Can't stop the earthquake, then think about it
Taking the car from Chengdu to Moxi Town, Luding County, Hu Kaiheng looked out the window almost all the way: the road was relatively smooth, and it seemed that only a small part of the surface of the mountain in the distance had collapsed.
This shows that the earthquake that occurred the day before may not have caused serious geological disasters, Hu Kaiheng thought.
On September 6, the Sichuan Province emergency response expert for geological disasters and colleagues from the Chengdu Institute of Mountain Disasters and Environment of the Ministry of Water Resources of the Chinese Academy of Sciences (hereinafter referred to as the Institute of Mountains) rushed to the earthquake area: September 5, Sichuan Ganzi Tibetan Autonomous Prefecture A 6.8-magnitude earthquake hit Luding County.
Previously, researchers from the Mountain Institute had made a quick assessment of the earthquake. Based on the magnitude, intensity, and population of the earthquake area, they applied past earthquake empirical models and formulas to estimate the casualties: 10-60 people may be killed.
After going deep into the earthquake area, Hu Kaiheng found that the consequences of the earthquake were more serious than expected: geological disasters occurred frequently, collapses and landslides cut off roads, affecting rescue and rescue. "Like a magnifying glass", secondary geological disasters aggravated the Luding earthquake zone. losses to come.
Subsequent data updates corroborate this.
As of 17:00 on September 11, the Luding earthquake had killed 93 people and lost contact with 25 people.
Why is the Luding earthquake more destructive than earthquakes of the same magnitude?
Starting from Moxi Town and walking towards the lower reaches of the Dadu River, Hu Kaiheng saw a series of landslides and collapses, which almost cut off the road through the township.
The thrilling journey of more than 40 kilometers did not end until he arrived at Wanggangping Township, Shimian County.
Hu Kaiheng recalled, "The earthquake loosened the slope, and it fell and smashed houses and cars."
According to a public article published by the public account of the Mountain Institute on September 8, according to the results of post-earthquake landslides interpreted by drone images, there were 151 earthquake landslides in the Wandong Village investigation area, and there were a large number of large landslides, with a landslide area of 0.68 square kilometers. ; There are 704 landslides in Moxi Town, with a landslide area of 2.5 square kilometers.
According to the detection of multi-source radar, there are 17 suspected landslide areas in Caoke Township, Shimian County.
After earthquakes, landslides and collapses are the most common types of geohazards.
These sudden falling rocks and clods can smash vehicles and damage roads, making it more difficult to rescue the Luding earthquake.
This is one of the tens of thousands of earthquakes that occur on Earth every day, caused by the squeezing and collision of two plates.
The movement of the Earth's crust creates fault zones where earthquakes often occur, consisting of faults that form on the ground and rock blocks on either side.
According to Hu Kaiheng's current investigation, the active fault zone caused by the Luding earthquake is about 20 to 30 kilometers long, runs north-south, and extends from Hailuogou to Wanggangping Township. Landslides and collapses are also concentrated on both sides of this active fault zone for 5 kilometers. range of places.
The Luding earthquake is a "strike-slip" earthquake. It is like two cars crossing each other face to face. The two plates slide close to the horizontal. The position and distance have changed, but the height has not changed.
The plate that slides first is called the active block and rubs against another passive block.
Hu Kaiheng visited and found that the landslides and collapses were concentrated on the side of the active block, on the west side of the fault zone.
"Large-scale landslides and deep rock landslides are rare, and most collapses and landslides occur in shallow layers." He worries that the possible future aftershocks and precipitation will cause the previously loose mountains to continue to destabilize and form more serious debris flows. .
Xu Qiang, vice president of Chengdu University of Technology and executive deputy director of the State Key Laboratory of Geological Hazard Prevention and Geological Environmental Protection, said that generally speaking, earthquakes of less than magnitude 7 will not cause large-scale landslides.
However, Luding is a high mountain and canyon area with complex terrain and prone to geological disasters.
Gongga Mountain in Luding County ranks the highest peak in Sichuan with an altitude of 7556 meters.
Kong Jiming, a researcher at the Institute of Mountains, said that in general, the occurrence of landslides is controlled by conditions such as landforms, lithology, and slope structure. Disaster conditions vary.
When various conditions that are not conducive to the stability of the mountain are combined together, an unstable slope with steep slope, low lithological strength and loose slope structure is formed.
He said that the Luding earthquake was located on the eastern margin of the Qinghai-Tibet Plateau and was obviously affected by strong tectonic activities. Mountainous areas generally have conditions for the formation of landslides and geological disasters, and landslides are prone to occur under the induction of the earthquake.
