Seismological Study of 920km-mantle Discontinuity and the Mantle Transition Zone

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We have shown that certain features seismologically observable do exist in the mid-mantle beneath several subduction zones in the western ocean hemisphere.By stacking hundreds of short-period seismograms observed in Japan ("J-array"), we have suggested that there is a seismic discontinuity at a depth of-920km beneath Tonga, Japan, and Indonesia subduction zones. Further study for Indonesia subduction zone, however, revealed a large depth variation of the discontinuty and thus we now cell it a "mid-mantle discontinuity". The depth variation of the mid-mantle discontinuity beneath the Indonesia arc, where the discontinuity deepens from 940 km at the eastern end to 1080 km at the western end, appears to be well correlated with the location of the high-velocity anomalies (HVA) in recent tomographic models. However, the mid-mantle discontinuity cannot be simply coincided with the bottom of the high-velocity anomalies as was suggested in our earlier paper as one possible explanation, because … More a velocity increase at the discontinuity is observed from the waveform analysis.We utilized near receiver P-to-S conversion waves (so-called receiver function) to study the effect of the presence of a slab on the mantle transition zone discontinuities. Broadband waveforms of deep events occurred in Tonga observed at three stations in the northwestern ocean hemisphere, HIA,MDJ,MAJO,are stacked to produce receiver functions beneath those stations. Beneath the station in the 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 inversion result indcates 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 (at 670km, 740km and 780km) is observed. We interpret these features as the first in situ evidence for the garnet*ilemenite*perovskite phase transformation, as well as for gamma-spinel*perovskite+ Mg- wustite transformation. Less