High resolution 3D S-wave velocity structure and radial anisotropy of the sedimentary layer around Tongzhou-Sanhe area
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 NE 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.
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