Huailai, Yanqing County And The Surrounding Areas Of Crustal Anisotropy Studies

Seismic anisotropy widely exists in the Earth’s crust and upper mantle. Due to the action of tectonic stress field, many cracks in the crust ranked directional advantage, which make the crust medium azimuthally anisotropic. Asthenosphere and mantle convection or flowing also caused anisotropy of lattice orientation arrangement in the upper mantle. The Earth’s crust and upper mantle anisotropy contains a lot of information about the tectonic movement, which provides the inference and valuable information to distinguish the lithology, stress state and tectonic evolution history. The observation and research about seismic anisotropy already become an important tool for people to probe the interior of the Earth information.This thesis introduces ray tracing equations in anisotropic medium and the relevant theory of weak anisotropy based on the elastic wave theory. We derived the qP wave phase velocity formula in the first-order approximation for weakly anisotropic medium. Compared with the exact solution by using the Christoffel equation, the qP wave phase velocity of the first-order approximation is applicable when anisotropy is less than20%. Using perturbation theory we also derived the linear inversion formulae. The weakly anisotropic (WA) parameters in the formulae are a linear function of the P-wave travel time. We designed an experiment to calculate WA parameters by using P wave travel time. The results show that this method is feasible. We can use the method to invert the medium azimuthal anisotropic parameters by using the P-wave travel time from an explosive source. The WA parameters are independent to the reference velocity when the reference travel time takes an average value of P-wave travel times. The WA parameters from inversion can be taken as effective anisotropic parameters concerning the medium within different source-distance and different depth, which reflect the variation of phase velocity with azimuth. The azimuthal variation of phase velocity was probably caused by the structure force in different history periods. An inversion for P wave data from a large explosive source in Huailai-Yanqing basin was carried out to study the azimuthal anisotropy in the crustal medium. In order to reduce the influence of the lateral heterogeneity to anisotropy, we divided the studied region centered at shot into two equal parts and inverted seismic anisotropy by using the travel time data of each concentric circle respectively. According to the total error between observed and the inverted results, we divided the studied region into W1(10°,170°) and W2(170°,350°). The results show that the regional crustal medium exist obviously azimuth anisotropy (8.9%) in the crust of studied region. Most of curve of the phase velocity variance with azimuth has two maxima that change with the increase of ray penetrating depth as well. The maximum of P wave phase velocity usually corresponds to the maximum principal compressive stress direction in general. The two different maxima of the phase velocity may represent principal compressive stress direction in different tectonic period, which vary with the ray penetration depth. The azimuth of phase velocity maximum in W1(10°,170°) region are mainly distributed in around65°or140°, while in the W2(170°,350°) region mainly in about180°or210°. On the whole, the inversion results are roughly consistent to regional tectonic stress field of north-east and east-west in North China.