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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. 93
JAMSTEC 2002 Annual Report Institute for Frontier Research on Earth Evolution (IFREE) Fig.11 The position of the Earth’s mantle and core in the view of simple thermal convection, and the range of our study; The color photos are the observed temperature field with viscous fluid. Thermal convection in a spherical shell constant viscosity, basal heating Ra 10 6 top middle 2700 2500 2300 2100 1900 1700 1500 K 1300 1100 900 700 500 300 bottom Fig.12 Numerical simulation for mantle convection in a spherical shell. We used the Earth Simulator for this calculation. The color indicates the temperature field. The horizontal resolution is about 30 kilometers at the surface, and the number of grid points is around one hundred million. 94
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