Application Of NTFT In The Study Of Mantle Anisotropy

The mantle anisotropy reflects information such as the deformation and stress state of the mantle beneath the plate,and has become an important tool for studying the coupling between the Earth’s inner spheres and the mantle convection model.Previous studies have demonstrated that the mantle is anisotropic.In this paper,normal mode coupling signals and long-period Love wave scattering signals are used to investigate the characteristics of the mantle anisotropy.Normal mode coupling signals are sensitive to the anisotropic structure in the deep mantle,while long-period Love scattering waves are sensitive to the anisotropic gradient in the mantle.The shear wave splitting method is a conventional method to observe the anisotropy of the mantle,but this method lacks the depth information of the anisotropy.Compared with the shear wave splitting method,the normal mode coupling signal and the long-period Love scattered wave have good depth resolution,which is an important supplement to the shear wave splitting method.Time-frequency transform can observe the time-frequency characteristics of signals in the time and frequency domains simultaneously.In this paper,normal time-frequency transform(NTFT)is used to analyze normal mode coupling signals and Love wave scattered waves.The normal time-frequency transform is a new continuous wavelet transform,and it can provide the instant frequency,instant amplitude and instant phase of a harmonic or a quasi-harmonic signal in the signal simultaneously.This paper focuses on the application of normal time-frequency transform in the above two signals,and has carried out the following work:1.We first introduce the similarity method(SC)based on normal time-frequency transform.This method is based on normal time-frequency spectrum coefficients and obtains the correlation coefficient of two signals through double integration in the time domain and frequency domain.It is the latest research results of NTFT.Simulation experiments show that the similarity method has good robustness in identifying similar signals comparing with the traditional cross-correlation method.Under extremely low signal-to-noise,the SC can estimate the correlation coefficients of the two signals and give accurate time delay.This paper applies the SC to the correlation analysis and delay estimation of Love wave scattered waves.2.Using the data from seismic station deployed in the Aleutian Islands,we observed strong coupling signals of 0S11-0T12.We exclude the effect of earth’s rotation on the coupling of 0S11-0T12 by expanding the observation,and confirm the existence of mantle anisotropy beneath the Aleutian Islands.Based on the Inaction method(IM)of the normal time-frequency transform,the coupled pair 0S11-0T12 is extracted into the time domain,and the strength of the coupling pair 0S11-0T12 is observed in the time domain.Based on the sensitive kernel function of the 0S11-0T12,we inferred that the depth of anisotropy in the Aleutian is about 400-700 km.The anisotropy locates in the upper mantle transition zone(MTZ),which indicate that the deep mantle in the Aleutian subduction zone is anisotropic.Finally,the Trench-Parallel mantle fast axis was estimated based on the coupling characteristics of 0S11-0T12,and we compared and discussed our results with previous shear wave splitting observations in the Aleutian area.3.The quasi-Love wave(Quasi-Love,QL)are generated by the scattering of long period Love wave,the QL is used to study the mantle anisotropy in the Ross Sea Embayment(RSE)of West Antarctica.In the ultra-long linear seismic array located on the eastern edge of RSE,QL waves with a dominant frequency of 10mHz and were observed from the 2011/3/09 Mw 7.3 and 2011/3/11 Mw 9.1 earthquakes.The sensitive depth of the QL wave is about 150 km,which is located in the upper mantle region,indicating that the upper mantle in the central RSE has anisotropic gradient structure.Based on the delays between the QL waves and Love waves,we speculated that these QL waves are formed in the middle of RSE.The scatterers of QL waves form an east-west boundary in the middle of RSE.It is worth noting that the boundary line of the mantle anisotropy gradient determined by the QL wave coincides with the boundary line of the magnetic anomaly obtained from the recent aerial magnetic survey in the RSE.The mantle and crust is coupled in the middle of the RSE.The anisotropic gradient boundary has great geological significance for explaining the formation of the Trans-Antarctic Mountains.There are few seismic stations in the Antarctic,so there are only a small number of shear wave splitting data sets.The observation of QL waves in the Antarctic provides a new way to investigate the mantle anisotropy in Antarctica.