The radiation of the "break-out phase" is studied. This phase is explained to be generated when a fracture initiated within the earth's interior reaches the earth's surface; when the fracture reaches a free surface an extremely large pulse can be generated. For an explanation of generation of the "break-out phase, " the displacement field when a propagating double-couple source reaches the free surface from depths in a semi-infinite homogeneous elastic medium is calculated numerically. Equations for such calculations are formulated. When the source propagates to the free surface, the effect of the free surface on the seismogram is found to be considerable.

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A large earthquake, magnitude 7.5, occurred at the western end of the Ryukyu Trench on March 12, 1966. The focal process of this earthquake is discussed in detail here on the basis of the moving lithosphere model. Its focal process and the features of the lithosphere in the Taiwan region are explained by means of a simple picture.
The seismic parameters of this earthquake, which were obtained by analyzing two distinct phases in the initial P-wave group and by analyzing G waves, are as follows. Dip angle and dip direction of the fault plane are 82° and 215°, and the type of this fault is unilateral. The earthquake moment is 1.6×10²⁷ dyne-cm; the average dislocation is 2.1×10²cm; stress and strain drop are 6.6×10 bars and 1.8×10^-4; strain energy is 1.4×10²³ ergs; fault length in the strike direction is 2.7×10km; fault width is 2.8×10km; and rupture velocity is 1.9km·sec^-1.

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The possibility of attenuation of microseismic waves due to the presence of an upward folding of the ocean bottom into the liquid is discussed in this paper. The upward folding of the ocean bottom has been idealized by means of an irregularity in the form of a rectangle intruding into the liquid and the evaluation of the displacement component at any point of the solid medium has been carried out in two stages. First, the change in the incident mode of the surface wave due to the presence of the rectangular irregularity has been determined, and second, the subsequent change of the resulting displacement components due to the termination of the liquid layer and the step change in elevation of the solid medium at the continental margin has been discovered. Finally, numerical calculation has been made to determine the attenuation of the microseismic waves of different periods due to the presence of the rectangular irregularity at the ocean bottom.

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Compressional wave velocities in an olivine nodule from Takashima Islet, Karatsu, were measured up to 900°C and 18kb. Piston-cylinder type high pressure apparatus was used. The sample showed strong elastic anisotropy, so that the velocities passing through the specimens in the three mutually perpendicular directions were observed, and the temperature derivatives of the wave velocity at constant pressure, (∂V_p/∂T)_P, were determined. The average of (∂V_p/∂T)_P of the three directions is about -0.73m/sec/deg. This value may give good approximation for the isotropic case. The pressure derivative of the wave velocity at constant temperature, (∂V_p/∂P)_T, is 11.0m/sec/kb for the present sample. Combining both derivatives, the critical thermal gradient for the compressional wave velocity, (∂T/∂P)V_p, is 15.1deg/kb.

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Studies on the crustal structure in the profile across central Japan close to the longitude 137°E were started in 1966 with 20 explosions made off the Atumi Peninsula. In 1967, twenty-one reverse shots off the Noto Peninsula and a supplementary three-ton explosion at the Ebisu Mine were observed at stations on land specially arranged for the derivation of crustal structure. Most of the stations were equipped with magnetic-tape-recording systems which provide us a filter technique to detect signals of low S/N ratio. The quality of record was extensively increased through filtering, and the following characteristic features were revealed:
A. Pn waves were extensively observed in the explosions off the Atumi Peninsula but not in those in the Sea of Japan.
B. Waves belonging to a granitic layer were clearly observed in the explosions off the Noto Peninsula but were absent or almost absent in the explosions off the Atumi Peninsula except for shots near the peninsula. These marked differences suggest an asymmetric crustal structure in the profile.
A time-term method is satisfactory for the derivation of sediments under the Sea of Japan, but not for the derivation of the Moho discontinuity. A new method utilizing long wave components, was developed and used in the present study resulting in many possible structures. The crustal structures thus obtained are, in general, continental under the mountain area, oceanic under the Pacific, and intermediate under the Sea of Japan. The most probable one out of many is shown Fig. 17. Gravity data were also used in the selection of the best model. The thickness of the basaltic layer remains unknown, but it should be very thin in the southern half of the profile. It forms a striking contrast with the structure in the profile along the longitude 139°E which is located within a short distance of only 220km.
A velocity gradient for the upper mantle was also obtained from the explosions off the Atumi Peninsula.

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A double-stage split-sphere apparatus has been developed for a series of phase-transformation studies at high pressures up to about 300kb and at high tempepatures of around 1000°C. First, the coesite-stishovite transition curve has been determined to calibrate the pressures at high temperatures. Then the olivine-distorted spinel transition in pure Mg₂SiO₄ has been investigated in the temperature range from 700 to 1200°C and at pressures above 100kb. The spinel phase of pure Mg₂SiO₄ has been finally synthesized at 1000°C at pressures higher than 220kb. Mg-rich spinels in the system Fe₂SiO₄-Mg₂SiO₄ have also been synthesized in order to complete the phase diagram at 1000°C.

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