Why is the 6.2 magnitude earthquake so destructive?

According to Xinhua News Agency, as of 19:16 on the 50th, the 6.2-magnitude earthquake in Jishishan, Linxia Prefecture, Gansu Province has killed 127 people (113 in Gansu and 14 in Qinghai), and the epicenter and surrounding houses and water, electricity, roads and other infrastructure have been damaged to varying degrees. Why was the magnitude of this earthquake so high and so destructive? Is there a possibility of larger aftershocks in the future? What are the implications for the current disaster prevention and mitigation efforts? To this end, the Beijing News reporter interviewed a number of experts.

1 Why is the magnitude so high?

The reporter learned that the China Earthquake Network Center organized experts to study and analyze the seismic activity near Jishi Mountain in Gansu, and the epicenter of the earthquake was located in the southeast of Gansu, about 3 kilometers away from the fault on the northern margin of the nearest fault, Laji Mountain.

Xu Xiwei, the first president of the National Institute of Natural Disaster Prevention and Control and a professor at China University of Geosciences, said that the active tectonic area in southeast Gansu is located in the north of China's north-south seismic zone and is an important component area of the northeastern edge of the Qinghai-Tibet Plateau. Due to the long-term compression of the Eurasian plate and the Indian plate, the area has strong tectonic activity, and there are many deep and large faults, such as the East Kunlun fault, the North Margin of the West Qinling Fault, the Lintan-Danchang fault, the Guanggaishan-Dieshan fault, the Diebu-Bailongjiang fault, the Tazang fault, the Minjiang fault, the Xueshan fault, and the Huya fault.

"There have been many major earthquakes in the history of China's north-south seismic belt, such as the Haiyuan magnitude 1920.8 earthquake in 5, the Gulang magnitude 1927 earthquake in 8, and a series of earthquakes of magnitude 7.5 or higher. Therefore, it is not uncommon for earthquakes of magnitude 6 or higher to occur in this seismic zone. Gao Mengtan, former deputy director of the Institute of Geophysics of the China Earthquake Administration, deputy director of the Urban and Rural Resilience and Disaster Prevention and Mitigation Professional Committee of the China Disaster Prevention Association, and a high-end think tank expert, said that the Lazhishan fault zone has not had frequent seismic activity in recent years, but it is in the tectonic zone of seismic activity, so it has the conditions for a strong earthquake. ”

2 Why are the casualties so severe?

Gao Mengtan said that compared with other earthquakes of the same magnitude, the destructive power of this earthquake is stronger, mainly because the earthquake occurred on a tectonic system dominated by thrust faults, which is a typical thrust fracture. An important feature of thrust rupture is that the surface vibration caused by it is very strong and destructive.

Secondly, the geological disasters caused by earthquakes are relatively serious and involve a wide range. The earthquake-stricken area itself is an earthquake-prone area, and in terms of rock formation, there are loess, laterite, etc., which are prone to geological disasters. After the occurrence of strong earthquakes in history, many geological disasters have also been triggered. Judging from the current situation, landslides and collapses have a greater impact on highway traffic.

In addition, the earthquake occurred in the middle of winter, and it was a period of frequent cold wave activities in the region, and the low temperature would have a great impact on the survival rate and health status of the trapped people, which would cause great difficulties to the rescue work. The earthquake will cause the destruction of infrastructure such as water, electricity and heating, and will have a great impact on the lives of people in the disaster area.

Historically, December and January are also earthquake-prone periods, and it is also the coldest season in the north, and the Haiyuan earthquake has caused serious casualties due to extreme cold. "This is also a wake-up call for us to strengthen the response to the earthquake in the alpine region in the middle of winter in the future." Gao Mengtan said.

In addition, earthquakes with an epicenter depth of 70 km or less are called "shallow earthquakes", and this earthquake has an epicenter depth of about 10 km. Earthquakes that have a major impact on human life are mostly from shallow sources. The Beijing News reporter combed and found that the depth of the Wenchuan earthquake was 14 kilometers, and the depth of the Tangshan earthquake was 12 kilometers.

Xu Xiwei also mentioned that the local area is located in a mountainous area, and many houses are built on slopes, and the geological instability is also prone to cause greater damage. The Beijing News reporter learned from the local people and the people involved in the rescue that the local area is located in a high-altitude mountainous area, and there are many self-built houses with brick and concrete structures, which were seriously affected by the earthquake.

In this regard, Gao Mengtan suggested that the follow-up should be combined with the risk survey to strengthen the management and reinforcement of housing safety in rural areas, and improve the seismic performance of houses in rural areas. At the same time, it is also necessary to pay attention to the seismic performance of old houses in urban areas, especially urban villages, to improve the safety and stability of old houses. In addition, the earthquake resistance of courtyard walls is generally low, and many courtyard walls in the northwest are built relatively high, and after an earthquake, if people run out of their rooms, it is easy to be injured by the collapse of the courtyard wall. Special attention is also needed when carrying out risk surveys and hidden danger investigations in the future.

