Seismic Azimuthal Anisotropy For The Eastern Tibetan Plateau Extracted By Wave Gradiometry Analysis

In recent years,the deployment of dense arrays has become a new trend,when the USArray and Chin Array have been deployed from early years,in addition,more and more other small size dense arrays are deployed by different agency in different region.New seismic data processing methods have also appeared with the occurrence of dense arrays,and the Waveform Gradiometry(WG)based on the plane wave assumption is one of the new and reliable methods for dense array.In order to make full use of the advantages of dense arrays,the WG method takes the waveform differences between stations into account,and only needs one earthquake can extract the velocity,seismic attenuation characteristics,and seismic wave propagation characteristics beneath the stations.Relying on the convenience of single event tomography of WG method,this paper proposes a new method for anisotropy measurement of Rayleigh wave phase velocity.We used the WG method to analyze the Rayleigh wave phase velocity of earthquakes from different azimuth and the related back azimuth,firstly.And then using the nonlinear least squares method to fit the anisotropic model to find the anisotropic parameters(fast orientation and anisotropic magnitude).Through theoretical experiments we found that the WG method has a good stability on anisotropy of Rayleigh wave phase velocity,especially on the measurement of anisotropic fast orientation,in addition,the theoretical experiments also found that the reducing velocity method can greatly suppress the error in computation of both the four WG parameters(phase velocity,azimuth variation,geometrical spreading and radiation pattern)and azimuthal anisotropy.We applied this new method to the measured waveforms of Temporary Western Sichuan Array(TWSA),and measured the four WG parameters and azimuthal anisotropy in eastern margin of Tibetan plateau at periods of T = 20 s,T = 40 s,and T = 60 s.Our results show that the Rayleigh wave phase velocity in three periods are bounded by the Longmenshan fault and Lijiang-Xiaojin fault while plateau area shows a low-velocity anomaly,the Sichuan Basin and Emeishan igneous provinces show a high-velocity anomaly;furthermore,this velocity boundary is also a sharp gradient band of azimuth variation,Therefore,we speculated that the Longmenshan fault and Lijiang-Xiaojin fault are deep-rooted faults that control the regional deformation of the lithosphere.The anisotropic fast orientations in this study area are generally consistent with large fault systems or orogenic belts.In Songpa-Ganzi block,the relatively high velocity,the abnormally positive azimuth variation,the abnormally negative geometrical spreading,and the anisotropy of three periods reflect the unusually complex tectonic environment in this area.The fast orientation of the boundary fault zone(BFZ)are basically consistent in three period;as block collision zones,the fast orientation of Longmenshan fault,Longriba fault and Xiaojin-Lijiang fault are different with period.When compared our results with results measured by different methods,we found that the anisotropic fast orientations of BFZ measured by the WG method have a systematical angle difference with the directions of maximum compressive stresses and directions of surface movement,while the anisotropic fast orientation measured by S wave splitting are more consistent with the directions of maximum compressive stresses and the direction of surface movement.Therefore,we suggest that the azimuthal anisotropy of low-frequency surface waves measured in BFZ is mainly affected by large folds or inverted stratum caused by paleotectonic movement,while anisotropy measured by high-frequency S wave splitting is more sensitive to tectonic stresses today,but all of those suggestions need further investigation when our results only present on a few frequency.