Why is this earthquake so harmful? What caused it?

Experts analyze the causes of Wenchuan earthquake. The concrete mechanism of this earthquake is mainly compression at first, and strike-slip energy may be released at the end of the earthquake. There has been no strong earthquake in Sichuan for a long time. In the words of the industry, it is called no earthquake, which means no strong earthquake. According to historical experience, the longer the absence, the greater the possibility of future (strong earthquake). Although it seems that the tectonic activity in Longmenshan area is not strong, it may be in the process of stress accumulation. To a certain extent, the earth's crust will break and earthquakes will occur. The public is generally concerned about why such a big earthquake happened in Wenchuan. Generally speaking, Wenchuan is located in the North-South seismic belt, a major seismic belt in China. Experts from the Seismological Bureau of China believe that due to the great differences in geological distribution, crustal thickness and crustal movement speed between the east and the west of China, the earthquake occurred in the zone with the greatest difference between the east and the west, that is, the North-South seismic zone, which includes from Ningxia to Yunnan via eastern Gansu and western Sichuan, which is an earthquake-intensive zone. The macro explanation is that "the Indian Ocean plate collided with the Eurasian plate from south to north, and the collision zone arched the Qinghai-Tibet Plateau." Professor Ni Sidao from the School of Earth and Space Sciences of China University of Science and Technology told Southern Weekend reporter, "The Qinghai-Tibet Plateau is rising, but it is also moving to the northeast, squeezing the Sichuan Basin to slide to the northeast. The Wenchuan earthquake occurred in the southeast of the Qinghai-Tibet Plateau." Ni Sidao said that the specific mechanism of the earthquake is squeezing. "At the beginning, it was mainly squeezing, and there may be a slip of energy release at the end of the earthquake." Ni Sidao said that the Wenchuan earthquake occurred on the southeast edge of the Qinghai-Tibet Plateau, in the center of Longmen Mountain in western Sichuan, and was located on the Wenchuan-Mao Wen fault zone. Erchie Wang, a researcher at the Institute of Geology and Geophysics of China Academy of Sciences, said that the Sichuan Basin is a relatively stable block. According to historical records, although there was no major earthquake in the main body of Longmen Mountain, there was a strong earthquake in Songpan in the north of the last century. Therefore, although the Longmenshan area seems to have weak tectonic activity, it may be in the process of stress accumulation. When the stress accumulates to a certain extent, the crust will break and an earthquake will occur. However, Erchie Wang also said that at present, we don't know the mechanism of this earthquake, and we don't know whether it was caused by the compression, extension or horizontal strike-slip of the earth's crust. To Xiaoli, Wenchuan is in the Longmenshan earthquake zone in Sichuan. Longmenshan is the intersection of the new Tibet uplift plate and the old Yunnan and Sichuan plates. Seen from this angle, this place is an earthquake-prone area. According to the Catalogue of Strong Earthquakes in China compiled by Forecast Management Office of Monitoring and Forecasting Department of Seismological Bureau of China 1999 in September, there were 8 earthquakes with magnitude above 7 1800 in Sichuan, among which 8 earthquakes with magnitude above 7 occurred since1800. An earthquake of magnitude 7.6 occurred in Luhuo, Sichuan in June of 5438+0973, and an earthquake of magnitude 7.2 occurred in Songpan-Pingwu, Sichuan three years later. In 2002, Chen Xuezhong, a researcher at the Institute of Geophysics, Seismological Bureau of China, analyzed the risk of earthquakes of magnitude 7 or above in Sichuan Province: "There has been no strong earthquake in Sichuan for a long time. In the words of the industry, it is called no earthquake, which means there is no strong earthquake. According to historical experience, the longer the absence, the more likely it will happen in the future. This is a qualitative estimate. " In this context, Chen Xuezhong studied the earthquake risk in Sichuan, and found that historically, there was a strong earthquake of about 8 years before a strong earthquake of magnitude 7 or above occurred in Sichuan. According to this phenomenon, an earthquake of magnitude 8. 