1. The hazards of earthquakes
The mantle material rises due to thermal convection. After reaching the bottom of the lithosphere, it first stretches horizontally in the opposite direction, and then sinks to both sides. They roll like a pair of gears, with each pair of teeth approaching, engaging and moving apart.
The crust in the upper part of the mantle is like a conveyor belt. Dragged by the movement of mantle magma, the crust stuck on it separates to both sides, thus forming a rift. It is like a piece of paper laid flat on a gear. Driven by the rotation of a pair of gears below, the paper is torn apart along the bisector between the gears and brought to both sides. This tearing process is the process of earthquake occurrence, which is a form of crustal movement. For the earth, an earthquake is a tiny crustal movement, and its displacement is so small that it can be ignored.
The 7.8-magnitude Tangshan earthquake that occurred in 1976 caused the upper and lower displacement of the earth's crust to be only 20 to 30 centimeters. It led to the total destruction of urban buildings and leveled a city in a matter of tens of seconds, causing 24.2 Ten thousand people died and 164,000 were seriously injured.
The magnitude 8 Wenchuan earthquake in 2008 caused the upper and lower displacement of the earth's crust to be only 3 to 6 meters. It destroyed a city and killed more than 60,000 people. The 2004 Indian Ocean tsunami was triggered by an earthquake. The waves were 20 meters high and swept across the entire Indian Ocean, leaving more than 280,000 people missing and dead. It is caused by a fault that is more than 1,000 kilometers long and has an upper and lower dislocation of 13 to 15 meters.
In Shanxi, we see faults and folds exposed on the cliffs. The vertical displacement of the strata is at least a hundred meters, and faults of several thousand meters are common. That was the result of the Yanshan Movement more than 100 million years ago. As for the Luliang Movement 1.8 billion years ago, it is inferred from the temperature of mineral formation in the rocks on both sides that the amplitude of the rise and fall was greater than 10 to 20 kilometers. The Wutai Movement, which occurred 2.5 billion years ago, caused the crust to subduct to a depth of more than 30 to 40 kilometers.
So earthquakes are “pediatric” in terms of crustal movement.
The degree of earthquake damage is generally related to the intensity of the earthquake. The higher the magnitude, the more severe the damage. It is also related to the depth at which the epicenter occurs. For the same magnitude, the shallower the earthquake, the greater the destructive power. Because the epicenter is deep and the upward expansion area of ??the power is large, the power intensity must become smaller due to dispersion. On the contrary, the epicenter is shallow, the diffusion range is small, and the power intensity is concentrated, so the damage is great.
The degree of earthquake damage is also related to the structure of the earth's crust. For earthquakes that occur in bedrock mountainous areas, the vibrations are directly transmitted to buildings through the rock surface. For earthquakes that occur on the plains, the vibrations are transmitted from the underlying rocks through the surface sand and mud layer. If the sand and mud layer contains more groundwater, and the time is slightly longer, the mixture of sand, mud and water will be shaken. They will "liquefy". They are originally a solid layer of mud, and when they vibrate, they will turn into liquid mud. It will amplify the destructiveness of the vibration waves to the buildings dozens of times, so all the buildings above will seem to be built on mud. It all collapsed with one shake. We can see the process of silt liquefaction in construction. The "vibration tamper" is driven by an electric wheel and inserted into the semi-solid cement. It immediately turns the semi-solid cement into a liquid and soon flows and becomes smooth. The following The large and small air cavities were quickly filled with flowing sand and mud.
The damage rate and casualty rate of the Tangshan earthquake were high. The reason is that Tangshan City was built on a layer of silt. Its foundation was completely liquid, and the shock wave (transverse wave) was immediately destroyed as soon as it passed through the foundation. It was broken into pieces and fell apart, and all the houses collapsed in an instant. If the epicenter of the Wenchuan earthquake had occurred in the Chengdu Plain, the number of casualties would have exceeded one million. Fortunately it happened in the sparsely populated Qiongxia Mountains in western Sichuan. It must also be pointed out that during the Tangshan earthquake, there were few casualties among coal miners at depths of hundreds of meters. This is because in the strata under the earth's crust, the rocks in the tunnel were squeezed together in all directions. During the earthquake, they had become a whole and followed the vibration waves. When moving together, only a few roofs and lane walls collapse.
This case tells us that if we live in a fully buried basement, the probability of survival is also very high. In ordinary residential buildings, the upper third of the basement is on the ground. It is easy to collapse, but the preservation space must be larger than that of a room where the upper part completely collapses.
2. Earthquakes in the Fenwei Rift Valley in Shanxi Province
Open the seismic map of Shanxi Province and find the contour map classified according to earthquake intensity. Contours with intensity levels above 4 are concentrated in the rift valley. area, its boundary almost coincides with the rift edge fault. All intensity zones above level 5 are in the rift valley. The highest intensity zone is level 6, which is located in the Linfen Basin. There are only two earthquakes with magnitude 8 in the province, and their epicenters are both. In the center of Linfen Basin. In addition to the Fenwei Rift, Shanxi seismic zones are also distributed along the Taihang fault zone in the direction of the Xiyang-Jincheng connection, but they are all low-intensity faults. In addition, there are also sporadic large Lishi faults running north-south.
