The Studies On Inversion Of Velocity And Attenuation Structures And Its Application To The Middle-Yunnan Region

The relativity between seismic tomography and fault structure is one of the important fields in seismology research. There are many researchers who have already carried out this work from many aspects. These can help us to discuss the relationship between seismic tomography and fault structure from new aspects, and are also a basis for us to do further correlative research.1. Research TechniqueIn the first place, we study systematically seismic tomography method, which includes four aspects: (1) model parameterization, (2) seismic ray tracing, (3) inversion, and (4) result estimation. The thesis includes velocity structure tomography and attenuation structure tomography.Velocity structure tomography is: (1) The grid of original model in level and vertical directions is set respectively. The grid division in vertical direction is based mostly on the velocity model of the research area, and the grid division in level direction mostly by the distribution of the stations, earthquakes and seismic rays in the research area. Then the checkerboard method is used to test whether the gridding scheme can obtain good resolving power. (2) The seismic ray tracing method is used to get the theoretical time according to the location and original velocity of each gridding point. (3) We inverse the true velocity of each gridding point combining the theoretical and observational time. Then we obtain images of velocity in different depth. (4) The checkerboard method is used to get the resolving power in different depth, which can judge the reliability of the velocity images.The attenuation structure tomography is: (1) The wave and noise data of seismic record are intercepted respectively. Then FFT method is used to transform the intercepted wave and noise data into displacement spectrum. (2) The source model is assumed as Brune'sω2 model, and the heredity arithmetic is used to get the three parameters f c,Ω0,t? . (3) Based on the known ray trace and 3-D velocity structure, we use the result of t? , and adopt the same method of the 3-D velocity structure inversion to obtain the 3-D attenuation structure.2. Research Content and ResultBased on the established tomography methods and program of velocity and attenuation structure, we carry out the research on the relativity between the seismic tomography and fault structure on two different areas in this paper.One is applying to a smaller area (point), (λE 101°～101.25°,φN 25.45°～25.65°). We use the data of Yaoan earthquake sequence to analyze its 3-D velocity and attenuation structures. The results show that: The earthquake sequence is distributed mostly along the direction of NW and in depth of 2-12km in the research area and agglomerated in the WN end of the NW strip. The Yaoan M6.5 earthquake is in sandwiched VP, VS and QS zones between higher and lower values. Most of the aftershocks are in the area with low-VP, low-VS and low-QS disturbance anomalies. The agglomerated aftershocks around the M6.5 earthquake seems distribute along the direction of NNE. It is conferred that there is a NW fault in the earthquake area of Yaoan, whose northwest end is deeper than the southeast end, from the characteristic of the aftershocks and the 3-D tomography results of VP, VS and QS. At the same time, there is another NW fault respectively in the north side and the south side of the NW fault. Furthermore, there seem two interlaced NE-NNE faults on the northwest end and the southeast end of the NWW fault. Considering the limit of the earthquake sequence data and the research area, that NE-NNE fault are not confirmed here.The other is applying to a bigger area (region), (λE 99.5°～102.5°,φN 24°～28°). We use the data of earthquakes in middle-Yunnan area, which includes the Yaoan M6.5 earthquake and the Dayao M6.2, M6.1 earthquakes, to analyze its 3-D velocity and attenuation structures. The results showed that: Most faults in research area, such as Chengjiang fault, Lvzhijiang fault, Jinpingshan-Yulongshan fault, and Chuxiong-Jianshui fault, et al., are showed obvious low-VP and low-QP disturbance anomalies in different depth. The strong earthquakes with M≥6.0 in history mostly occurred in the transition area with high-low VP and QP disturbance anomalies. From the characteristic in the whole research area, it can be seen that most faults are near SN direction, or NW-NNW direction, and or NE-NNE direction. It may be related to the result of the whole region stress field.In Yaoan earthquake area, there exist NW and NE low-VP, low-QP disturbance anomalies. The mainshock locates around their point of intersection. In Dayao earthquake area, there also exist NW and NE low-VP, low-QP disturbance anomalies. The two mainshocks locate around their point of intersection too. The NE low-VP disturbance anomaly result in the NW low-VP disturbance anomaly presents two segments. Earthquakes occur in the transition area with high-low VP and QP. It may be that the high velocity and low attenuation can accumulate energy more easily than their circumstance, where the earthquakes are gestated. The results confirm the doubtful point in earthquake sequence research that there not only exist NW fault, but also exist interlaced NNE fault, which extends to Dayao earthquake area, on the northwest end of the NW fault in Yaoan earthquake area.In addition, we provide four different schemes to do some primary discuss on the research of the original attenuation model. We use the method that from same value model to changed value model with depth gradually, which is more reasonable, and then adjust it by the results. As a result, we take the fourth scheme as the final research method that the original QP model is delaminated as four layers, the first layer (-4km) set 200, the second layer (1.5km) set 400, other layers set 600, and the final layer set 800.3. Research ConclusionsCombining the results of 3-D velocity and attenuation structure tomography in different regions, and other research results, our conclusions are that there are NW concealed faults in Dayao and Yaoan earthquake areas, which may be the results of the level NW-NNW and NE-NNE main press stress respectively. It is conferred that there are NE-NNE concealed faults, which extends from Yaoan earthquake area to Dayao earthquake area. The faults blocks the break of the NW fault in Yaoan earthquake area and results in the two Dayao earthquake sequences in Dayao earthquake area, whcih break towards opposite directions.Based on the tomography results in this paper and the fault background of Yaoan earthquake area, it is inferred that the fault movement of Yaoan M6.5 earthquake is that the NW and NE-NNE concealed faults is firstly formed inside the block under the clockwise rotation of the middle-Yunnan block, then the stress increasing area in a intersection of NW and NE-NNE concealed faults is formed due to the change of deep structure and constitution on the influence of the faults around the block, such as Chuxiong-Jianshui fault and Tonghai-Mouding fault, finally, the fault of Yaoan earthquake is formed..4. Meaning of researchThe thesis results show that it can explain better the elaborate conformation of Yaoan and Dayao earthquake areas and the holistic conformation of middle-Yunnan region that combine the results of 3-D velocity and attenuation structure tomography in different regions. The combination of velocity and attenuation structure can restrict better the physical characteristic of underground medium. So that, we can understand the nonsymmetrical characteristic of underground medium, deduce the distributing of faults, and study the conformation of earthquake area. At the same time, we can establish a method that how to explain simultaneous the elaborate conformation and region conformation by comparing the similarities and differences between earthquake sequence and region earthquake in tomography research. And it is a basis on applying to other areas in the future.