Determination of the absolute depths of
the mantle transition zone discontinuities beneath China:
Effect of the stagnant slabs on the mantle transition zone discontinuities
Fenglin Niu, Hitoshi Kawakatsu
Earth, Planets, and Space (OHP special issue),
50, 965-975, 1998.
Abstract
Broadband seismic waveform data are stacked to
investigate the mantle discontinuities beneath a station.
A polarized filter is devised to remove the psudo-signals in
the stacked traces, which may be explained to be a discontinuity.
As the determined depth of a mantle discontinuity
in the previous studies depends on the used reference model,
we suggest to use different data sets which have different
epicentral distance to the investigated station.
The observed travel time of the P-to-S converted phases
as a function of
the epicentral distance can be used to constrain the used reference
model.
When the technique is used to the real data, we can determine the absolute depth
of a discontinuity to an accuracy of approximately +-10km.
The method is applied to the broadband data of the CDSN stations.
There is no significant depression observed for all the stations
except the station BJI.
Therefore the real lateral scale of the trough in the `660-km' discontinuity
under the northeast China must be much smaller than that
suggested by the previous SS precursors studies.
Beneath the station BJI,
the `410-km' and `660-km' discontinuities are elevated 10 km and
depressed 30 km, respectively,
which results an extremely thick transition zone. The phenomenon
may be attributed to the cold pacific plate that exists in the
transition zone of the same region.
Meanwhile, at the station MDJ, where the subducted pacific plate
is also found in the mantle transition zone depths,
a multiple-discontinuity structure rather than a depressed `660-km'
discontinuity is observed. At the station SSE, there is
no depression of the `660-km' discontinuity, suggesting
that there is no significant difference of temperature at
depths around the 660 km between
the SSE and the average mantle.