Depth variation of mid-mantle discontinuity
Fenglin Niu, Hitoshi Kawakatsu
Geophys. Res. Lett., 24, 429-432, 1997
Abstract
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 arc show clear S-P arrivals observable even in
individual seismograms.
The mid-mantle discontinuity is characterized by large depth variation
(900 ~ 1080 km) and velocity contrast variation in
different subduction zones.
Especially, 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 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.