Extraction Of Higher Mode Dispersion Curve From Ambient Noise Based On Frequency-bessel Transform Method And Inversion Of 3D Shear Velocity Model In The US Continent

The seismic surface wave contains the structure information of the interior earth,whose dispersion information can be used to inverse the shear wave velocity structure of the interior earth.Seismic ambient noise technology is not limited by the spatial and temporal distribution of seismic sources.Through cross-correlation of seismic ambient noise data,the approximate Green's function between two stations is obtained.Then,the dispersion curve is obtained by F-K transform,narrow-band filtering and time-frequency analysis.It not only broadens the way of extracting the dispersion information of seismic surface wave,but also has the advantages of high horizontal resolution.Ambient noise cross-correlation technology has been widely used in dense array.Similar to the earthquake surface wave tomography method,ambient noise itomography is a nonlinear inversion problem.The inversion results are usually non unique.By adding independent constraint information,the non uniqueness of inversion results can be reduced or even completely overcome.In the past,ambient noise surface wave tomography methods are basically based on the fundamental dispersion information.Although compared with the earthquake surface wave tomography,they have achieved great success and has been widely used,the non uniqueness of the inversion results still limits the further improvement of the resolution and reliability of the tomography results.In order to solve this problem,the research team led by Chen Xiaofei recently proposed the frequency-Bessel transform method(Wang,Wu&Chen,2019;Wu et al.,2020),which can systematically extract multi-mode surface wave dispersion energy spectrum including higher mode dispersion from seismic ambient noise data.In this paper,the fundamental mode and higher mode surface wave dispersion information extracted by this method is applied to the shea-wave velocity inversion,which effectively constrains and solves the non uniqueness and instability of the nonlinear inversion problem.Firstly,the method is applied to three subarrays in the continental area of the United States to discuss the feasibility and advantages of the F-J method.In this study,a large amount of high-quality dispersion information is obtained from ambient noise data by using F-J method.The information includes the dispersion curve of continuous fundamental mode surface wave with dispersion energy ranging from 0.01 Hz to 0.5Hz and the dispersion information which can reach at least the third higher mode at the same time.Then,the shear-wave velocity structure is inversed by using quasi Newton method based on sensitivity analysis of phase velocity of dispersion curve to medium parameters.Finally,the convergence and stability of the inversion results are verified by adding higher mode surface wave dispersion information.The results show that when the initial model deviates from the real model greatly,the iteration is easy to fall into local minimum,and the unstable results are obtained.However,when the second higher order surface wave dispersion information is added,the instability of the calculation results is obviously eliminated.Because the seismometers in US Array array moving from west to East,we divided the U.S.mainland into 9 large arrays and each large array contains about 350 broadband seismometers.The cross-correlation function of continuous waveform records in one half year is calculated and stacked by days.In each large array,the subarrays of 5° × 7°and 6°× 8° are selected,and they move around the large array in steps of 1°.When two stations of cross-correlation function are in the same subarray,the cross-correlation function is taken as the cross-correlation function of subarray.Finally,a 0.5 ° x 0.5 °grid composed of central points can be obtained by calculating all station coordinate centers of subarray as center points instead of subarray.We conduct frequency Bessel transform for the cross-correlation function of each subarray to get the dispersive energy spectrum,and then we can get the dispersion energy spectrum of each central point.We have made up the fundamental surface wave phase velocity map of the whole American continent from 2s to 150s.We found that the phase velocity profiles of subarrays with the same size moving in different lengths are almost the same,and subarrays of different sizes moving at the same step length are also very similar.The phase velocity map of fundamental surface wave is also in good agreement with the results of previous studies.In addition to the broad-band fundamental surface wave phase velocity maps,we also obtain the first-order,second-order and third-order surface wave phase velocity maps in higher frequency band,and calibrate the coordinates of the phase velocity values.After determining the phase velocity map,we use the quasi Newton method to invert the one-dimensional shear-wave velocity structure at each central point,and then we use the simple-Kriging interpolation method to construct a 3-D S-wave velocity model of the U.S.mainland under a depth of 0-150 km on a 0.25 ° x 0.25 ° grid.From the inversion structure,we find that the three-dimensional S-wave velocity structure is similar to the previous results in general,but there are more new discoveries in details.The biggest feature of the U.S.mainland is the low-velocity zone.In the middle and lower crust of the western United States,there are obvious large scale low velocity zone,such as the basin and range area and Yellowstone/Snake River Plains area.After comparing with the previous research results,we confirm that there is crustal flow and crustal fluid channel in the middle and lower crust of the western United States.In the shallow part of the upper mantle,there is a negative gradient zone of S-wave velocity in the lithosphere of the whole U.S.continent at a depth of 70-120 km,and the shear wave velocity decreases by 2%-10%within 40 km.We believe that there are mid-lithospheric discontinuities(MLD)in the craton area in the eastern part of the United States continent at a depth of 70-100 km.The lithosphere asthenosphere boundary(LAB)exists at the depth of 60-90 km in the western United States,and the LAB and MLD overlap and cannot be distinguished.The low velocity zone determined in depth is of great significance to lithospheric tectonics,plate motion and continental dynamics.