Kong Jiming said that the degree and scope of geological disasters are affected by the magnitude and intensity of the earthquake.
The higher the magnitude, the greater the intensity, the stronger the damage to the mountain, the larger the scope of influence, and the more secondary geological disasters that will occur later.
Hu Kaiheng said that in the future construction planning and road site selection, especially the "hidden points" of the slopes with loose rock mass should be avoided.
That's a geological disaster-prone area.
Gao Jianguo, a researcher at the Institute of Geology of the China Earthquake Administration, said that in 2022, an earthquake with a magnitude of 6.1 occurred in Lushan County, Ya'an City, Sichuan Province.
The natural topography brings difficulties to the rescue and rescue of the Luding earthquake.
Gao Jianguo said that some villages in the epicenter are sandwiched between two mountains, and the area is narrow, and it is difficult to find a flat land that can be used as a parking pad for drones.
Provincial and national roads are built on the slopes of the mountains, and residential buildings are built along the roads.
He analyzed that the lack of emergency shelters in rural areas makes it difficult to prevent geological disasters caused by earthquakes. In the future, the group measurement and group prevention of geological disasters should be combined with the group measurement and group prevention of earthquakes, and it should be written into the earthquake prevention and disaster reduction plan.
"Do a good job in the prevention of geological disasters in rural areas, this is the last mile."
The Luding earthquake occurred in the active Xianshuihe fault zone
Many earthquake and geology experts interviewed by China Youth Daily and China Youth Daily said that the Luding earthquake had been predicted, and the magnitude of 6.8 was within the range estimated by experts; Luding County is in the central and southern part of the Xianshuihe fault zone. On this fault zone, a magnitude of 6.8 is neither too small nor too big.
Looking at several previous studies, the Xianshuihe fault zone has always been the fault zone with the most frequent seismic activity in Sichuan. Since 1700, there have been 8 earthquakes of magnitude 7 or above, which occur on average once every 40 years.
Xu Qiang said that before the 6.8-magnitude earthquake in Luding, many experts predicted that a 6.5-7.2-magnitude earthquake would occur in this fault zone, which "may happen at any time."
After all, the last earthquake in this area was a magnitude 7.6 earthquake in Luhuo County in 1973. It has been 49 years, exceeding the average time interval since records began.
It is the focus of seismologists and geologists all over the country, and relevant scholars have started relay research as early as the 1970s.
Gao Jianguo said that the arrangement, excavation and research of the historical data of the Xianshui River was the fastest and most productive of the more than 20 earthquake zones in China at that time.
Spread out the map of Sichuan, those earthquake-prone areas are like a Y-shape, the main trunk from Anning River, Zemu River all the way north, to Kangding City, Shimian County near the two fork roads.
One fork is called the Longmenshan fault zone, which extends to the northeast and passes through Mianyang and Guangyuan cities; the other is the Xianshuihe fault zone, which leads to the northwest and extends from Kangding all the way to Liangzhou.
The most famous earthquake in the Xianshuihe fault zone occurred on June 1, 1786, from Kangding to Moxi.
Gao Jianguo said that the earthquake triggered severe landslides and landslides, which blocked the Dadu River and formed a huge dammed lake.
Aftershocks continued, the dam of the dammed lake broke, washed away the downstream county towns and villages, and killed more than 100,000 people. "At that time, the dammed lake was more harmful than the earthquake."
In 1984, the China Earthquake Administration cooperated with the United States and conducted intensive research here.
Gao Jianguo said that the Xianshuihe fault zone is similar to the San Andreas fault zone in the United States, which is an area where earthquakes are frequent in the United States and faults can be seen on the ground.
However, until today, the scientific community still has not fully grasped the movement law of the Xianshuihe fault zone.
Warning and forecast are not the same thing
A researcher from the China Earthquake Administration in his 80s recalled that when the Tangshan earthquake occurred in 1976, more than 100,000 aftershocks were recorded manually. Dozens of researchers gathered in a large conference room and set up several tables. The instrument was attached to the drum, and the probe-shaped pen bounced back and forth, drawing waveforms on the paper on the surface of the drum, and they calculated the location of the earthquake based on the waveform.
After the "5.12" Wenchuan earthquake, the National Earthquake Intensity Rapid Report and Early Warning Project was launched, and the number of earthquake monitoring stations across the country increased from more than 1,200 to more than 15,000 today.
Monitoring stations are generally built in densely populated locations with good communication signals.
