EVIDENCE FOR AN ANISOTROPIC LOWER MANTLE BENEATH EASTERN ASIA - COMPARISON OF SHEAR-WAVE SPLITTING DATA OF SKS AND P660S

Anisotropy in the lower mantle is studied by measuring the difference of the splitting parameter tau (the delay time between two split shear -waves) between the two phases SKS and P660s. These two phases are the P-to-S conversion waves at the CMB and the 660-km discontinuity, and therefore are affected by the accumulated anisotropy of whole and uppe r mantle, respectively. An approximately 0.4 sec time difference of ta u value between SKS and P660s is observed at the station HIA, a CDSN b roadband seismic station located in central China. This difference is supposed tb be caused by the accumulated anisotropy in the lower mantl e beneath the eastern Asia region. If the anisotropy is assumed to be distributed uniformly in the whole lower mantle or a lowermost 300-km D '' layer, the resulted anisotropy will be 0.08% and 0.5%, respective ly. However, the real intensity of the existing anisotropy in the stud ied region may be much larger than the above values, depending on its size and location in the lower mantle.