Crustal Seismic Anisotropy Study In The Northern Section Of North-South Seismic Belt

Seismic anisotropy is observed in different layers of the earth, and the stress-aligned fluid-saturated grain-boundary cracks are the main reasons for the crust anisotropy. Shear-wave splitting is an effective approach to analyze the seismic anisotropy. The North-South Seismic Belt is located in the western region of China with complex structure and frequent seismic activity, especially the moderately strong earthquakes. According to the digital seismic data from the seismic network, using the shear-wave splitting, the crust anisotropy was studied in the Northern Section of North-South Seismic Belt. Taking the parameters of the shear-wave splitting in the different seismic stations as constraint, we analysis the characteristics of the crustal deformation and its characteristics before and after the Wenchuan earthquake.The North-South Seismic Belt is mainly located in the Northeastern Margin of Qinghai-Tibet Plateau, the principal stress changes from the NE-trending in the northwestern, gradually to NWW in the southeastern. The active faults are well developed, especially the NWW-trending faults, showed the partition characteristics. Therefore, we could divide it into 3 partitions, Gandongnan Active Tectonic Zone, Longzhong Basin and Qilianshan-Hexi Corridor Active Tectonic Zone, respectively.According to the digital seismic data from the Gansu Seismological Network from Jan 2006 to December 2010, using the SAM method, the shear-wave splitting was studied. When selecting the useful records, we picked out the records which ML≥1.0 and within the shear-wave windows, also took in the ones which have not known the focal depth but the epicentral distances are less than 10km. Finally, we get 23 stations 242 available records analysis results.The average polarization direction of fast shear-wave shows the direction of the aligned cracks under the seismic station, reflects the principal stress direction. The spatial distributions of the average polarization of fast shear-wave reflected the crustal seismic anisotropy exist obvious difference in this study area. Take the polarizations of fast shear-wave of all stations in 3 partitions to average calculation, showed the average polarization directions of fast shear-waves are consistent with the principal stress directions. The dominant polarization directions of fast shear-waves at stations near the active faults are always consistent with the strike of active faults, and in the active faults junction zone the polarization directions of fast shear-wave will be relatively complicated.The time delays of slow shear-wave are related with anisotropic degree, and the result in this study area is uneven. Analysis suggests the tectonic stress of Qilianshan-Hexi Corridor is consistent with the Qinghai-Tibet Plateau’s, while in the Gandongnan Active Tectonic Zone is affect by regional stress and active faults, shows the characteristics of regional tectonic stress.