It turned out that the stations were separated by more than 100 kilometers, and it was necessary to monitor the first-degree earthquakes in long distances, and the construction cost was very high.
After the stations became denser, the monitoring range was reduced, and the construction cost of each station was also reduced from about 1 million yuan to several thousand yuan.
The aforementioned researcher recalled that when the Wenchuan earthquake occurred in 2008, he and his colleagues did not know that Beichuan was seriously affected until they reported the situation on May 13, after the Beichuan communication was restored.
"Only the epicenter was known at the time, but the distribution of the vibration intensity was inaccurate."
Now, two minutes after an earthquake occurs, the earthquake intensity quick report can outline the range of larger vibration intensity, which is convenient for rescue teams to determine rescue plans.
The construction of intensive earthquake monitoring stations is also to achieve more accurate earthquake early warning.
Earthquake early warning and earthquake prediction are different: the former is the earthquake information sent to the surrounding area through the time difference after the earthquake occurs, which can be realized by current technology; the latter is to predict the time, location and magnitude of the earthquake before the earthquake occurs. Still a worldwide scientific problem.
In an earthquake, the main shock usually lasts for more than 10 seconds.
According to Wang Tun, director of the Chengdu High-tech Disaster Reduction Research Institute, if people can receive an early warning 3 seconds after an earthquake, the number of casualties can be reduced by 14%, and if people receive an early warning 10 seconds in advance, the number of casualties can be reduced by 39%, and 20 seconds in advance, It can be reduced by 63%.
However, there are blind spots in earthquake early warning.
Wang Tun said that in the Luding earthquake, the blind spot of the early warning system of the Chengdu High-tech Disaster Reduction Research Institute was a circle with a radius of about 15 kilometers centered on the epicenter of Moxi Town.
In this range, the public will feel the earthquake first before receiving the warning message.
Before the earthquake wave arrives, the farther away from the epicenter, the earlier the people receive the warning: Kangding City received the warning 7 seconds in advance, Ya'an City received the warning 20 seconds in advance, and Chengdu City received the warning 50 seconds in advance.
To reduce the dead zone, it is necessary to shorten the system response time and optimize the communication signal.
In the past 10 years, the average response time of the earthquake early warning network of the Chengdu Hi-Tech Disaster Reduction Institute has dropped from 9 seconds to within 5 seconds.
A more ideal state is to realize earthquake prediction before the earthquake.
The Chengdu High-tech Disaster Reduction Research Institute plans to build another 2,000 underground cloud map monitoring stations in Sichuan and Yunnan, of which 650 have been built so far.
Wang Tun described the "underground cloud map" as taking "X-ray films" of the internal forces and energy of the earth, and dynamically monitoring the stress and energy of 5-20 kilometers underground through ground sensors.
Underground cloud map monitoring stations are more precise and are mostly set up in the wild, which can reduce errors caused by human activities.
As of 11:20 on September 10, the underground cloud map recorded a total of 1,061 aftershocks of the Luding earthquake, most of which were concentrated in asbestos.
Shimian County is located at the junction of three fault zones, only a few kilometers away from the epicenter.
"Whether these aftershocks will trigger or accelerate the movement of other fault zones still requires continuous attention." Wang Tun said.
The capture of aftershocks also helps to achieve early warning of geological hazards that occur simultaneously with earthquakes.
Wang Tun told China Youth Daily and China Youth Daily that the dammed lake formed by the Luding earthquake is small, but the risk of rockfall, landslides and debris flows is greater, and a wider disaster chain will be formed in mountainous areas.
The follow-up disaster reduction institute will combine data capture to release early warning information of geological disasters to relevant regions.
Of these aftershocks, except for 42 aftershocks of magnitude 3.0 and above, the vast majority were tiny movements 10 kilometers underground.
In the future, these data will also help to calculate the underground stress and energy change process and realize earthquake prediction.
Improving earthquake resistance of home buildings in high-risk areas
At the request of the Sichuan Provincial Department of Housing and Urban-Rural Development, Lei Jie, an expert from the Sichuan Institute of Building Research, arrived at Moxi Town at 3:00 a.m. on September 6 to conduct an emergency assessment of housing construction and municipal infrastructure.
There was no electricity in the town, and only the faint light around the tents shone on the ruins of the buildings on the road.
During the inspection the next day, Lei Jie found that many self-built houses lacked professional guidance, had problems with their structural systems, and were severely damaged. Most of the shops facing the street were gone on the first floor: such a top-heavy building, the upper three or four floors were used for It is a brick-concrete structure, supported by a few pillars below.