1 occurred in Kunlun Mountain in 200 1 year. If this rule continues, this kind of earthquake will happen in Sichuan in a few years. According to experts, there are earthquakes of magnitude 7 or above 18 times and earthquakes of magnitude 8 or above 1-2 times every year in the world. China is pushed by Indian plate and Pacific plate, and there are frequent earthquakes. Zhang Guomin said that from a big perspective, the Wenchuan earthquake is located in a large earthquake zone in China-the North-South seismic zone, and the central axis seismic zone is between 100 and 105, covering the areas from Ningxia to eastern Gansu, western Sichuan and Yunnan, belonging to the earthquake-intensive zone in China. To Xiaoli, Wenchuan is in the Longmenshan earthquake zone in Sichuan. Therefore, the probability of an earthquake here is very high. According to Zhang Guomin, a researcher at the Institute of Earthquake Prediction, Seismological Bureau of China, Wenchuan Earthquake is an internal earthquake in Chinese mainland, which belongs to shallow earthquakes and has great destructive power. Zhang Guomin said that earthquakes can be divided into shallow earthquakes, moderate earthquakes and deep earthquakes according to focal depth. Most shallow earthquakes occurred at a depth of more than 30 kilometers below the surface, and the deepest earthquake could reach about 650 kilometers. Among them, shallow earthquakes has a high frequency, 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 the Indian plate, which has the greatest influence on human beings, sneaked into the Asian plate, leading to the rapid uplift of the Qinghai-Tibet Plateau. The plateau material slowly flows eastward and squeezes eastward along the Longmenshan structural belt on the eastern edge of the plateau, encountering the tenacious resistance of the rigid block under the Sichuan basin, resulting in the long-term accumulation of tectonic stress energy, and finally suddenly released in Beichuan-Yingxiu area of Longmenshan. Thrust, dextral and compressional fault earthquakes. The seismogenic structure is the central fault zone of Longmenshan structural belt, which thrusts from southwest to northeast under the action of compressive stress. The earthquake was a one-way rupture earthquake, which moved from southwest to northeast, causing aftershocks to expand to northeast. After the main earthquake, the process of stress propagation and release is slow, which may lead to strong aftershocks and long duration. This is a shallow earthquake. Wenchuan earthquake does not belong to the deep plate boundary effect, but occurred in the brittle-ductile transition zone of the crust with the focal depth of 10 km -20 km, so it is extremely destructive. Earthquake prediction is a worldwide problem, and precursory observation before a major earthquake is very important. Although we have observed some pre-earthquake information this time, it is very different from the precursors of the great earthquakes in the 1960s and 1970s, and it is not as rich as the previous precursors. Moreover, some earthquakes with relatively small magnitude were surprisingly calm before the big earthquake. Since the Kunlun Mountain M = 8. 1 earthquake in 20065438, there has been no M = 7 earthquake in seven years, and even the M = 6 and M = 5 earthquakes have been "quiet" for a long time. According to our experience in the past decades, there should have been some connections before the earthquake, but none of them appeared. 1976 There was no precursor before Tangshan earthquake, but there were still many fluid anomalies before Tangshan earthquake, including one month before Tangshan earthquake. The groundwater levels in Beijing and Tianjin were found to have risen a lot, but none of them appeared this time. Therefore, this earthquake is the one with the least information before the earthquake since China began to make earthquake prediction in 1966. The reporter learned from the Ministry of Land and Resources on June 5438+08 that since June 5438+05, the China Geological Survey held an analysis meeting on Wenchuan earthquake and its induced secondary geological disasters. Experts from China Land and Resources Aerogeophysical Remote Sensing Center, China Geological Environment Monitoring Institute, Chinese Academy of Geological Sciences, Institute of Geology of Chinese Academy of Geological Sciences, Institute of Geomechanics and other units initially formed three disaster conclusions based on the results of investigation, monitoring and evaluation. First, the Indian plate subducted into the Asian plate, which led to the rapid uplift of the Qinghai-Tibet Plateau. The plateau material slowly flows eastward and squeezes eastward along the Longmenshan structural belt on the eastern edge of the plateau, encountering the tenacious resistance of the rigid block under the Sichuan basin, resulting in the long-term accumulation of tectonic stress energy, and finally suddenly released in Beichuan-Yingxiu area of Longmenshan. Second, thrust, dextral and compressional fault earthquakes. The seismogenic structure is the central fault zone of Longmenshan structural belt, which thrusts from southwest to northeast under the action of compressive stress. The earthquake was a one-way rupture earthquake, which moved from southwest to northeast, causing aftershocks to expand to northeast. After the main earthquake, the process of stress propagation and release is slow, which may lead to strong aftershocks and long duration. The third is shallow earthquakes. Wenchuan earthquake does not belong to the deep plate boundary effect, but occurred in the brittle-ductile transition zone of the crust with a focal depth of 10 km-20 km, which is extremely destructive.