A Study On Inhomogeneity Of Crust Medium In Changbaishan Tianchi Volcano By Scattering Body Wave Inversion Method

Observation and research results show that inhomogeneity of medium exitsts in the interior of the earth with varying degress, the difference of inhomogeneity scale of medium revealed by seismic wave is about 8 orders of magnitude. According to the size of scatter body and seismic wavelength, the inhomogentiy of medium can be classified two types, one is lagre scale inhomogeneity (long wavelength inhomogeneity), and other is small scale inhomogeneity (short wavelength inhomogeneity). In contemporary seismology, the determintic method used to study the large scale inhomogeneity of medium is know very well, and its method and technique have been approved by theory and practice. The small scale inhomogeneity of medium is a new research area, and its theory and method study is on starting stage. The aim of paper is to discuss 3D small scale inhomogeneity.Geological structure movement is the main cause of inhomogeneity. Studying the inhomogeneity of medium in the interior of the earth, it is very important to study the producing process, force, distortion and movement stage of inhomogeneity structure boundary for knowing the strucute of earth, resource exploration, and volcano and earthquake observation and prediction.Main research contents and conclusions in the article are following.First, random medium model was described using statistical method. Model building, types and characters of ramdom medium model were discussed. The relationship between heterogeneous body and model parameters (autocorrelation length a and b, the fractional velocity fluctuationε) were also analysed.Second, the phenomena and the theory of S wave envelope broadening based on Von Kármán type random medium were discussed. The theory expression of power spectral density function of Von Kármán type random medium is deduced, and the factors influencing duration time of S wave envelope, such as the travel distrance of Swave in random medium, the fractional velocity fluctuation of the medium and the frequency of Swave, were analyzed. Third, the relationship between the duration time of S wave envelope in multi-layer and the parameters of random medium is developed. The equivalent travel distrance is the main factors to make multi-layer random medium to single layer random medium, which set up the relation between the duration time of S wave envelope in multi-layer and the parameters of random medium, and is a base of discuss the inhomogeneity of medim by quantitive or qualitative method.Four, the forward simulation of S wave in 2D Von Kármán type random medium were modeled. The phenomena of S wave envelope broadening, the frequency of S wave, the fractional velocity fluctuation of ramdom medium and the travel distance of S wave were analysised.Five, the 3D inhomogeneity of medium in Changbaishan Tianchi volcano were qualitatively analysised by the peak delay time of S wave envelope. The results show that the medium of Changbaishan Tianchi volcano presents strongly inhomogeneity. The inhomogeneity of medium in Changbaishan Tianchi volcano presents multi-scale character. Earthquakes usually occur around the contact band of strong and weak inhomogeneity.Six, the 3D inhomogeneity of medium in Changbaishan Tianchi volcano were quantitively analysised by the peak delay time of S wave envelope. The parameters ofHurst index k and log (the main reason is the relationships between the fractional velocity fluctuation of ramdom mediumεand the autocorrelation length a and b are undecoupling) were accept to describe the inhomogeneity of medium. The result also show that the medium of Changbaishan Tianchi volcano presents strongly inhomogeneity, the inhomogeneity of medium in Changbaishan Tianchi volcano presents multi-scale character, and earthquakes usually occur around the contact band of strong and weak inhomogeneity.