave). Surface waves can make the ground fluctuate. " Since the nature of surface wave is consistent with shear wave, it can't be the main factor of earthquake damage, and whether the so-called "surface wave" exists is still a problem.
The destruction of an earthquake is neither longitudinal wave nor shear wave, nor surface wave. What wave is it? Is it possible that some unknown wave is at work and has not been discovered? There are three ways to find the unknown wave. First, conduct on-site investigation to study the nature of failure stress. The second is to reanalyze the working principle and image of seismograph; Thirdly, the formation mechanism of elastic wave is discussed theoretically, and the relationship between elastic vibration and elastic wave is clarified. So far, source vibration and elastic propagation are still in a state of separate discussion. Not yet connected into a whole.
(2) Five key issues
There are many problems, which can be summarized as the following five points:
① Why are people more earthquake-resistant than factories?
② How to destroy the earthquake?
③ Does surface wave exist?
④ Are there any unknown waves?
⑤ How are elastic waves generated and what is the formation mechanism?
Understanding the formation mechanism of elastic waves is the key to solve five problems, and other problems can be solved easily.
Second, the formation mechanism of elastic waves
1, elastic vibration
The volume or shape of an object changes under the action of external force, and after the external force is cancelled, the object returns to its original volume and shape, which is called elastomer.
The circle (solid line) in fig. 33 represents a spherical elastic body. If external pressure FF' is applied along a certain diameter direction of the sphere. The sphere is elastically deformed by compression and becomes an ellipsoid (dotted line), which is called a deformed ellipsoid. The specific change (displacement) of each point in the deformed ellipsoid is different, and the magnitude and direction of the local force that promotes the change of each point are also different. These local forces actually cause the deformation (displacement) of each point of the sphere, which is commonly called internal force. There is also a reaction force that is balanced with internal forces, called resilience or restoring force. When the external force and internal force are eliminated, the deformed ellipsoid will return to its original state under the action of rebound force and become a sphere. Rebound force and internal force are equal in magnitude and opposite in direction, and act on the same point. The internal force or rebound force per unit area is called stress, which can be measured by stress unit.
Figure 34-A shows the original state of a spherical elastic object. Add a pair of external forces FF' to make it strain (Figure 34-b), and the generated internal forces are balanced by resilience with equal magnitude and opposite direction. Once the external force (FF') is cancelled, the internal force disappears, and the object will return to the original state (A) of the elastic body under the action of the rebound force. If the external force gradually disappears, the energy will be gradually consumed, and all points of the object will gradually return to their original positions, directly returning from (b) to (a), which is called "vibration-free recovery" or "static recovery". If the external force is suddenly cancelled, all points of the object will generate acceleration under the action of the rebound force, and the movement will be accelerated from (b) to (a); The closer we get to the original position (a), the greater the speed, and when we reach (a), we will reach the maximum speed, so we can't stop. We will continue to move forward at this initial speed, giving the object an internal force in the opposite direction (B), making it strain to (C), accumulating new rebound force, gradually transforming kinetic energy into elastic potential energy (strain), and gradually reducing the speed. When all kinetic energy is converted into elastic energy, if there is no energy loss during the movement, the absolute value of strain (c) should be equal to the original strain (b), but the strain direction is opposite. After reaching (c), it immediately transforms to (a) under the action of new toughness. The evolution process is the same as before, but in the opposite direction, from (c) to (a) and finally back to (b); After reaching (b), go from (b) to (a) and then to (c). This cycle is endless. This periodic change of strain and resilience of elastic objects is called "elastic vibration". If the medium is completely elastic and has no energy consumption, it will always vibrate and never stop. In fact, due to friction or other resistance, energy will be gradually consumed, and the absolute values of maximum strain and maximum rebound force in each cycle will gradually decrease. Finally, the rebound force will disappear completely, the vibration will stop and the object will return to its original position (A). This elastic strain recovery process is called "vibration recovery".
2. Elastic waves
Generation mechanism of (1) elastic wave
The vibration of an object can propagate outward from an elastic medium to form elastic waves. The vibration of the object that causes elastic waves is called "source". By analyzing the elastic vibration process shown in fig. 34, the waveform of elastic wave can be obtained. If (a), (b) and (c) in Figure 34 are drawn in the form of Figure 35, elastic wave waveforms in any direction can be directly obtained from the figure.
The circle in Figure 35 is the original sphere before strain, that is, the vibration source. Elastic waves propagate outward along the radius of the sphere. When an object vibrates, the strains in all directions are different, and the elastic waves are also different. The generation of elastic waves follows the following rules: the vibration of any point on the spherical surface of the vibration source propagates along the radial direction, and its waveform depends on the vibration trajectory of that point. This law is called the law of elastic wave generation. The second half period of vibration on the vibration source body is the same as the first half period, and the evolution direction is opposite, but it is called a complete periodic continuous wave train when it propagates.