If it is changed to a wall, the usable area will be reduced. Restaurants and supermarkets pursue economic benefits and often do not use them.
Xu Qiang, vice president of Chengdu University of Technology, said that the Gongga Observation Station of the Mountain Institute was located in the epicenter, and the first floor collapsed.
He explained that the source of the earthquake was 16 kilometers deep underground, and the seismic waves traveled vertically upwards in the epicenter.
In 2008, a similar house collapse occurred in Beichuan: several floors were stacked on top of each other due to up and down vibrations.
Lei Jie lives in a hotel in Moxi Town. After investigation, the building has good seismic performance.
But the aftershocks continued, and Lei Jie still had to jump up twice a night, "reasonably told me (the house) was okay, but I still wanted to run when the aftershocks came."
On September 8, he went to Baiyangping Village, which is only a few kilometers away from the town, to conduct a house assessment. The roads were mainly winding mountain roads, and the villagers' main means of transportation were motorcycles. The road was seriously damaged by landslides and falling rocks. It takes more than two hours in the village.
There are "almost no complete houses left" in the village. The adobe and old brick houses are basically tilted or collapsed, and two elderly people were killed in the collapse.
Lei Jie found that after the Wenchuan earthquake, the local villagers basically stopped using prefabricated slabs with simple construction technology but prone to cracks. Cast-in-place slabs with high cost and complex technology but strong stability are now more popular.
But the masons in small places are still unprofessional when it comes to the structural system of house construction.
Some houses in the village are built on landslides, and cracks have appeared in the ground in the courtyard.
"The hills over there are relatively steep. Building a house on the only piece of flat land is a very steep hillside." But even if the house is marked "forbidden to use", many people are still reluctant to go to the town's For the resettlement site, a temporary shed was built on the mountain with colorful strips of cloth and plastic cloth, because of "the kind of complex that is hard to leave the homeland".
"China Earthquake Parameter Zoning Map" stipulates building fortification requirements according to the different earthquake risks in each region, and the regions are divided into 6, 7, 8, and 9 degrees according to the seismic risk from low to high.
According to the fifth-generation "China Earthquake Parameter Zoning Map" specification, Moxi Town is located in a 9-degree seismic fortification zone.
Lei Jie said that the "China Earthquake Parameter Zoning Map" is a macro concept in the design specification, generally accurate to the county level, but it is difficult to be accurate to the village.
According to Wang Tun, there are two types of damage caused by the earthquake. One is the collapse of the house caused by the vibration, and the other is the strong pulling force of the fault, which directly destroys the house above the fault.
He said that many of the damaged houses at the epicenter of the Luding earthquake were built near active faults.
Xu Xiwei, president of the National Academy of Natural Disaster Prevention and Control, said in an interview with China Science and Technology News that it is necessary to draw a map of the distribution of active faults across the country through careful mapping, to "dig out" the source of earthquake disasters in the city, and try to "avoid" during construction. .
Xu Qiang said that the construction projects near the Xianshuihe fault zone have a high seismic level: the seismic fortification intensity of buildings in Kangding has reached level 9; the Dagangshan Hydropower Station has reached level 10; Sichuan-Tibet Highway and Sichuan-Tibet Railway are also high. level of seismic fortification level.
However, most of the collapsed houses in the epicenter area of the Luding earthquake are brick-concrete, stone and wood structures with poor seismic performance.
He suggested that my country should incorporate the seismic fortification of buildings in high earthquake risk areas into medium and long-term disaster reduction plans, "Should we set up mandatory standards to include buildings in high earthquake risk areas, including self-built houses of local residents, into mandatory should be considered in the seismic fortification?"
Lei Jie said, "Small earthquakes can be repaired, medium earthquakes can be repaired, and large earthquakes will not collapse. This is the basic goal of seismic design." He introduced that Sichuan Institute of Building Research is planning to prepare relevant guidelines for rural housing construction, which will increase construction costs. Control it within 10%-20%, and try to enhance the earthquake resistance of the house when the economic cost is reasonable.
He believes that as long as the seismic design is strictly implemented, even if there is an earthquake of relatively large magnitude, at least the house can be guaranteed not to collapse, and there will be no large-scale casualties.
Lei Jie has returned to Chengdu from Luding. He learned from his colleagues that the prefabricated houses in the earthquake area are ready to be built to help the people support the day when their homes are rebuilt.
He wrote in the circle of friends, "The deceased has passed away, and the living are like this. May the heaven and earth be peaceful."
China Youth Daily, China Youth Daily reporter Wei Xi Jiao Jingxian Source: China Youth Daily
September 14, 2022 Version 06