Seismic evidence for a chemical heterogeneity in the mid-mantle:
a strong and slightly dipping seismic reflector
beneath the Mariana subduction zone
Fenglin Niu, Hitoshi Kawakatsu, & Yoshio Fukao
J. Geophys. Res., in press, 2003
(PDF preprint)
Abstract
A clear later phase approximately 80 s after the direct P-wave
is observed in most
of individual seismograms recorded by a short-period seismometer
network in Japan (J-array)
from a cluster of deep earthquakes that occurred at
the northern Mariana subduction zone.
This phase 1) shows a P-wave particle motion;
2) arrives later from earthquakes with shallower focal depths;
3) has a steeper incident angle than that of P wave; and 4) shows a
deviation of a few degrees in the arrival azimuth from that of P wave.
We interpret it as an S
to P converted wave which takes off downward from the source and is
reflected at a velocity
discontinuity (reflector) below the earthquakes.
Applying an inversion technique to the data
set shows that the seismic reflector dips toward southwest by
about 20 degrees at 24.25N 144.75E and a depth of
1115 km with a lateral extension at least 100 x 100 km.
The location corresponds to the lower
edge of a high-velocity anomaly in global tomographic models.
Amplitude and waveform
analyses suggest a decrease of S-wave velocity by 2-6% and an
increase of density by 2-9% within the reflector.
There is almost no difference in P-wave velocity (<1%) between the
reflector and the surrounding mantle.
The estimated thickness of the reflector is about 12 km.
These observations indicate that the observed seismic structure is
more likely to be a
chemical reservoir rather than a purely thermal anomaly.
The seismic reflector might be a
piece of subducted oceanic crust, as suggested by a previous study.
It also could be related to
the break down of the D-phase of dense hydrous magnesium silicates (DHMS)
at mid-mantle pressure condition reported by recent mineral physics studies.
Both scenarios imply that either mechanical or chemical segregation
might occur within the subducted slab at
mid-mantle condition.