Method Of Seismic Interferometry And Its Application In The Eastern Margin Of The Tibetan Plateau

In recent years,seismic interferometry has a great development in the study of seismology.Seismic interferometry has important applications in monitoring velocity change of the earth's subsurface media,interferometric source estimation and seismic wave velocity imaging.The interferometric waveform carries some properties that the original waveform does not possess.It makes the interferometric waveform have some unique characteristics that the original waveform record does not have.This provides an unprecedented view of the Earth's subsurface structures for seismology and enriches the knowledge and study of seismic wave propagation.The eastern margin of Tibetan plateau is located among Songpan-Ganzi block,Chuandian block and Sichuan basin.Under the influence of the collision between the Indian and Eurasian continents,the crust material of Tibetan plateau flows horizontally.In the eastern margin of Tibetan plateau,this eastward flows of material runs into the strong crust of the Sichuan basin forming a complex tectonic environment.The mechanism of crustal deformation and the mode of material migration have been one of the hot topics in seismology and geodynamics.Many strong earthquakes occurred in this area.Wenchuan M8.0 earthquake,Lushan M7.0 earthquake and Jiuzhaigou M7.0 earthquake occurred in the eastern margin of Tibetan plateau.The use of seismic interferometry to study the changes of the subsurface medium wave velocity,interferometric source imaging,crust medium velocity structure and anisotropy in this area has important scientific significance for understanding the complex tectonic environment and seismogenic mechanism in the region.Ambient noise interferometry can be used to study the variation of seismic wave velocity before and after strong earthquake.In this study,the Lushan M7.0 earthquake was used as an example for monitoring velocity change of the earth's subsurface media.The cross-correlate functions of ambient noise recorded at 14 seismic stations are computed,and the 24 inter-receivers empirical Green's functions(EGFs)were retrieved.The velocity change of seismic waves is measured by comparing the current EGFs with the reference EGFS.In this study,we compare the increase and decrease areas of wave velocity with the Coulomb strain expansion and compression region and find that the Rayleigh surface wave velocity increase region in the 10-20 s period and the Coulomb strain compression region are basically the same.This study concludes that there is a relationship between coseismic deformation and surface wave velocity changes during the 10-20 s period.Seismic interferometry can be used to study the source imaging.In this paper,taking the Jiuzhaigou M7.0 earthquake as an example,the main shock and some aftershocks have been imaged by using seismic interferometry.Seismic interference source imaging has a wide range of applications in the field of seismic exploration,but it is less widely used in natural seismic fields.The main reason is that the natural seismic waveform record is greatly influenced by the source radiation pattern.At different azimuth angles,the seismic waves radiated by different focal mechanisms are different at the receiving stations.In particular,differences in the positive and negative polarities of the initial motion,etc.,affect the offset and superposition of the interference waveform.In order to eliminate this effect,we first used the long-short-window amplitude ratio method to calculate the eigenfunction of the original waveform before doing waveform interference.The eigenfunction is calculated and the horizontal position and depth of the seismic source are got by cross-correlation interferometry and convolution interferometry respectively.The location of the Jiuzhaigou M7.0 main shock is(103.81°E,33.21°N,9km)?The location of the main earthquake and some aftershocks was compared with seismic information released by USGS,GFZ and CENC.This study has certain scientific significance to determine the seismogenic fault and its mechanism.Coda interferometry imaging is a passive imaging method.The coda records at receives are the scattered energy summed.The formation mechanism of coda wave is consistent with the theory of stochastic interferometry.In this paper,the shear wave velocity structure and radial anisotropy of crustal media on the eastern margin of Tibetan plateau are imaged by coda wave interfereometry.Through different vertical profiles,it is observed that crustal material is moving eastward as a channel beneath the eastern Tibetan Plateau.It is also observed that when this eastern eastward flows of material runs into the strong crust of the Sichuan basin,a large amount of materials insert under Sichuan basin and only a few part of materials thrusting to the surface.The radial anisotropy is inversed by using the Rayleigh wave and Love wave empirical green function dispersion data.The tilt angle of anisotropic symmetry axis of HTI medium has a major influence on the strength and positive of radial anisotropy.According to the distribution of radial anisotropy at different depths in the study area,the relationship between the anisotropy strength of the area and the geometric structure of the rock formation is concluding.