The current earthquake distribution map shows that the more recent the earthquakes are, the more earthquakes there are. This does not mean that the earthquakes are more active in recent times. It is a reflection that the older the earthquakes are, the fewer records there are. First, there are continuous wars, and who cares about recording earthquakes (especially low-level earthquakes) in the midst of swords and swords? Second, governments change frequently and there is a lack of full-time personnel for stable periods. Without instruments, the location of the epicenter is more difficult to determine. For example, for an earthquake, 5 to 6 counties recorded the earthquake. The epicenter must be determined by comparing the degree of damage in several counties. Otherwise, the only recorded (or saved) recorded) which county is regarded as the epicenter. Although there was a seismometer in ancient my country, it has not been imitated so far. Even if there is such an instrument, perhaps at least two of them should be installed at the same time so that the two lines can intersect into an intersection point.
In this earthquake map, only two earthquakes were recorded in BC, one during the Warring States Period and the other in the second and third centuries BC. The reasons for the low number of earthquake records include: first, the vast territory and sparsely populated areas, and insufficient sensitivity; and the most important reason is that in the past, only the central government recorded earthquakes in the Kyoto area, while local officials had no record-keeping duties. Only when a major earthquake occurs and there are too many casualties will the earthquake be reported to the central government. Little earthquakes are not mentioned.
In ancient Chinese mythology, Emperor Gong’s head touched Buzhou Mountain, which caused the western pillars to break, the ground to sink to the northwest, and the water to flow eastward. This should be a huge earthquake that occurred in ancient times, which could change the flow of rivers. In mythology, the Gong family ranked after the Yellow Emperor and before Dayu. At that time, the Central Plains region had entered the agricultural period. Some people have verified that his activity area is in the Yishui and Luoshui areas in northwestern Henan, which is close to the Fenwei Rift Valley and slightly south. There is also research that the war between Jiang Gong and Zhuan Xu took place on the east side of Taihang Mountain. No matter where mentioned above, they are all very close to Shanxi and within the Central Plains farming area.
From the point of view of water flowing eastward, it is more likely that it is on the east side of the Taihang Fault Zone (still within the Taihang Fault Zone). Due to fault activity, the east side of the block was broken off, forming a gap and causing water flow. Flow eastward. This kind of earthquake that changed the course of many rivers should be a major earthquake caused by super fault activity with a fault distance of at least several hundred kilometers. Its impact can be strongly felt in all parts of the Central Plains. It is deeply imprinted in the minds of the ancients, so It has been passed down orally and was recorded in history by later historians. It won't be fabricated out of thin air. It should be the first giant earthquake in our country's human history.
Since the Tang and Song dynasties, there have been frequent major earthquakes in Shanxi. The famous ones include the collapse of the Yingying Tower in Yongji, the cracking of the top of the Anyi Tower in Yuncheng, the earthquake in Quwo, the Guoyang earthquake in the 1950s, and the earthquake in the 1990s. Yanggao earthquake, there are people who specialize in studying these earthquakes.
Earthquakes, as far as the current scientific level is concerned, cannot yet be predicted because there are too many unknowns. The general principle is that the ground stress is unbalanced. In order to achieve balance, the stress is released by vibrating in the stress concentration zone. Using scientific methods, there are now instruments that can measure the magnitude and direction of geostress. If a network can be established across the country, for example, one for every 100 to 1,000 square kilometers, the magnitude and direction of the stress in the normal field can be found first. When an earthquake occurs, There may be signs of stress, and it can be concluded that when the abnormal value is several times the normal value, an earthquake will occur. Unfortunately, this work is currently limited to a few earthquake observation stations and has not yet been launched.
There is a legend about "earthquake clouds" in Japan. When stress is concentrated to a certain extent, certain ionization anomalies will be emitted into the atmosphere, thus forming clouds of special shapes. Unfortunately, the researcher did not report the major earthquake in Japan a few years ago, which caused tsunamis, damaged nuclear power plants, and triggered nuclear leaks.
Many animals will run out before an earthquake, such as mice fleeing, chickens not entering their nests, and loaches being disturbed and coming out of the water to breathe in groups. However, there is no conclusion as to whether these phenomena are inevitable or accidental.
The earth’s crust is a collection of broken bodies, and there are countless large and small fractures in the earth’s crust. Some are healed scars, and some are still ulcerated wounds, which may cause recurrence for unknown reasons.
For example, after the construction of a large reservoir, the amount of groundwater infiltration increases and the water pressure in the reservoir increases, which will cause small earthquakes of magnitude 2 to 4, which may occur hundreds of times. But it has never been known to cause devastating earthquakes.
When trains and trucks pass by, people will also feel the ground shaking. Let me say here that people can only feel earthquakes with magnitude 3 to 4 or above, so earthquakes with magnitude 4 or above are called felt earthquakes, and only earthquakes with magnitude 6 or above will cause disasters. The Shanxi Rift Valley is an area prone to earthquake disasters, so the construction department requires newly built houses to be protected against magnitude 8 earthquakes.
The earthquake-prone areas in Shanxi are the five major basins, which are the rift valley areas. The soil in the rift zone has a high water content and a thick sand and mud layer. Once it shakes like the Tangshan earthquake, it may not be a negative consequence of the Tangshan earthquake. Generally speaking, bedrock mountainous areas are safer than basin areas.