Seismic anisotropy and its variation in the upper crust beneath the Longmen Shan fault zone

Both the Wenchuan M(s)8. 0 earthquake in 2008 and the Lushan M(s)7. 0 earthquake in 2013 occurred on the Longmen Shan fault zone. The purpose of this work is to study the spatial distribution and variation of seismic anisotropy in the upper crust beneath this area before and after the Lushan earthquake and the influence of major templors on polarization direction of fast shear waves. Using the systematic analysis method for shear-wave splitting and data from the permanent seismic stations from January 2010 to October 2017 and the Western Sichuan Seismic Array from October 2006 to July 2009, we obtain anisotropic parameters of upper crust in the study area. The results show that the time delay of slow shear-waves ranges from 0. 65 to 7.39 ms . km(-1), implying obvious lateral inhomogeneity. Media above depth 20 km has major contribution to the anisotropy. Dominant polarization directions of fast shear-waves (FPS) are NW ,NWW and NE, showing an obvious zoning pattern. Dominant FPS from north to south along the Longmen Shan fault zone changes from NWW to NW or NE direction. However,it is in NWW or NE direction in the south part. Tectonically it indicates segmentation of the Longmen Shan fault zone. Based on data of effective events available,seismic parameters of 19 of 49 stations show some correlations with the back-azimuth, source depth, magnitude and ray paths. The Lushan earthquake and the other major earthquakes in the study area might have influenced locally polarization directions of fast shear-waves. This paper indicates more effective data will be helpful to quantitatively analyze variations of the local tectonic stress field and fault attributions, furthermore beneficial to study seismic features of fault zones and earthquake prediction.