Surface Wave Tomography Of The Crust And Upper Mantle In Sichuan-Yunnan Region

Sichuan-Yunnan region lies in the southwest of continent in china, which tectonic is connect with the collision of Indian block and European-Asia block 45ma ago, the collision of two blocks strong deform s the crust of Sichuan-Yunnan region; besides, Sichuan-Yunnan region locates the south end of the seismic south and north belt, which frequently occurs many strong earthquakes and is one of the most active seismic main regions in china; and this region is located on the part of east tectonic connection of Tibet and close in the channel of material escape and three big blocks. Recently, Sichuan-Yunnan region becomes the focal point that geophysicists perform research because of these especial environments.Peculate and Aki founded the idea of crust equilibrium since the center of 19, the form and evolvement and uplift about Tibet became the topic one of international geoscience's research, which geodynamics model is always the important problem that geoscientists are concerned about. The processing of uplift is one of the core topics during Tibet study, currently, all certain conclusions about the uplift and level of Tibet are from tests and theoretical results of different subjects, but for the complex of the shape and evolvement of Tibet, which makes the acquire of the velocity model important during the study on uplift of Tibet. However, study on the deep media structure is the key of study on uplift geodynamics mechanism. At the same time, through the much research for deep media structure in Sichuan-Yunnan region, to reconstruct the speed structure of crust and upper mantle, which does not only favor to make clear the geodynamics effect on deep media structure of the region during the co-collision with India block and European-Asia block, but also provides the more and trustful evidences for the idea of east escape of material.Based on the above consideration, workers engaged in geosciences did much research for the region by using different ways and the knowledge of different subjects and got many effect fruits. In order to renew the prototype of the media with high resolution, more and more persons do research for earth fine structure through all information in the seismic wave with the development of seismic multi-wave and multi-weight prospecting technology and the progress of computer computation ability.In seismic prospecting, seismic surface wave is dealt with as a kind of interferential wave and much not made use of. As a matter of fact, all kinds of seismic wave immingle together during the propagations of seismic wave, we may use phase velocity and group velocity and the attenuation character to inverse the structure of crust and upper mantle of different region on earth because seismic surface wave take much information of inner structure of the earth and seismic focus. The research reconstructed the speed structure of crust and upper mantle in Sichuan-Yunnan region by making use of the dispersion character when seismic wave propagates in the media. Before this, we review the theory of seismic surface wave and the research progress on the region and fruits; and introduced tectonic background of the area and the chosen resource; finally we modeled synthesize picture and dispersion curve by the way of seismic surface wave type.Recurring to seismic dispersion curve and the theory of seismic surface wave, we analyzed seismic resource in the domain of frequency and time, then got mixed path group velocity of Raleigh surface wave, and got group velocity of the pure path through the technology of grid dispersion, finally, we got transverse wave velocity of the media in the area. After interpreting velocity structure of the studied area, the following is what we arrived at conclusions:1) Sichuan and Yunnan provinces are typical tectonic activity area. The range of crust thickness variation is about 30km. It is thought commonly that this kind of variation is related to many tectonic factors, such as crustal extension or squeeze, isostasy and magma under plating or intrusive etc. There is high heat flux in the west of Yunnan and PanXi provinces. And this phenomenon has great correlation with tectonic activity. Furthermore, it is confirmed that there exists high conductivity layers of upper mantle in the west of Yunnan and Sichuan. Also it is linked to partial melting and slippage tectonic.2) From the velocity maps of Sichuan and Yunnan provinces, we can infer some complex deformation and activities like crust thickness increasing and shorting, plateau uplifting, blocks rotation and lateral extrusion. Compared with the outer of Sichuan and Yunnan diamond blocks in the top of crust and upper mantle, although the inner's velocity has local negative anomaly, it presents relative high velocity as a whole. There are many strike slip faults in the vicinity of the blocks, such as Xianshuihe fault, Xiaojiang fault and Honghe fault. Those faults emerge negative anomaly of velocity extending to the depth of top of upper mantle. And that will deduce lateral extrusion of crust blocks along fault.3) In the provinces of Sichuan and Yunnan, the deep crustal ambience of most strong earthquake is like that it has normal velocity or high velocity in the location of focus and low velocity reversely beneath the focus. The presence of low velocity is helpful to amass stress in the top of fragile crust. The fact is that velocity of crust and upper mantle beneath Xianshuihe seismic zone is bigger than in Anninghe seismic zone, Xiaojiang seismic zone and Honghe seismic zone. So it is more likely strong earthquake will occur in Xianshuihe seismic zone. However, due to the tectonic complexity in Sichuan and Yunnan provinces, we cannot overlook there are strong earthquakes in other zones by many other outer factors.4) The crust has apparent layered structure in the studied area, and it is divided into upper crust and lower crust. Through our calculation, we find the thickness variation of upper crust is small, and it is between 10km and 15km. So we conclude that the thickness variation of crust is invoked by lower crust.5) In our studied area, group velocity appears high in north and in east and low in south and in west. Dispersion structure emerges zone trait along main tectonic curve in vertical direction. And those results can explain that faults have great affection to global tectonic pattern in Sichuan and Yunnan provinces.6) Not only can we acquire fine structure of earth's surface for each block and tectonic unit from pure path dispersion curves, but also we can identify strong lateral heterogeneity and zone trait in Sichuan and Yunnan provinces.7) The latitude and longitude sections have showed that the average depth of Moho is about 48km in Sichuan basin. The depth of Moho is not coinciding along east-west direction in Yunnan Provinces. The depth is 53km in the east of Yunnan and 51km in the west. The above results indicate different tectonic blocks will undergo different influence degree during tectonic activity.8) Dispersion inversion of surface wave can be used to study fine structure of subsurface. Our results will be helpful information for studying Tibet in the field of other disciplinary. Since media under the earth is complex and variability, especially influenced by anisotropy, the inversion solution is multiple. So the author think we should strength the research of anisotropy and combine other technique when study this area. At the same time, it is very important to utilize information from other seismic waves sufficiently.