Mantle anisotropy above the deflected Pacific slab beneath Northeast China

Systematic spatial-variations of SKS splitting parameters (fast polarization direction PHI and splitting time DT) have been observed at global and regional broadband seismic stations in NE China and Mongolia. Based on the characteristics of the resulting splitting parameters, the study area can be divided into western and eastern regions by the western boundary of the Song-Liao Basin (SLB). A DT of about 1.5 s is observed at the eastern stations, corresponding to an anisotropic layer of about 150 km thick. Most western stations show significantly smaller DT (ranging from 0.5 to 1.0 s). The dominant PHI measurements at stations in the eastern region are about 100 degrees from the north, which are almost exactly the same as the relative plate motion directions between the Eurasian and Pacific plates calculated at each of the stations. Spatially inconsistent PHI measurements are observed at stations in the western region. The boundary between the eastern and western regions is consistent with the western limit of the subducted Pacific plate in the mantle transition zone (Widiyantoro, 1997). The slab is deflected horizontally by the 660 km discontinuity to the west by several hundred kilometers. While the difference in lithospheric deformation between the eastern and western regions could result in the observed spatial variations in anisotropy, the observations can be best explained by the nearly E-W flow in the mantle transition zone induced by the subducting slab.