| Seismic ambient noise tomography is the most active research field and frontier direction of Seismology in recent years.It has an important scientific significance in detecting the structure of the crust and lithosphere,also in further studying the process of geodynamic evolution,as well as the seism tectonic environment and carrying out seismic risk assessment.The key problem of tomography is how to extract high resolution surface wave dispersion information from ambient noise cross-correlation function.To solve this problem,Chen Xiaofei research group proposed the Frequency-Bessel transform method(F-J)based on seismic array data,which already successfully extracted multi modes surface wave dispersion curve from ambient noise data.Compared with traditional methods,dispersion curve extracted by F-J method has higher resolution and wider frequency range.For the situation that the region structure is very complex and the data quality is relatively poor,even if F-J method can't extract the high model dispersion curve,it can also get the wider band and very effective fundamental dispersion information.Therefore,F-J method has achieved satisfactory results in both shallow surface exploration and large-scale velocity structure research of crust and upper mantle.It has good effect both on active and passive source research,and has a very broad application prospect.However,as for the area with large lateral structure changes,there are some contradictions in the current ambient noise tomography methods.In other words,the small-scale seismic array is effective for the tomography of local shallow structure,and can obtain the shallow velocity structure with higher resolution,but it is not sensitive to the deep velocity structure.However,large-scale seismic array is effective for deep velocity structure research,but not enough for shallow structure.Therefore,in order to get high resolution velocity structure from shallow layer to deep layer,this paper proposes a way of nested combination of multi-scale arrays,that is,extract the dispersion curve with relatively high frequency part from small-scale array data,and the dispersion curve with relatively low frequency part from large-scale array data,and then compose the dispersion curves extracted from different scale arrays according to the region size to obtain dispersion curve with wider frequency range,and then inverse the velocity structure with wider depth range in the target area.Yunnan is located in the southeastern margin of the Tibetan Plateau.Due to the collision of the plates,the regional horizontal structure is extremely complex,and the moderately strong earthquakes continue to be active.In order to detect and monitor the change of regional underground medium structure,a Fixed Airgun Signal Transmission Station(FASTS)has been built in Binchuan County,Yunnan Province in 2012.In order to explore the near-surface velocity structure around the launcher,a dense seismic array experiment was carried out around the launcher from March to May 2017,and a large number of continuous data were recorded,which provided abundant ambient noise information for inversion of velocity structure by ambient noise tomography.As a small scale array,dense array can extract relative high frequency dispersion curve from ambient noise data.In order to obtain high resolution deep velocity structure,we expand the array scale with adding Binchuan airgun source local seismic network and Yunnan local seismic network,to obtain lower frequency dispersion curve from relatively larger scale array data.The addition of low-frequency information can reduce the non-uniqueness of inversion results and make the inversion results more accurately to reflect the underground medium structure.In this paper,firstly we study the one-dimensional velocity structure of a single sub region in the dense seismic array.We use the F-J method to extract the surface wave dispersion curves from the ambient noise correlation function of the dense array sub region,dense array,Binchuan airgun source local seismic network and Yunnan seismic regional network.Then the nested combination of multi-scale arrays method is used to obtain the wider frequency band dispersion curve.Finally,we use the quasi Newton method to inverse the one-dimensional shear wave velocity structure of the sub region.Compared with the velocity model obtained by predecessors,the result of this paper confirmed the feasibility of using F-J method and way of nested combination of multi-scale arrays,even more accurately reflected the real velocity structure of underground medium.Then,we divide the dense array into several sub regions,obtain the combined dispersion curve of each sub region by using F-J method and nested combination of multi-scale arrays,and then inverse the one-dimensional shear wave velocity structure of each sub region,project these one-dimensional velocity structures to the central point of the sub region,next use the Kriging interpolation method to obtain the three-dimensional shear wave velocity structure of the airgun source area.Finally,the three-dimensional shear wave velocity structure is sliced to several profiles along the direction of depth and latitude.The results show that the thickness of sedimentary layer is less than 1km,and the buried depth of basement layer is about 4km.The S-wave velocity distribution near the surface corresponds well with the surface structure,showing the phenomenon of low velocity in the fault basin and high velocity in the mountain uplift.The regional fault activity has a very significant impact on the velocity structure,and the shear wave velocity structure on both sides of the fault is obviously different.The phenomenon of clockwise rotation of high-speed abnormal body appears in the rhombic sedimentary center area formed by the joint action of NE trending fault group and NW trending fault,which is speculated to be related to the left-lateral extensional and torsional movement mode of the whole region,and that is consistent with the clockwise rotation deformation movement characteristics of Northwest Yunnan fault depression belt.In this paper,the high-resolution three-dimensional shear wave velocity structure of Binchuan,Yunnan is obtained by using F-J method and multi-scale arrays nested combination method,which provides the basis for underground structure exploration in this area,and has important guiding significance for further understanding the seism tectonic background and seismic risk analysis in this area. |
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