I saw news two days ago that nearly half of China's major cities are sinking.
At first, I was still wondering how this method of sinking was. I wouldn’t know it unless I searched. When I searched, all kinds of pictures of cracking and collapse of the ground popped up.
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I was already quite annoyed because I had a single day off this week, but when I saw this kind of news, my anxiety suddenly came back.
I also immediately went to find out the source of the news.
It turns out that the cause was a blogger who introduced a paper titled “Assessment of Land Subsidence in Major Cities Nationwide” recently published by Peking University in Science.
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After putting some of the viewpoint data from the paper into the video, I came to the conclusion that “nearly half of China's big cities are sinking.” The topic even reached the top of the hot search list.
As soon as the popularity exploded, many marketing accounts also took advantage of it, saying that this is indeed the case:If you don’t believe me, the ground at Hangzhou East Railway Station has subsided so much that the train doors and platform are no longer on the same level. (Actually, it’s because the models are different)
Related videos also start with millions of views.
Anyway, after this matter became popular, basically everything was said on the Internet. What about “No matter where you and I are buried, we will eventually meet in the sea in 200 years”, and there are also people who want to buy a house in the countryside as soon as possible.
Out of curiosity, I asked AI to help me check the papers, and found that the hot search conclusion that “nearly half of China's major cities are sinking” was more or less a flurry of news.
First of all, the statement “nearly half” is a bit loose.
Because the samples studied in this paper are actually cities, provincial capitals, and major industrial cities in China with a population of more than 2 million, a total of 82 cities.
According to the conclusion in the paper that “45% of areas in China's major cities have a subsidence rate of more than 3 mm per year, and 16% of areas have a subsidence rate of more than 10 mm”, in fact, only more than 30 cities have been affected.
Moreover, according to the city size classification standard, urban areas with a permanent population of more than 1 million are called large cities. There are currently 105 large cities in the country.
Therefore, the value range of “near” in the hot search “nearly half of the major cities are sinking” is a bit too generous.
and,This matter is somewhat of a myth, because humans have discovered the phenomenon of ground subsidence for hundreds of years, and the principles behind it have been basically thoroughly studied.
Let's put it this way, in fact, both natural and man-made factors may cause the ground to sink.
Let’s start with natural factors, including crustal plate movement, such as subduction and uplift. There are also geological conditions, such as the depth of the bedrock. If the bedrock is deep, the soil layer above it will be thicker. The soil layer, especially the softer clay layer, will be easily compressed. On the contrary, if the bedrock is shallow, the support capacity will be better.
In addition, changes in groundwater levels caused by rainfall and drought may also cause land subsidence.
But in fact, for the urban sinking that everyone has discussed in recent years, more of the blame lies with humans, especially the exploitation of groundwater, which is the main “culprit behind the scenes.”
Generally speaking, groundwater refers to the water stored in the pores of rocks and soil in underground aquifers. When we dig a well, we usually dig to the location of the aquifer.
Under normal circumstances, the aquifer has a certain degree of support for the surface.
But when groundwater is exploited, the lack of water will lead to a reduction in the pore water pressure of the aquifer.
It doesn’t matter if you really can’t understand it. Let’s simply understand it asThe aquifer is compressed, causing the ground to sink and that's it.
In my country's North China Plain, more than 75% of water demand is supported by groundwater.
According to the statistics,The area of land subsidence greater than 200 mm in the North China Plain reaches 64,000 square kilometers. Among them, the maximum cumulative subsidence in Tianjin even reached 3.25 meters. (2022 data from China Geological Survey)
In addition to groundwater extraction, too many and heavy buildings can also cause ground subsidence.
Now that we have figured out why it sinks, is this matter serious?
To be honest, if it sinks too much, something will really happen.
Last year, there was a community in Tianjin. Due to ground subsidence, the road surface suddenly cracked, and even cracks appeared in the residential buildings, prompting the emergency evacuation of nearly 3,000 people.
It is also mentioned in the paper that disasters related to land subsidence in China cause direct economic losses of more than 7.5 billion yuan every year.
Moreover, ground subsidence is not unique to our country. On a larger scale, this is a global problem.
Take Mexico City, which has the fourth largest amount of groundwater extraction in the world. According to foreign media reports, it sinks by about 50 cm every year.
As well as Jakarta, the capital of Indonesia, some organizations predict that by 2025, 26.86% of Jakarta will be submerged in sea water.
So as early as 2022, Indonesia passed the capital relocation bill and prepared to run away.
At this point, you guys may be wondering again. If it's so serious, why don't most of us feel anything about it? There is a saying, just because we can't see it with the naked eye, it doesn't mean that the ground subsidence hasn't happened.
One is because the subsidence velocity in most areas is on the order of millimeters.
For example, if we use the data in the paper that the annual settlement rate exceeds 3 millimeters, then it will be 6 centimeters in twenty years, which may be less than a curb height.
The algorithm is not very rigorous, but it should be able to explain some problems. Just like chronic rhinitis, this disease may not be life-threatening, but it has always been quite torturous.
In addition, compared with other countries, the overall rate of land subsidence in most of our areas is indeed slow.
The paper mentioned,The median land subsidence rate in 82 large and medium-sized cities across the country is about 2 mm per year, and the extreme domestic subsidence rate is 2.2 cm per year.
Compared with Mexico City, which has about 50 centimeters per year, and Tehran, Iran, which has about 36 centimeters per year, it pales into insignificance.
Moreover, our country has also been controlling over-exploitation of groundwater in recent years, replenishing groundwater, and installing subsidence monitoring points on the ground. In some places, satellite remote sensing technology is also used to monitor the dynamics of ground subsidence in real time.
in addition,The land subsidence problem in many major cities in China has also been controlled to a certain extent. For example, the amount of land subsidence in Shanghai has dropped from 105 mm in 1958 to less than 6 mm per year. The prevention and control effect can be regarded as a world benchmark.
In short, land subsidence is like boiling a frog in warm water, but we are not sitting still and waiting for death, but are constantly trying to put out fires and control the temperature.
Therefore, for us ordinary people, there is no need to be too anxious about this matter. We should do whatever we should.
The ground is sinking, but humans have never stopped moving forward, right?