COMPLEX STRUCTURE OF MANTLE DISCONTINUITIES AT THE TIP OF THE SUBDUCTING SLAB BENEATH NORTHEAST CHINA - A PRELIMINARY INVESTIGATION OF BROAD-BAND RECEIVER FUNCTIONS

Broadband seismic waveform data recorded at stations in the western Pa cific region are analyzed to investigate mantle discontinuities. By us ing teleseismic deep events, we observe unambiguous P-to-S conversion waves associated with the mid-mantle discontinuities in many of the in dividual seismograms. The commonly used method of stacking receiver fu nctions is not so effective in this case due to the limited number of deep events. An inversion scheme is developed to determine a discontin uity response function at each station by fitting observed waveform da ta with superpositions of the P-to-S converted waves. Beneath the stat ion in northeast China (MDJ) where the subducted Pacific plate appears to stagnate along the ''660-km'' discontinuity, the discontinuity res ponse function has more complicated features than those of other stati ons. The preliminary results indicate no depression of the ''660-km'' discontinuity at the tip of the subducting slab beneath MDJ; instead a multiple-discontinuity structure down to a depth of 780 km is observe d.