Tomography Of Scattering Coefficient In The Crust

Seismic tomography is an effective method of studying heterogeneousstructure of the earth. Velocity tomography has got extensive applicationin studying long-wavelength inhomogeneous structure, however, instudying short-wavelength inhomogeneous structure, coda waves have theparticular advantage. In this thesis, Before introducing scatteringcoefficient tomography, we first introduced the basic theory of codanormalization and estimated attenuations of P and S waves in Yunnan region.The results show that our attenuation values are very close to thoseobtained by using other methods, Proving that coda normalization methodis effective in attenuation research of Yunnan region. And then, weestimated the scattering coefficient and intrinsic absorptionattenuation in Dayao strong earthquake area and nearby area by usingmultiple scattering theory and multiple lapse time window analysis method.Consequently, we prepared initial values for the research of spatialdistribution of scattering coefficient.Synthetic envelopes are calculated on the basis of the singlescattering theory, assuming spherical radiation sources and aone-dimensional depth-dependent velocity structure. An observationequation, which connects synthetic and observed energy densities, isobtained employing coda normalization method. Finally, we obtain thespatial distribution of scattering coefficient of research areas usinga standard ]east squares technique iteratively. The research region ofDayao strong earthquake area is 60km×40km. We set 1680 Grid points with5km spacing in the central part of research area and 10km spacing in thesurrounding area and there are total 1224 blocks. Obtained results showthat most aftershocks of M_S6.2 and M_S6.1 earthquake sequences occuredwithin large scattering coefficient zones and two main earthquakesoccured on the edge of large scattering coefficient zone. The extension of M_S6.1 earthquake sequence towards northwest and southeast ends at lowscattering coefficient zones. The aftershock focal depth of M_S6.1earthquake is relative shallow. M_S6.2 earthquake sequence bends downwardsto round a low scattering coefficient zone and forms an arc shape insection. The large scattering coefficient zone at depth of 10km isdistributed along the main shock faults, which implies that there arefault-damaged zones under Dayao strong earthquake area. The main shockfaults are supposed to be the extension of Tonghai-Mouding fault. Theresearch region of Jiashi strong earthquake swarm area is 72km×60km. Weset 2725 Grid points with 6km (horizontal) and 5km(vertical) spacing inthe central part of research area and 12km and 10km respectively in thesurrounding area, thus there are total 2106 blocks. The results show thatthere is obvious inhomogeneous structure in middle and upper crust, A lowscattering coefficient zone exists at depth of 10-25kmin the central partof research area and forms a good spatial correlation with high P-wavevelocity zone. Seven M≥6.0 earthquakes in 1997 occurred at the centeror edge of the low scattering coefficient zone.