Complex Structure of Mantle Discontinuities at the Tip of the Subducting Slab beneath Northeast China. A Preliminary Investigation of Broadband Receiver Functions.

Broadband seismic waveform data recorded at stations in the western Pacific region are analyzed to investigate mantle discontinuities. By using teleseismic deep events, we observe unambiguous P-to-S conversion waves associated with the mid-mantle discontinuities in many of the individual seismograms. The commonly used method of stacking receiver functions is not so effective in this case due to the limited number of deep events. An inversion scheme is developed to determine a discontinuity response function at each station by fitting observed waveform data with superpositions of the P-to-S converted waves. Beneath the station in northeast China (MDJ) where the subducted Pacific plate appears to stagnate along the "660-km" discontinuity, the discontinuity response function has more complicated features than those of other stations. 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 observed.