Crustal anisotropy of Huailai--Yanqing region in North China

A method for inversion of seismic anisotropy in crustal medium is presented by using the P-wave travel times from an explosive source. Perturbation theory is used to derive the linear inversion formulae when the medium is taken as a weakly anisotropic (WA) medium. The WA parameters in the formulae are a linear function of the P-wave travel time. The WA parameters are independent of the reference velocity when the reference travel time takes an average of P-wave travel times. The WA parameters from inversion can be considered as effective anisotropic parameters concerning the medium within different source-distances and different depths， which reflect the variation of phase velocity with azimuth. The azimuthal variation of phase velocity was probably caused by the tectonic force in different history periods. A large exploration was carried out in Huailai--Yanqing region near Beijing by China Earthquake Administration in 2007. A high density seismic network was deployed around the explosive point with azimuthal coverage of 360°， which provides an abundant data set for studying P wave seismic anisotropy of crustal medium. Travel time tomography in weakly anisotropic medium was used to calculate WA parameters. In order to get a comprehensive understanding of the studied medium， the inverted WA parameters were transformed to a new coordinate system related to a HTI (transverse isotropy with a horizontal axis of symmetry) medium. Three WA parameters and the orientation of symmetry axis of the medium were obtained. The relation of the WA parameters to tectonic stress field was discussed. The result shows that there exists an obviously azimuthal anisotropy (maximum 4.6%) in the crust of studied region.