Search for seismic discontinuities in the lower mantle
Lev Vinnik, Mamoru Kato, Hitoshi Kawakatsu
Geophys. J. Int., 147, 41-56, 2001.
Abstract
Indications of lower mantle discontinuities are known for decades, but still
little is known about their properties, and their origins are enigmatic. In
the present study broad-band seismograms of deep events are explored
with the aid of a novel technique (Vinnik et al. 1998).
We deconvolve vertical component of the P wave coda in the period
range around 10 s by the S waveform and stack many deconvolved traces
with time shifts, which depend linearly on epicentral distance.
In synthetic seismograms for the Earth's model without lower mantle
discontinuities, the strongest signal thus detected in the time
window of interest is usually s'410'P phase, which is generated as
S and reflected as P from '410 km' discontinuity above the source.
In actual seismograms there are other phases, which can be interpreted as
converted from S to P at discontinuities in the lower mantle beneath
the seismic source. We summarize the results of processing the
seismograms (1) of deep events in Sunda arc at seismograph stations in
east Asia, (2) deep Kermadec-Fiji-Tonga events at the J-array and
FREESIA networks in Japan and stations in east Asia, and (3) deep
events in the north-west Pacific region (Mariana, Izu-Bonin and the
Japan arc) recorded at stations in north America.
In our data there are indications of discontinuities near 860-880 km,
1010-1120 km, 1170-1250 km, 1340-1430 km and 1670 - 1800 km depths.
The clearest signals are obtained from the discontinuity at a depth of 1200 km.
%Waveforms of these signals favour a sharp discontinuity
%rather than a complicated transition zone.
We argue that '900 km', '1200 km'
and '1700 km' discontinuities are global, but laterally variable.
Seismic stratification of the lower mantle may have bearings on the
patterns of subduction, as revealed by tomographic models.
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