Ultrasonic velocities of single crystal MgO (periclase) and MgAl₂O₄ (spinel) were measured up to 7.78 GPa and 6.16 GPa, respectively, under hydrostatic pressure. The present success was attained by making various technical developments, such as a liquid-solid hybrid system and indium bond between sample and buffer-rod. Analytical procedures were also developed both in required corrections for apparent travel times, and in formulation of pressure effect on elasticity. A distinct shear softening was found in MgAl₂O₄, and was predicted to occur in MgO beyond 30 GPa. Isothermal equation of state of MgO was derived by combining the present results with shock hugoniot. Heat capacity, Grüneisen constant, and coefficient of thermal expansivity of MgO at high pressures were calculated by means of a new model equation of state. Negative thermal expansivity is expected to take place in MgO above -50 GPa. The pressure derivative of rigidity of γ-Mg₂SiO₄ is estimated to be 0.56-1.0 by means of a lattice dynamic analogy with MgAl₂O₄; the lower margin of the estimate leads to the pressure-induced shear softening of γ-Mg₂SiO₄ in the earth's upper mantle.

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A detailed shear wave velocity structure in the Pacific lithosphere for an ocean-floor age greater than 100 m.y. has been obtained in high resolution by means of several isotropic and anisotropic surface wave inversions. The vertically heterogeneous velocity structure of the vertically polarized shear wave has the following 3 characteristics: (1) has a small-scale low velocity zone near a depth of 30 km; (2) reaches a maximum value at a depth of 70 km; (3) obtains a minimum value at a depth of 130 km in the large-scale low velocity zone. The velocity increases with depth between 130-220 km. The velocity structure of the horizontally polarized shear wave does not possess Characteristics 1 and 2. Characteristics 2 and 3 give strong constraints on the rigorous estimation of the plate thickness. If the bottom of the heterogeneous lithosphere is tentatively defined as a mid-point between the depths indicating Characteristics 2 and 3, the plate thickness is estimated to be 100 km for the region where the age is greater than 100 m.y. A relationship between Characteristic 1 and the double-planed seismic zone is also discussed.

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