Marine Ecosystems Research Department - jamstec japan agency ...

Japan Marine Science and Technology Center
Institute for Frontier Research on Earth Evolution (IFREE)
3.1. Experiments in the region of the mantle downwelling
We have conducted both seismic and electromagnetic
experiments in the central Mariana trough and
surrounding area, since (Fig.). Larger experiments
under international collaboration are planned
for the future. From June , a large and long-term
ocean bottom seismic observation is being conducted
for one year as a part of a multi scale study of the
NSF-funded MARGINS project. The main aim is -D
imaging of mantle structure in the island-arc and backarc
basin by using seismic waves from active deep
events. In addition, active monitoring of a serpentine
sea mount in the fore-arc, surface wave analysis of
teleseismic waves, and other studies are planned as
part of this experiment. As a pilot study for this, a
long-term seismic observation using ocean bottom
seismometers was performed (Fig. ) from October
Deep Events (1990-2000, PDE)
and
Mariana LT-OBS Array (2001/10-2002/10)
to February . An example of the records for
a large event is shown in Figure , and analysis of this
data set will be made in FY. The precise activities
of deep events in the subducted slab, which cannot
be obtained from land data alone, will be investigated.
For electromagnetic studies, we deployed ocean
bottom electromagnetometers (OBEMs) in along
a line crossing the Mariana trough (Fig. ). This
experiment was in collaboration with the Deep Sea
Research Department of JAMSTEC, the Earthquake
Research Institute, Kobe University, and the National
Institute of Polar Research. Two of the OBEMs,
which were located on the island arc and eastern
Mariana trough, were recovered in April .
The other three OBEMs, which were located in the
Parece-Vela basin, in the western Mariana trough,
and in the Pacific, were recovered in November .
Preliminary analysis of the acquired data suggests an
interesting feature: that the electric conductivity is
Ev: 2001/10/31 09:10:20.00 -5.91 150.20 33.0 7.00
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20˚N
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18˚N
mr7 BZ
16˚N
mr6 BZ
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14˚N
mr4 BZ
12˚N
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140˚E 142˚E 144˚E 146˚E 148˚E 150˚E
100 200 300 400 500 600
Focal depth (km)
Fig. 5 Site locations of the seismic (large circles) and electromagnetic
(crosses) observations and hypocenter distribution
(colored filled circles) are superimposed.
Fig. 6 Vertical component records of seismic waves obtained by
OBSs. Blue lines are theoretical travel times based on the
iasp91 model.
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