In the history of many countries, earthquakes have left behind some painful national memories, including the Tangshan earthquake in 1976 and the Wenchuan earthquake in 2008, the Northeast earthquake in Japan in 2011, and the San Francisco earthquake in the United States in 1906.

earthquake is one of the most important natural disasters faced by human society. In the history of many countries, earthquakes have left behind some painful national memories, including the Tangshan earthquake in 1976 and the Wenchuan earthquake in 2008, the Northeast earthquake in Japan in 2011, and the San Francisco earthquake in the United States in 1906. So, what are the rules for earthquakes? Can earthquakes be predicted? In fact, this is not only a question of concern to the public, but also a question that seismologists have been thinking about. Unfortunately, earthquake prediction is still a global scientific problem. However, this does not mean we know nothing about earthquakes. For example, we already have a certain understanding of the "old nest" fault zone of earthquakes.

What is a fault zone

Natural earthquakes occur on fault zones. The fault zone can be said to be the home of earthquakes. Then how did the fault zone come about? We know that the outermost layer of the earth is the lithosphere, but the lithosphere is not integrated, but is "breaked" into seven major plates, and the boundary between the plates is a large fault zone. Driven by factors such as mantle convection and the gravity of the plate itself, each plate has been in the relative motion of . Since there is relative movement, friction will inevitably occur on the boundaries of the plates, and one of the results of this friction is an earthquake. This is the basic understanding brought by the greatest achievement in earth science in the 20th century - plate tectonic theory .

In short, the fault zone can be regarded as a large fracture in the lithosphere with relative displacement. There is a sector boundary, is there any inside the sector? Some, similar to the concept of a plate, geoscientists have discovered more subdivided blocks inside the plate, called plots. Various types of fault zones have also been formed on the boundary of the plot. Compared with the fault zone at the plate boundary, the scale and relative sliding rate of the fault zone inside the plate are smaller. However, this does not mean that the earthquake magnitude or the harm in the plate will be relatively small. Long-term stress accumulation (hundreds of years or even thousands of years) can cause major earthquakes of magnitude 7 or even 8 or above within the plate.

my country's fault zone

Figure 1. Distribution of major fault zones in my country ( Zhang Peizhen et al., 2013, Chinese science -Earth science)

To understand the distribution rules of earthquake disaster , we must figure out the distribution of fault zones. Figure 1 shows the distribution of major fault zones in my country. In the eastern part of my country, the most obvious one is the Tanlu Fault Zone (called the Yilan-Yitong Fault Zone in the northern section), which is the main source of earthquake threats in the eastern part. The 1975 Haicheng earthquake, which is internationally recognized as a successful prediction, occurred in the Tanlu Fault Zone. There is a fault zone perpendicular to the Tanlu Fault Belt in North China, called the Zhangjiakou-Bohai Fault, which was the culprit of the Tangshan earthquake in 1976.

In Figure 1, you may have noticed that most of the dazzling many fault zones in western my country exist inside and around Qinghai-Tibet Plateau. The Alkin Fault and Haiyuan Fault in the north (the 8-magnitude earthquake occurred in Haiyuan in 1920) to the Xiaojiang Fault and Honghe Fault in the south, causing serious earthquake disasters to Tibet and Qinghai, as well as surrounding provinces such as Sichuan, Yunnan, Gansu, and Xinjiang. Why are the earthquake activities on the Qinghai-Tibet Plateau and surrounding areas so severe? The main source of power is the squeezing of the Indian Ocean plate on the Eurasian plate. On the one hand, it caused the uplift of the Qinghai-Tibet Plateau, and on the other hand, it brought huge pressure to the internal and surrounding plots of the Qinghai-Tibet Plateau. When the friction force on the fault cannot withstand the pressure, it will cause a staggered movement and cause an earthquake.

Fault zone of recent earthquakes

Since 2021, 7 earthquakes of magnitude 6 or above have occurred in my country (Figure 2). Among them, the Yangbi Mw 6.1 earthquake in Yunnan in 2021 (near the Weixi-Qiaohou fault), the Maduo Mw 7.3 earthquake in Qinghai ( Kunlun Pass -Jiangcuo fault), the Menyuan Mw 6.7 earthquake (Lenglongling fault), the Lushan Mw 5.8 earthquake in 2022 (Longmenshan fault) and the Luding Mw 6.6 earthquake (Xianshuihe fault). These earthquakes all occur on different fault zones inside and around the Qinghai-Tibet Plateau.Inside Sichuan, there are three main fault zones in the shape of Y, namely, the Longmenshan fault along the northwest edge of in the Sichuan Basin, the Xianshui River fault extending from the inside of the Qinghai-Tibet Plateau to the southeast, and the Anning River fault to the south. The Wenchuan earthquake in 2008 and the Lushan earthquake in 2013 occurred on the Longmen Mountain fault. The Luding earthquake on September 5, which has attracted widespread attention recently, occurred at the southernmost end of the Xianshui River fault (Moxi section).

Figure 2 The survey, nature description and risk assessment of major earthquakes and fault zones in my country since 2021 have always been the top priority in earthquake disaster research. Earth scientists have conducted long-term multi-angle and multi-scale research on the fault zone through various means such as seismology, geology, geodesy, and geodynamics. While achieving fruitful results, they have also stimulated various new scientific problems. On the way to find out the old nest of earthquakes, earth scientists are still exploring unremittingly.

Source: University of Science and Technology of China