Fine Crustal Structure In Yinchuan Basin, The Seismotectonics And The Shallow Seismic Surey Of Buried Active Fault

Yinchuan basin is a region with the condition of damage earthquake structure, which is located in the north of the South-North seismic belt. In this area, the new tectonic movement is very active, active faults are developed and earthquakes occur frequently. The seismic preparation and occurrence are related with complicated shallow and deep crustal structures. In order to obtain an efficient probability evaluation of the potential earthquake in a region, the sounding research must be done, including the dynamic environment and material property, as well as the relationship between the shallow and deep structures of the deep seismotectonics in earthquake source zones. The further study of the fine crustal structure and tectonic in Yinchuan basin, will provide a good method to understand the deep tectonic background about the occurrence of earthquakes.In this paper, the crustal structure of Yinchuan basin from shallow (covering strata of the Quaternary) to deep(Moho) is analyzed, by use of the data of deep seismic reflection profile crossing the Yinchuan basin, as well as the high resolution shallow reflection seismic profiles with different scales crossing the Yinchuan fault. The results are as follows:1. The crust of Yinchuan basin is obviously characterized by vertical delamination and horizontal block. In the top part of the upper crust, the delamination is clear, the layers is developed, the reflection energy is strong and the continuity of stratum is fine in different segments, which is characterized by the structure of typical grabon sedimentary basins. But in the lower part of the upper crust, the significant delamination feature isn't obvious, and less horizen.The reflections from the lower crust generally appear as disordered, and some inclined and arcurate reflections can be found only in some segments along the profile.The reflection energy from Moho is very strong, which shows dense reflection layer with the duration more than 1s and horizontal discontinuity. The Moho is deeper in the middle (about 43.5km) and shallower in the east and west sides (about 41.4km) along the profile. It shows the Yinchuan basin suffered strong converging and squeezing actions.2. A series of faults are developed in Yinchuan basin, and these faults are characterized not only by normal faults, but also by shallow up-breakpoints. The Yellow River fault with steep angle of inclination is the biggest fault in them, which is characterized by flowerlike strike-slipping, it extends down with a bundle-shaped fault, and it dislocates the Moho. The fault in the eastern piedmont of Helanshan mountains is the major fault with smaller scale than the Yellow River fault. It is a listric normal fault, dipping to the east. Some of faults in the west part of the profile converge to this fault, while this fault itself converges to the Yellow River fault in the lower crust.3. Based on the results of deep seismic reflection profile, the shallow reflection seismic profiles are carried out in this region. It shows that the T02 reflection interface with 12-15m depth is dislocated by Yinchuan fault. The Yinchuan fault is Recent active fault, that is proved by drilling.4. Based on the former research, the Yinchuan fault or Helanshan Mountain fault is the seismogenic tectonic of the earthquake Ms=8.0 in 1739. But the results in this paper show that the seismogenic tectonic is not limited to single fault and maybe located at the intersection of eastern Helanshan mountain fault and Yellow River fault. The energy is concentrated on the place of the convergence of different faults and is transferred to the upper part through these two faults under the action of regional stress.In this paper, the data acquisition and processing of deep seismic reflection profiles are also discussed, combining the deep seismic reflection profile launched in Yinchuan basin. According to the environment features of urban area, the shallow reflection seismic exploration with anti-noise method is studied under the background of urban areas. Based on the theoretical analysis and testing results, the different data acquisition parameters are put forward according to different exploration depths.As the integrative research for the fine structures of the crust from shallow to deep, the method of the shallow refraction and transverse wave exploration are studied specifically, combining the active faults by which earthquake could be triggered. Besides, two examples are provided, one is the shallow refraction seismic exploration experiment in Sichuan Qingchuan area, and the other is transverse wave exploration experiment in Beijing.