Formation of Longmenshan earthquake fault zone

200 million years ago, the Qinghai-Tibet Plateau began to rise as the Indian plate pushed northward and inserted under the Eurasian plate. Subsequently, the Himalayas were born. At the same time, there is also a geological fault zone located on the edge of the Qinghai-Tibet Plateau. First of all, we must understand how mountains are formed. Mountains are generated by the uplift of blocks due to compression. Then, on the one hand, the mountain expands due to extrusion, and the underground breaks due to extrusion, which is a twin relationship. There are more than 20 geological fault zones like this in China, most of which are concentrated in the Qinghai-Tibet Plateau. There are geological fault zones inside the Qinghai-Tibet Plateau and on the boundaries of surrounding blocks interacting with the Qinghai-Tibet Plateau. The most representative is the north-south fault zone between the Qinghai-Tibet Plateau and the Sichuan Basin. This belt runs from Ningxia, Gansu and Sichuan to Yunnan, just forming a north-south broadband. Not a single fault zone, but several fault zones have a common feature in this place: some turn and some interact, which happens to be the north-south zone. This geological fault zone is also called the north-south seismic zone. Seismologists call it that because it is an earthquake-prone area in China. The north-south seismic belt mainly refers to a feature of seismic activity, with relatively high seismic frequency and relatively high intensity in some areas. According to the survey of geologists, Longmenshan seismic belt in Sichuan Province is the most active in the north-south seismic belt. Longmen Mountain stretches for hundreds of kilometers, starting from Lixian and Wenchuan in the south and reaching Pingwu and Qingchuan in the north. According to historical records, there have been many large and small earthquakes in the past two or three hundred years, including more than a dozen earthquakes of magnitude 6.

Why do earthquakes occur so frequently in Longmenshan seismic belt? From Chengdu Plain to Dujiangyan, it soon entered the mountainous area, and the terrain changed dramatically. Longmen Mountain still represents the interaction between the Qinghai-Tibet Plateau and the eastern block. There is a relative movement of one to three millimeters between Longmen Mountain and Sichuan Basin every year. The movement of Longmen Mountain indicates that the Qinghai-Tibet Plateau is moving eastward. Once blocked by the hard Sichuan basin, there will be a strong collision between them. When two objects move relatively, the relatively soft object in the west pushes the relatively hard object in the east, so we call it thrust. Backward slip is the dislocation of both sides, and normal fault is the fall of the hanging wall on the fault, which belongs to a relaxed state and then falls. Thrust is the upward extrusion in the case of tight extrusion. In other words, the movement form between plates is by no means single. In the process of thrust movement, the energy released by the earth is the largest, which directly leads to the occurrence of earthquakes.

In fact, earthquakes, like wind, rain and lightning, are natural phenomena that often occur on the earth. Seemingly calm underground, there is always a hidden crisis. The earth is divided into crust, mantle and core. Materials in the core, mantle and crust are all moving. Especially in the mantle, convection is formed in many places, and the result of mantle convection drives the crustal movement. In the 20th century, scientists discovered the internal structure of the earth with seismic waves, which is called egg-shaped structure. Among them, the shell is equivalent to the eggshell, the mantle is like protein, and the kernel is like egg yolk. The upper part of the earth is mainly composed of rocks, which closely cover the main body of the earth. However, the rock formation is not monolithic, but consists of six major plates and several small plates: the Pacific Ocean, Eurasia, America, Indian Ocean, Africa and Antarctica. The interior of the plate is relatively stable, but the edge zone and contact zone of the plate are unstable, which is the main place for violent crustal movement such as earthquake and volcanic activity. The plate floats on the asthenosphere at the top of the mantle. Because the upper mantle temperature is low and the lower mantle temperature is high, thermal convection will occur. Although this flow is very slow, less than 5 cm a year, the power it produces is earth-shattering. When the flow of mantle material is hindered by rock strata, it will produce powerful forces. Let this fracture appear on the surface. When two opposite mantle material flows meet, trenches or ridges can also be formed under strong compression. The mantle material flow near the rock layer is called asthenosphere. The asthenosphere itself looks like the part before molten iron that we see in this steelmaking furnace. It is molten iron, and iron itself can have certain fluidity. Therefore, the asthenosphere carries all the floating plates. Plates can drift away from each other, but they can also be thought of together. This kind of submarine expansion of the ocean crust will be formed when they drift away, and when they meet together, it will also have a strong orogeny. Like the uplift of the Himalayas. This is the result of this effect 70 million years ago.