The Study On Anisotropy In The Upper Mantle Beneath Northeast China

The mantle deformation is essential for understanding the mantle dynamics,and seismic anisotropy is an effective tool to study the mantle deformation.Northeast China is located in the back-arc of the western Pacific subduction zone and the Pacific plate beneath the study area is stagnated in the mantle transition zone.Here,benefiting from deep-focus earthquakes developed within the slab,we measured shear wave splitting using both the SKS and local S waves recorded by the National Network from 2009 to 2015 and the NECESSArray from 2009 to 2011 to study the anisotropy at different depths under Northeast China.The results of SKS splitting measurements in Northeast China vary greatly in different regions.The NW-SE fast axis direction in the northern Daxing'anling and the Songliao Basin is consistent with the direction of the motion of the Eurasian plate.There is no obvious regularity in the fast axis direction and delay time of the southern part of northeast China,which may be related to the hot upwelling flows beneath the Changbai volcano and the lithospheric delamination.The circular fast axis direction obtained by the local S measurements may explained by the slab gap and the retreat of the subdcuting plate.The sub-slab mantle materials have an access to the mantle wedge along the gap,forming a circular mantle flow which is similar to that on the sides of the subducting plate.Both SKS and local S splitting results were obtained in the eastern part of Northeast China,and the SKS and local S splitting parameters obtained at MDJ station are relatively consistent,which indicates that the anisotropy in Mudanjiang area is mainly located in the upper mantle.The SKS splitting measurements show apparent azimuthal variations that can be modeled by two?layer anisotropy.The NE?SW fast axis direction of the upper layer is consistent with the strike of the Dunhua-Mishan fault,and the NW?SE fast axis direction of the lower layer is consistent with the motion of the Eurasian Plate;the linear relationship between the delay time and the focal depth in local S splitting measurements may be explained by the aligned metastable olivine in the subducting slab.