Ambient Noise Love Wave Tomography Of The Chinese Mainland

Located on the southeast of Eurasia,the Chinese mainland is the intersection of the Indian plate and the Pacific plate.It is also a region with strong tectonic and earthquake activity.The underground velocity structure that can be obtained by the tomography at the continental scale is of great importance to the continental structure and dynamics,and also provides velocity model for the near-field seismic observation.Compared with the traditional seismic tomography method,ambient noise tomography uses the noise cross-correlation to extract the empirical Green's function of the station pair and hence extract the surface wave dispersion,which is not limited by the spatial distribution of the earthquake,and improves the resolution of the tomography.The existing ambient noise tomography is mainly for Rayleigh waves,and the ambient noise Love tomography is relatively few.In order to extract Love wave signal by the ambient noise cross-correlation,the waveform data of two horizontal components must be processed at the same time.The signal-to-noise ratio of the Love wave extracted by the noise cross-correlation is relatively low,so the data processing amount is greatly increased.The advantage of Love wave tomography is that it is more sensitive to shallow shear wave velocity structures,better constrains shallow velocity structures,and provides velocity model for near-field seismic observations.The published results of ambient noise Love wave tomography are aimed at localized areas and lack of continental-scale high-resolution v_SH?transversely polarized shear wave velocity?structures.These research results are published by different authors.The processing flow and parameters used are different,and it is difficult to form a unified and comprehensive velocity structure of China at continental scale.The new generation China national and regional digital seismic network includes more than a thousand permanent seismic stations.Since its operation in 2008,it has produced 10 years of continuous waveform data.The observation conditions of these permanent seismic stations are good,and the recorded seismic waveform records are continuous and stable,providing high quality data for ambient noise Love wave tomography.We collected three-component waveform data from 878 broadband seismic stations,and the data was recorded from 2013 to 2017.The seismic waveform data of up to 5 years guarantees the reliability of the Love wave empirical Green's function obtained from ambient noise cross-correlation.To extract the Love wave signal,it is necessary to process the horizontal waveform data simultaneously,rotate the north-south and east-west component continuous waveform data to the tangential direction,and then calculate the cross-correlation of the tangential components to obtain the Love Wave Green's function.By using the linear operation property of convolution,calculate the cross-correlation of horizontal components first,and then rotates them to tangential and radial directions by azimuth and back-azimuth of station pair path,which improves the operation efficiency.This requires the same data pre-processing parameters to be applied to both north-south and east-west component data.Pre-processing includes filtering,mean removing,deconvolution of instrument response,time domain normalization,and spectral whitening.The station-to-station Love wave empirical Green's function is obtained by the 5-year waveform data cross-correlation.For quality control,signal-to-noise ratio criteria,the 3 wave length criteria and data consistency principle are applied.The time-frequency analysis tool is used to extract the group velocity dispersion from the empirical Green's function,and the phase-matching filter is used to suppress the contaminated signal in time-frequency domain.Based on the 8 to 50 s period group velocity dispersion data,the velocity structure of the high-resolution Love wave group velocity maps in China are showed for the first time,covering the South China and Xinjiang province which are not reported by previous studies.The checker-board experiments show that the resolution in the eastern region reached 1°,the resolution in the northeast reached 1° to 2°,and the resolution in the west is 2° to 3°.The group velocity structure has a good correlation with the main geological units and the thickness of the crust,which verifies the validity of the group dispersion maps.Based on the 8-50 s period group dispersion maps,the three-dimensional v_SH structure of the Chinese mainland is inverted by linear inversion method.The error estimated by the "bootstrap" method shows that the v_SH error in the eastern region is less than 0.03 km/s,and the v_SH error in most regions in the west is not exceed 0.06 km/s.The lateral variation of the sedimentary layers in the Songliao Basin,the Bohai Bay Basin and the Sichuan Basin can be observed in this structure,which verifies the understanding of the Love wave is more sensitivity to shallow velocity.The structure fills the v_SH structure gaps in Xinjiang and South China,and facilitates the comparison of velocity structures of different structural blocks.It can also provide model support for seismic observations,especially for the inversion of source parameters and seismic location using near-field data.The difference between the v_SH structure and the existing v_SV?vertical polarized shear wave velocity?structure provides subsurface radial anisotropy information.The main characteristics of radial anisotropy in China are as follows:?1?It shows strong positive radial anisotropy under the main basins,which suggests extension under these basins.?2?It shows two different deformation mechanisms of the Tibetan Plateau.The northeastern margin of the Tibetan Plateau exhibits weak and negative radial anisotropy,corresponding to the growth mechanism of lateral crustal shortening and vertical thickening.The southern and southeastern margins of the Tibetan Plateau exhibit positive radial anisotropy,corresponding to the lateral expansion and vertical shortening of the crust.?3?The top of the uppermost mantle in North China shows weak or negative radial anisotropy,while the top of the uppermost mantle in South China shows positive radial anisotropy.This difference indicates their distinct tectonic settings.