There has been no strong earthquake in Sichuan for a long time. In the words of the industry, it is called lack of earthquake, which means lack of strong earthquake. According to historical experience, the longer the lack of time, 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. When it reaches a certain level, the crust will rupture and an earthquake will occur.
What the public is generally concerned about is why such a big earthquake happened in Wenchuan.
Generally speaking, Wenchuan is located in a major earthquake zone in China-the North-South earthquake zone.
Experts from the Seismological Bureau of China believe that because of the great differences in geological distribution, crustal thickness and crustal movement speed between the eastern and western parts of China, the earthquake occurred in this belt with the greatest difference between the eastern and western parts, that is, the north-south seismic belt-including from Ningxia via eastern Gansu, western Sichuan to Yunnan, which is an earthquake-intensive belt.
The macro explanation is that "the Indian Ocean plate collides with the Eurasian plate from south to north, and the collision area arches the Qinghai-Tibet Plateau." Professor Ni Sidao from the School of Earth and Space Sciences of the University of Science and Technology of China told the Southern Weekend reporter that "the Qinghai-Tibet Plateau is uplifting, but at the same time it is moving to the northeast, squeezing the Sichuan Basin to slip to the northeast, and the Wenchuan earthquake occurred in the southeast of the Qinghai-Tibet Plateau."
Ni Sidao said that the specific mechanism of the earthquake was extrusion. "At first, it was mainly extrusion, and at the end of the earthquake, there may be energy release from strike-slip." Ni Sidao said that the Wenchuan earthquake occurred on the southeast edge of the Qinghai-Tibet Plateau and in the center of Longmen Mountain in western Sichuan, located on the Wenchuan-Maowen fault zone.
Erchie Wang, a researcher at the Institute of Geology and Geophysics of the Chinese Academy of Sciences, said that the Sichuan Basin is a relatively stable land mass. 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 it at the beginning of 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 accumulation reaches a certain level, the crust will rupture and an earthquake will occur.
However, Erchie Wang also said, 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.
In a small way, Wenchuan is in the Longmenshan earthquake zone in Sichuan. Longmenshan is a place where the new Tibet uplift plate intersects with the old Yunnan and Sichuan plates. From this perspective, this place is an earthquake-prone area.
According to the Catalogue of Strong Earthquakes in China compiled by the Forecasting Management Office of the Monitoring and Forecasting Department of China Seismological Bureau in September, 1999, there were 18 earthquakes of magnitude above 7 in Sichuan, among which 8 earthquakes of magnitude above 7 occurred since 1800. 1973 An earthquake of magnitude 7.6 occurred in Luhuo, Sichuan, and an earthquake of magnitude 7.2 occurred in Songpan-Pingwu, Sichuan three years later.
In 2002, Chen Xuezhong, a researcher from Institute of Geophysics, Seismological Bureau, China, made an analysis on the earthquake risk 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 absence of earthquake, which means absence of strong earthquake. According to historical experience, the longer the absence, the more likely it will happen in the future. This is a qualitative estimate. "
Under this background, Chen Xuezhong studied the earthquake risk in Sichuan, and found that historically empirically, a strong earthquake of magnitude 8 or so occurred around Sichuan a few years before the occurrence of a strong earthquake of magnitude 7 or above. According to this phenomenon, an earthquake of magnitude 8. 1 year has occurred in Kunlun Mountain, and if this law continues, it will happen in Sichuan a few years later.
Experts say that earthquakes of magnitude 7 or above occur every year 18 times, and earthquakes of magnitude 8 or above1-2 times. China is pushed by the Indian plate and the Pacific plate, and the seismic activity is frequent. Zhang Guomin said that, from a big perspective, Wenchuan earthquake is located in a large earthquake zone in China-the North-South seismic zone, and the central axis seismic zone is located between longitude 100 degrees and 105 degrees, covering areas including Ningxia, eastern Gansu, western Sichuan and even Yunnan, belonging to China's earthquake-intensive zone. In a small way, Wenchuan is in the Longmenshan earthquake zone in Sichuan. Therefore, the probability of an earthquake here is high. According to Zhang Guomin, a researcher at the Institute of Earthquake Prediction, Seismological Bureau of China, the Wenchuan earthquake is an internal earthquake in mainland China, belonging to shallow earthquakes, and its destructive power is great. Zhang Guomin said that earthquakes can be divided into shallow earthquakes, moderate earthquakes and deep earthquakes according to focal depth. Most of shallow earthquakes occurred in the depth of more than 30 kilometers below the surface, and the deepest earthquake can reach about 650 kilometers. Among them, shallow earthquakes has a high frequency of earthquakes, 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.
The Indian plate subducted into the Asian plate, resulting in the rapid uplift of the Qinghai-Tibet Plateau. The plateau material flows slowly eastward, squeezing eastward along the Longmenshan structural belt at the eastern edge of the plateau, and encountering the stubborn resistance of the rigid block under the Sichuan basin, resulting in the long-term accumulation of tectonic stress energy, which is 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 belongs to one-way rupture earthquake, which moved from south-west to north-east, causing aftershocks to expand to the north-east; After the main shock, the process of stress propagation and release is slow, which may lead to the strong aftershock and long duration. It's shallow earthquakes. Wenchuan earthquake did not belong to the effect of deep plate boundary, but occurred in the brittle-ductile transition zone of the crust, with a focal depth of 10 km-20 km, so it was extremely destructive.