Marine Ecosystems Research Department - jamstec japan agency ...

JAMSTEC 2002 Annual Report
Institute for Frontier Research on Earth Evolution (IFREE)
Depth [km]
Parece-Vela Basin West Mariana Trough East Mariana Trough
10 0
10 0
10 1
10 1
10 2
10 2
10 3 10 -4 10 -3 10 -2 10 -1 10 0 10 1 10 2
10 0
10 1
10 2
10 3 10 3
10 -4 10 -3 10 -2 10 -1 10 0 10 1 10 2 10 -4 10 -3 10 -2 10 -1 10 0 10 1 10 2
Conductivity [S/m]
Conductivity [S/m]
Conductivity [S/m]
Fig. 7 1-D electrical conductivity structure model at the three sites. Blue lines are the model that the electric current
flows in the east-west direction. Red lines are the model that the current flows in the north-south direction.
anisotropic between axis-perpendicular and axis-parallel
directions at depths of ~km (Fig. ). Further
analysis is on going, and includes adding data collected
in two past experiments. Moreover, we plan to conduct
additional experiments in the same area in -
. Over instruments will be used with international
collaboration, in order to reveal more detailed
features beneath the trough axis and the island arc.
We also started observations using OBEMs in the
eastern Japan sea from October . This experiment
is in collaboration with Toyama University and the
Earthquake Research Institute (Fig. ). The Japan sea
Fig. 8 Site locations of the electromagnetic observations in the
eastern Japan sea, superimposed on a bathymetric map.
is also a back arc basin but it has features that contrast
with the Mariana trough: it is no longer active and contains
a large non-oceanic crust area that is considered
to be thinned arc crust. Comparing the mantle structures
beneath the two back arc basins is useful for
understanding diversity in plate subduction styles and
back-arc basin generation processes. In , we plan
to deploy two OBEMs along an elongated line in the
Pacific to produce a transect of the northeast Japan arc.
3.2. Seismic experiments in the region of mantle
upwelling
A two year long broadband ocean bottom seismic
observation was started in Jan. in the French
Polynesia area where a large-scale uprising mantle
flow (plume) is predicted. This observation (Fig. ) is
in collaboration with the DSRD (JAMSTEC), the ERI
(Univ. Tokyo) for ocean bottom observations, and
with French scientists for temporary land observations,
which have already started. Due to the sparse
land observatories in this area, it is difficult to construct
a finely resolved image of the plume and the
details of the core mantle boundary. This experiment
will solve these problems by using seismic ray paths
as schematically illustrated in Figure .
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