Analysis Of Deep Structure And Seismic Characteristics Of Longmenshan Fault Zone

The Longmenshan fault zone is located at the Eastern Tibet, north from the Qingchuan, go through the Nanba, Beichuan, Yingxiu, Baoxing to LuDing, total length is about 560 km, width is 30-50 km. This belt is mainly composed of 4 faults from the west to the east, which are Maoxian-Wenchuan fault (Back Longmenshan fault), Beichuan-Yingxiu fault (Central Longmenshan fault), Anxian-Guanxian fault (Frontal Longmenshan fault) and Guangyuan-Dayi (hidden fault), respectively. The whole fault zone shows the characteristics of the earth’s physical field change abruptly, such as landform, crustal thickness, tectonic stress field, density and velocity. The complex surface morphology and lithology of the belt makes it still lacks a comprehensive and accurate understanding about the deep structure of Longmenshan fault zone. The Longmenshan fault zone is an active seismic zone, have occurred large earthquakes many times in history, but until now, people don’t have a unified understanding about the nature of the earthquakes on the Longmenshan fault zone. Such as how is the fault fracture and trigger earthquake? After the main quake, what’s the law of aftershock happened? What’s the factor that make the earthquake stopped? Is there existing patterns or laws which are recognized by human beings before catastrophic rupture of the fault, and so on? Therefore, research and analysis of Longmenshan fault zone’s deep structure and the characteristics of earthquakes are the keys to solve the above problems and understand the mechanism of the earthquake.This dissertation is based on the "Wenchuan Earthquake Fault Scientific Drilling" of the National Science and Technology Planning Project and the project of China Geological Survey "Longmenshan tectonic belt three-dimensional geological mapping". By analyzing artificial seismic reflection profiles and aftershock sequence space distribution of large earthquake, deep structures and the characteristics of earthquakes of Longmenshan tectonic belt are studied here.Through research and analysis, it reveals the fine crustal structure and construction of 3 sections of Longmenshan fault zone:north, middle and south, discusses the relationship between deep structural features and seismic, formed as below:1, The north segment of the Anxian-Guanxian fault with NW dipping angle of 45° at ≥ 14 km depth, and begin to slow in the depth of 8km, and this fault could extend to decollement surface, it shows the imbricate fault mostly,and the bottom may exist the hidden fault. The all middle segment of this fault show the characteristic:North East steep south west slow, and develop various structural types:dual structure, step fault, punching concave structure, V-belt and flower shaped structure. the south segment of this fault with NW dipping angle of 50°at≥10 km depth, and begin to slow in the depth of 4km, the angle change to 25°,According to the shape of the fault in the section, it is inferred that it extends from the low angle to the deep, and the low angle geometric feature constrain the pure thrust movement properties. From the aftershock distribution map, we can see that the earthquakes are less in Anxian-Guanxian fault northern part, the fault distance is relatively far away from the aftershock concentrated area. There is a large number of earthquakes in the middle section of the fault, which may be related to the development of the hidden faults. The southern section of the vast majority of the aftershocks are distributed along both sides of the fault, are located below the Anxian-Guanxian fault Most of southern part aftershocks are distributed on both sides of the fault, located below the Anxian-Guanxian fault.2, The study found that the north segment of the Yingxiu-Beichuan fault dip 55° to NW in ≥19 km depth, and begin to slow in the depth of 6km, extend to decollement surface at low angle, the middle segment of this fault dip 55° to NW in ≥17 km depth, and begin to slow in the depth of 9km, extend to decollement surface at low angle, From the aftershock distribution map, we can see that the earthquakes in northern fault are distributed along both sides of Yingxiu-Beichaun fault, the junction of Anxian-Guanxian fault and Yingxiu- Beichuan fault is dense. It reveals that Ying Xiu- Beichuan two measuring lines are located in middle of the aftershock concentration areas, show the characteristic:more in the east and less in the west, the bottom of this fault extends to the deep stretch through the aftershocks concentration areas3, It reveals that the middle segment of the Wenchuan-Maoxian fault dip 55° to NW in >10 km depth, and begin to slow in the depth of 5km, the angle change to 35°,From the aftershock distribution map, we can see that the earthquake near this is sporadic, only developing a NW-SE trending seismic zone in the South West of the middle segment, this seismic zone covered in Wenchuan-Maoxian fault, it’s near to the main quake of Wenchuan earthquake, this NW-SE trending seismic zone maybe is the activated product of Wenchuan earthquake.