Depth variation of the mid-mantle seismic discontinuity

Short-period array seismograms of deep events that occurred in the Indonesia, Japan and Izu-Bonin arcs are stacked and beam-formed to identify the near-source S-P converted waves that result from the mantle transition discontinuities. Most of the resulting images reveal the existence of a mid-mantle seismic discontinuity (''920 km discontinuity'') in these regions. Of the 15 events analyzed, three that occurred at the western end of the Indonesia are show clear SP arrivals observable even in individual seismograms. The mid-mantle discontinuity is characterized by large depth variation (900 similar to 1080 km) and velocity contrast variation in different subduction zones. Especially, the depth variation of the mid-mantle discontinuity beneath the Indonesia are, 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 in recent tomographic models. However, the mid-mantle discontinuity cannot be simply coincided with the bottom of the high-velocity anomalies, because a velocity increase at the discontinuity is observed from the waveform analysis.