The waveform of elastic wave propagating outward along Oa direction is determined by the vibration trajectory of point A, which is a straight line A 1-A-A2, which is consistent with the propagation direction (OA), and the tangent (tangent plane) of any point in the trajectory on the corresponding ellipsoid is perpendicular to the radial OA. Therefore, the vibration at point A propagates along the direction of OA, and the elastic wave generated is longitudinal wave (P wave) or compression wave, and the propagated particle vibrates back and forth along the propagation direction (Figure 35-a).
The vibration at point B propagates along the OB direction, which is also a longitudinal wave (Figure 35-B). The waveform is in the same direction as OA, but the time difference is half a cycle. At the same time, one moves outward and the other moves inward, but because the starting points of the two longitudinal waves are also different by half a period, the waveforms of the two are still the same on each sphere centered on O, but they are not synchronized.
In the OC direction, the strain ellipsoid is an inextensible surface. During the whole vibration process, the position of point C remains unchanged, but the instantaneous tangent of point C on the ellipsoid will change with the deformation of the ellipsoid, and the intersection angle of tangent and OC will change at any time. The trajectory of point C is a torsion pendulum, so the wave propagating in the OC direction is a torsion wave. Its propagation mode is similar to a clock balance wheel. The torsion angle of the propagating particle deviating from the original position is called the torsion angle, which can be expressed by the included angle α between the instantaneous tangent of point C on the ellipsoid and the tangent on the original sphere. The tangent of point C on the original sphere is perpendicular to OC, and the torsion angle α is zero. The torsion angle of point C on the ellipsoid is greater than zero at every moment. The greater the amplitude of the vibration source, the greater the strain, and the greater the variation range of the torsion angle α, and the maximum value of the torsion angle is proportional to the intensity of the torsion wave. Therefore, the maximum torsion angle can be used to express the intensity of torsion wave. Except for eight special points, such as A, B and C, the vibration trajectories of other points are S-shaped curves. For example, the trajectory of point D is d 1-d-d2 (Figure 35-D). Its vibration propagates outward along the OD direction. For the convenience of analysis, enlarge the trajectory of point D vibration as shown in Figure 36: dk is any position on the trajectory when the particle D vibrates to d2, and the movement from D to dk can be decomposed into three kinds of partial movements.
① The component motion (dk) along the OD direction propagates along the OD direction to form longitudinal wave (P).
② The component motion (Kdk) perpendicular to the OD direction propagates along the OD direction to form a shear wave (S) vibrating in the vertical propagation direction.
③ Torsion wave (N) is formed when the component motion of torsion pendulum propagates along OD.
It can be seen that any original wave with bending vibration trajectory can be decomposed into three basic body waves: longitudinal wave, transverse wave and torsional wave (Figure 35). The three body waves are homologous and born in the same place at the same time. It is only because of the different propagation speeds of the propagation media to the three waves that they gradually pull apart and operate independently.
Most actual vibration sources are not spheres. In this case, the elastic wave propagates outward along the curvature radius direction of each point on the surface of the vibration source, and the vibration trajectory of each point on the surface of the vibration source can be S-shaped, including three basic components, and there is no special direction that only produces one waveform.
(2) the propagation of elastic waves
P-wave is a kind of compression wave, and all objects with compression elasticity can propagate P-wave. Solids, compressed gases and liquids all have compressive elasticity and can propagate longitudinal waves. Vacuum has no compressive elasticity and cannot transmit longitudinal waves, such as sound waves. The vibration direction of longitudinal wave is consistent with the propagation direction, and the propagation particle propagates in one-dimensional space and vibrates linearly, so the propagation speed is the fastest.
The vibration of shear wave is perpendicular to the propagation direction, and only when the medium has compressive elasticity in the vertical propagation direction can shear wave propagate. Solids have compressive elasticity in all directions, and shear waves can pass through solids in any direction. Fluids such as liquids and gases are not. In large water bodies on the surface, such as the ocean, water droplets have to work hard to move upward due to gravity, so they have compressive elasticity, so they can spread shear waves along the horizontal direction, especially near the water surface, where gravity plays the most significant role and has the greatest ability to spread shear waves, such as water waves. Because of the large water body, the potential energy of two adjacent points is equal, and the resistance is very small, it is easy to produce flexible deformation after being stressed, consuming horizontal vibration and having little compression elasticity. Therefore, it is difficult for shear waves to pass through large water bodies vertically. The vibration of shear wave propagates vertically in two-dimensional space, and the propagating particles vibrate in the plane, so the propagation speed is slower than that of longitudinal wave. Seismic shear waves are generally slower than longitudinal waves 1/3 or so [1].
Torsional wave is an elastic wave formed by the torsional motion of an object particle, which can only propagate in an object with torsional elasticity.