What are the detailed reasons for the Wenchuan earthquake?

Zhang Guomin, a researcher at the Institute of Earthquake Prediction of the Seismological Bureau of China, told this reporter that the Wenchuan earthquake belongs to shallow earthquakes and has great destructive power.

Earthquakes can be divided into shallow earthquakes, moderate earthquakes and deep earthquakes according to focal depth. Shallow earthquakes mostly occurs at a depth of more than 30 kilometers below the surface, accounting for more than 70% of the total number of earthquakes, and the released seismic energy accounts for 85% of the total released energy. It is the main producer of earthquake disasters and has the greatest impact on human beings.

In seismology, the higher the magnitude, the greater the destructive power. The magnitude of Wenchuan earthquake is 7.8, and the destructive power in the epicenter area is about 10 degree, which will cause disasters such as house collapse, geological landslide and ground fissure.

At present, there is no detailed data about the earthquake and the distribution table of earthquake intensity reflecting the destructive power of the earthquake (the US Geological Survey published a rough distribution map of earthquake intensity on its website, which was obtained without field investigation). Chen Changlin from the Seismological Bureau of China told the reporter that after two days, the earthquake intensity distribution table was estimated to be 1. The relevant expert group has arrived in Wenchuan and the investigation has started.

Fortunately, although the earthquake intensity is the same as the Tangshan earthquake 32 years ago, it seems that the loss should be smaller than the Tangshan earthquake now.

1976 Tangshan earthquake is the biggest natural disaster since the founding of the People's Republic of China, causing 240,000 deaths. At that time, the epicenter was below the central area of the city, which was a direct earthquake in the city, so the damage was very serious. There are no big cities near the epicenter of this Wenchuan earthquake, and the local population is a little more than 65,438+10,000, which is sparsely distributed. Most of the polar seismic belts, including Wenchuan county, which are about 200 kilometers long and more than 0 kilometers wide 10, are in mountainous areas, and the losses will be smaller than those of Tangshan earthquake.

Another key point is that the earthquake happened during the day. "Most people are active outside, which greatly reduces the possibility of death, so you will also find that in some areas, schools have more serious casualties because students are in the building." An expert from the Seismological Bureau of China told Southern Weekend.

Regarding the reason why the earthquake affected most provinces in China, Ni Sidao of China University of Science and Technology said that the earthquake with the magnitude of 200 1 8. 1 at Kunlun Mountain Pass in Qinghai was felt in the west of China, but not in the east. The Sichuan earthquake is relatively east, about 1500 km from Beijing and Shanghai. For other areas, it belongs to teleseism, that is, it can be felt from 1000 km away. The surface wave of this earthquake has great influence on high-rise buildings, but little influence on low-rise buildings. There are many high-rise buildings in most cities now, so many cities are affected.

Erchie Wang, a researcher at the Institute of Geology and Geophysics, Chinese Academy of Sciences, said that strong earthquakes were felt in many parts of the country, and another important reason was that the Sichuan block east of the Wenchuan-Mao Wen fault zone where the earthquake occurred was relatively hard and had strong seismic wave propagation ability.

Forecasts are rarely accurate.

In response to the rumor that the earthquake prediction was concealed, Zhang Hongwei, spokesman of China Seismological Bureau, said on 13 that it was "unreasonable" to conceal the earthquake prediction results in order to ensure stability before the Olympic Games.

Earthquake prediction or prediction in China can be divided into four types: medium, long, short and imminent. At present, the accuracy of medium-and long-term prediction in China is about 30%, while the short-term prediction rate is very low. Moreover, earthquake prediction itself is a worldwide problem, and it is difficult to "go to heaven" and "go to the ground", which determines the difficulty of earthquake prediction.

At present, there are many kinds of earthquake monitoring in China, one is to monitor the earthquake activity itself, and the other is to monitor the changes of groundwater, gravity and deformation. The monitoring network in each province consists of many monitoring stations. "There was no prediction before this earthquake. It's normal from the current technical level. There are very few accurate predictions, unless there are many foreshocks in the earthquake, which can be alarming. " Chen Xuezhong, a researcher at the Institute of Geophysics, Seismological Bureau of China, said. "In terms of earthquake prediction, developed countries such as the United States and Japan are not more advanced than us. Their advancement lies in the safety of building fortification, and the location and magnitude can be determined quite quickly after the earthquake. " Ni Sidao said.

In addition, no unit or individual except the government has the right to publicly release information about the earthquake in China. "After being summarized by professionals and demonstrated by expert jury, government departments will release forecasts based on this information. Medium-and long-term forecasts should be reported to the government for preparation every year. What really works is short-term and imminent forecasting, such as the scale of half a month, which can achieve good results. " Chen Xuezhong said.

According to the long-term forecast and the local earthquake situation in previous years, the local government has a fortification standard in urban planning, and takes into account the economic cost to do a good job in urban building prevention. There is no unified standard for seismic grade of buildings in China, and the specific standards are mainly led by local governments. Fortification is a government act, and fortification work is carried out according to local financial resources.

"Wenchuan and other counties in the extreme earthquake zone were originally earthquake-prone areas, and fortification preparations in this area are indispensable. Obviously, the preparations for this huge earthquake are not sufficient. " Chen Xuezhong said, "Judging from the current situation, Chengdu is close to the epicenter, but there are no major casualties and impacts. It can be seen that this is related to the relative norms of prevention in large and medium-sized cities. "

The fortification situation in rural areas varies greatly across the country. "Some places are still very problematic, basically unprotected, and the government needs to guide them."

Reservoir induced earthquake problem

Another question that the public is very concerned about is whether the earthquake is related to the Three Gorges Project, Dujiangyan and other water conservancy projects. Conversely, does the earthquake have an impact on these water conservancy projects?

"I think they are geographically far apart, and contact is unlikely. There is no clear basis for saying that they are related. It is impossible to induce earthquakes, and it is impossible to induce earthquakes so far. This is a pure tectonic earthquake. Didn't there be an earthquake before the Three Gorges Project? " Chen Xuezhong said.

Earlier, Zeng Xinping, director of the Earthquake Division of the Yangtze River Committee Design Institute, said that the Three Gorges Dam and Wenchuan are not in a geological structural unit. The Three Gorges reservoir area is mainly located in the mountainous area of western Hubei and the eastern Sichuan Basin, and Wenchuan is located in the Longmenshan seismic belt, which has no geological connection. So the Wenchuan earthquake has nothing to do with the Three Gorges Project.

Zeng Xinping said that the earthquake induced by reservoir impoundment is mainly in the reservoir area, generally about 5 kilometers away from the reservoir bank, and the farthest is not more than 15 kilometers. At present, the backwater of the Three Gorges Reservoir is less than that of Chongqing, and even if an induced earthquake occurs, it is in the area below Chongqing. Previously, the earthquakes induced by the impoundment of the Three Gorges Reservoir were mainly in the reservoir area east of Wushan.

Chongqing spokesman Zhou Bo 13 said that there were 17 reservoirs with potential safety hazards after the earthquake, but no potential safety hazards were found after the monitoring of the second and third phases of the Three Gorges reservoir area by the land department.