High-mode Surface Wave Inversion,and 3D S-Wave Velocity Structure And Radial Anisotropy Of The Sedimentary Basin In Tongzhou-Sanhe Area

At local scale,the S-wave velocity of the structure is an important factor affecting the amplification and classification of sites.Therefore,it is of great significance to obtain the three-dimensional S-wave velocity structure and radial anisotropy of the sedimentary basin for utilization of underground space,earthquake disaster assessment and seismic zoning in urban area.At regional scale,the S-wave velocity of the Earth could provide seismic evidence for geodynamic studies.For example,the low S-wave velocity is usually associated with the high-temperature melting state of the mantle or the crustal channel flow.Since Rayleigh waves are sensitive to S-wave velocity of the structure,inversion based on the dispersion curves of Rayleigh wave is one of the main means to obtain Swave velocity of the earth at different scales.In traditional surface wave inversion,especially at the regional scale,only the fundamental mode Rayleigh wave is usually considered.Because of the complexity of the formation structure and the multi-solution of the inversion,the S-wave velocity structure constrained only by the fundamental mode Rayleigh waves suffers the uncertainty,especially the structure with low-velocity layer.In this thesis,the inversion of Rayleigh waves with higher modes is studied.The roles of higher modes on the constraint of low-velocity is investigated.The 3D S-wave velocity structure and radial anisotropy of the sedimentary basin in Tongzhou area are constructed based on the ambient noise at a dense array.The main conclusions of this thesis are as follows:(1)It is found that for a class of model containing low-velocity layer,the characteristics of low-velocity cannot be retrieved if the frequency band is limited for the observed data and only the fundamental-order mode Rayleigh wave is used in the inversion.The low-velocity can be retrieved by adding the higher modes at the same frequency band.For the model containing a low-velocity layer,the dispersion curves of the fundamental and higher modes show two typical characteristics.One typical characteristic is that the "cross-mode" would be observed between different modes,and the fundamental mode shows an obvious indication of low-velocity characteristics in interested frequency ranges.For the other kind of model with low-velocity layers,the dispersion curves have no visual crossing phenomenon in the frequency ranges of interest,and the low-velocity characteristics may not be observed in the measured dispersion curves.For the latter model containing a low-velocity layer,which is often encountered in practice,investigations on the inversion of multi-mode Rayleigh waves are conducted in this thesis based on seismic reflection data.The studies show that if the observed fundamental-mode dispersion curve does not include the frequency band sensitive to the depth of the low-velocity layer,the inversion based on the fundamental mode alone may not be able to recover the low-velocity characteristics of the model.But the low-velocity layer can be reconstructed accurately by inversion considering both the fundamental and higher mode Rayleigh waves even the observed fundamental mode dispersion curve has no obvious indication of low-velocity characteristics.(2)The effect of mode discrimination of surface wave dispersion curves on the inversion of radial anisotropy is investigated.It is found that in some cases,even the lateral variation of the inverted velocity structure is similar by different mode discrimination,the significant difference in radial anisotropy would be caused with different mode discrimination.In surface wave exploration,"cross-mode" could be observed in the frequency-phase velocity domain.For surface wave inversion based on dispersion curves,mode identification is important.However,we found that in some cases,the identification of low-frequency observations as different mode branches is sufficient to cause meaningful radial anisotropy variations even though the lateral variation of the inverse velocity model is similar.This means that the effect of mode mis-identification on radial anisotropy may be greater than that on the lateral variation of velocity,especially for the inversion on the shallow structure of the sedimentary basin with small absolute S-wave velocities.(3)The 3D high-resolution SH wave velocity structure and radial anisotropy of the sedimentary basin in Tongzhou area are constructed based on the ambient noise data at a dens array.Beijing metropolitan area,which is covered by the Quaternary soft sedimentary layer,is located in the transition region around Jizhong Depression,Taihangshan Uplift and Yanshan Fold Belt.The seismicity shows that this area is at a risk of the significant earthquake.Seismic risk assessment and strong ground motion simulation rely on accurate 3D S-wave velocity model of the sedimentary layer.Based on the data from the unprecedented dense array deployed in Tongzhou area(sub-center of Beijing),a high-resolution 3D S-wave velocity structure and the radial anisotropy characteristics under the array are studied in this paper.We first divide the dense array consisting of 919 stations into 1485 subarrays.The Modified Cross-correlation Beamforming(MCBF)is used to extract the fundamental mode dispersion curves of Love wave for each subarray.The 3D high-resolution SH-wave velocity structure is then obtained by depth inversion.Combing with the 3D SV-wave velocity structure constructed by fundamental and the first overtone of Rayleigh wave,the radial anisotropy model under the array is built.The model shows that the Daxing fault extends toward to NNE direction after passing through Niubaotun.Bounded by the Daxing fault,the Daxing high shows different characteristics and sedimentary process.On the northwest of the fault,the shallow Quaternary sedimentary layer with 100-400 m thickness is inferred.Xiadian and Gantang Sags are developed on the southeast of the fault,where the thickness of the Quaternary sedimentary layer varies greatly.The radial anisotropy of the sedimentary basin can reach ± 20%.Near the surface,a covering layer shows a strong negative anisotropy(VSH<VSV)within 0-60 m,which may relate to the development of vertical cracks in this area.Positive radial anisotropy(VSH>VSV)is the reflection of sedimentary stratification of horizontal sequence arrangement due to sedimentation.