Marine Ecosystems Research Department - jamstec japan agency ...

Japan Marine Science and Technology Center
Institute for Frontier Research on Earth Evolution (IFREE)
Fig. 10 Schematic image of this experiment. Earthquakes at
Tonga, Fiji and circum Pacific zone are used to study the
deep structure beneath the Tahiti island.
Fig. 9 Site location map. Japanese broadband ocean bottom seismometers
will be deployed at 8 locations (yellow circles).
Temporary French land sites are indicated by red squares,
and other symbols show permanent Japanese and international
observatories. Tahiti is located at PPT.
4. Modeling
The physical and chemical processes occurring in
the Earth's mantle and core are caused by the transport
of heat from the deep interior to the surface. In the
Earth's mantle, the dynamics are mainly controlled by
the convective motion of mantle material, and this
convection generates a pattern of the density and temperature
anomalies. By coupling the results of seismic
tomography with a fundamental understanding of
convection, we can propose a new view of the global
dynamics of the Earth's interior. We are studying the
physics of convection, and our method involves both
analogue fluid experiments (Fig.) and numerical
simulations (Fig.).
Using viscous fluid, we studied the nature of thermal
convection at high Prandtl number, which is
important for the dynamics of the Earth's mantle. We
observed the evolution of patterns and the mixing
process through laboratory experiments, in particular,
investigating the influence of inhomogeneous boundary
conditions, and the dynamics of layered convection.
These are the idealized models for drifting continental
tectospheres at the surface of the Earth, and for
the coupling between layered structures, respectively.
On the other hand, it is important to understand the
thermal convection at low Prandtl number for the
Earth's core dynamics. As the outer core is composed
of molten iron, the viscosity is very low and the thermal
diffusivity is very large. We can use gallium
metal as an analogue material: its melting temperature
is about K. We are now preparing the convection
experiment with gallium. Molten metals are opaque
fluids, so any optical methods of flow measurement
cannot be applied. We will utilize the Ultrasound
Velocity Profiling method to measure the convective
flow. Our aim is to observe the convection pattern and
to quantify the statistical features of turbulence.
Numerical simulation is essential for the creation of
a realistic view of the Earth. Mantle convection in the
Earth has many aspects, such as complicated rheology
in the uppermost part of the mantle in particular, phase
transitions, radiogenic heating, and chemical layering.
Using the Earth Simulator, we carried out calculations
of simple Rayleigh-Benard convection in a spherical
shell, and succeeded in calculations with a Rayleigh
number up to . The spatial resolution is sufficient to
contain the complicated aspects of the mantle, so we
can proceed to a 'very close to the real Earth' setting.
We aim to recreate plate motions, super hot plumes,
and sheet like subduction zones, which can be detected
by seismic observations.
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