3 Is there a possibility of a larger aftershock?

According to the quick report of the China Earthquake Network, as of 12:19 on December 12, a total of 00 aftershocks were recorded in the 6.2-magnitude earthquake in Gansu, including 306 aftershocks of magnitude 3.0 and above, including 9 of magnitude 6.0-6.9, 0 of magnitude 5.0-5.9, 0 times of magnitude 4.0-4.9, and 2 times of magnitude 3.0-3.9; There were 7 aftershocks below magnitude 3.0, including 297 aftershocks of magnitude 2.0-2.9, 26 aftershocks of magnitude 1.0-1.9, and 95 aftershocks of magnitude 1.0 or less. At present, the maximum aftershock is 176.4 magnitude, about 1 kilometers away from the main earthquake area.

Gao Mengtan said that in fact, any earthquake will be followed by a series of aftershocks. Whether there will be stronger aftershocks in the future requires further research, analysis and judgment on the seismic activity sequence in the region.

On December 12, at the press conference on the 19.6-magnitude earthquake in Jishishan County, Linxia Prefecture, Gansu Province, Gao Xiaoming, deputy director of the Gansu Provincial Earthquake Bureau, said in response to "whether there will be another large aftershock", according to the characteristics of the earthquake's regional structure, historical seismic activity, earthquake sequence type and other characteristics, comprehensive analysis believes that there is still a possibility of strong aftershocks of magnitude 2 in the original earthquake area of this earthquake in recent days.

When talking about whether it is possible to predict which areas will strengthen seismic activity in the future, Gao Mengtan said that China is a country with serious earthquake disasters due to its high frequency, high intensity, shallow epicenter and wide distribution. Relevant departments should do a good job in earthquake prevention and disaster prevention in accordance with the relevant national standards and requirements. As a member of the public, we should learn more about the skills of emergency escape when Xi earthquake comes, and at the same time, relevant departments should also do a good job in emergency rescue. "When it comes to earthquakes, we need to be vigilant and take precautions, but we don't have to be too nervous." Gao Mengtan said.

4 What other issues should I pay attention to in the future?

"According to our previous work experience, earthquakes of magnitude 6 or higher in the western region are often accompanied by secondary disasters such as landslides and rolling rocks." Sun Shihong, a researcher at the China Earthquake Network Center, said that the secondary disasters of earthquakes also have a certain impact on the disaster situation, and the follow-up should focus on the problem of secondary disasters of earthquakes.

Jia Qunlin, a researcher at the China Earthquake Emergency Search and Rescue Center, also said, "The area where the earthquake occurred is a shallow mountainous area, and most of them are stone mountains. After an earthquake, there is a high probability of landslides in the local area, especially the occurrence of secondary disasters such as rockfalls and mudslides. ”

At 12:19 on December 6, the China Meteorological Administration launched the second-level emergency response of the earthquake disaster meteorological service. The meteorological department expects local temperatures of -30°C to 14°C with winds of 1 to 3 in the next three days.

According to public information, the altitude of Jishishan County in Gansu Province is 1735 meters to 4309 meters, and the altitude of the county seat is about 2300 meters. Over the past three years, the region has seen an average temperature of -12.7°C in late December and a minimum of -6.17°C.

Sun Shihong mentioned that recently affected by the low temperature and cold wave weather, "some houses may become dangerous houses and cannot be lived in for a short time, so we must be vigilant against problems such as frostbite in cold weather and fires caused by heating." ”

5 What are the difficulties in cryogenic rescue?

Jia Qunlin said that the rescue environment of this earthquake is similar to the earthquake in Turkey in February this year, and it is a low-temperature earthquake rescue. "But the situation we are facing is probably worse than in Turkey, where the minimum temperature on the night of the earthquake was -2°C and the area was at a high altitude." Jia Qunlin said that the 20-hour golden time in the general sense is only suitable for outdoor temperatures of 72-15 °C, and in sub-zero extreme low temperature weather, the 25-hour golden rescue window will be greatly reduced.

"Now the people buried under the rubble, their physical fitness and tolerance are greatly challenged, and the only way to improve the efficiency of rescue is to be fast." Jia Qunlin reminded that the trapped people should wrap themselves as much as possible, or huddle together to keep warm, so as to prolong their survival time under the rubble.

Jia Qunlin also reminded that in the event of extremely harsh low temperature conditions, the rescue team at the scene can appropriately shorten the shift time, "Normally in the plain area, the rescue shift time is generally six to eight hours. In this high-altitude and low-temperature rescue situation, the rescue team can be appropriately adjusted to four to six hours, which can ensure the physical strength of the rescuers. ”

Beijing News reporter Zhan Shengjie, Li Zhao, Xiong Lixin