A three-dimensional shear velocity structure beneath the Japan Islands and the Sea of Japan have been investigated by applying an inversion technique developed by Aki and others. The obtained structure has been compared with the three-dimensional compressional velocity structure obtained earlier to discuss the internal physical properties and tectonics of the downgoing Pacific plate and the surrounding upper mantle.
The results show that high shear velocity (V_S) anomalies generally coincide well with high compressional velocity (V_P) anomalies, where there are high seismic activities of intermediate and deep-focus earthquakes. The descending Pacific plate down to 450km and the inner low-V zone extending under the Sea of Japan to southwest Japan have been well revealed by the S-wave observations as in the previous results from P-waves.
The highest V_S anomalies within the plate are 6-8% (50-250km), 5% (350km) and 2-3% (450km), which are somewhat greater than the corresponding V_P anomalies. The inner low V_S anomalies are -2-4%, which also somewhat stronger than the V_P anomalies. The velocity contrasts of both V_S and V_P between the plate and the inner low-V zone require large temperature differences and small amount of partial melt, which are estimated approximately as 600-800°C, 0.4-5% (50-250km), and 200-400°C, 0.5-7% (350km), although these values have large uncertainties depending on the assumed parameters such as the geometry of the partial melt zone and thermal coefficients of the seismic velocities. The three-dimensional structure of Poisson's ratio obtained from the V_P and V_S anomalies indicates that the descending Pacific plate has a lower ratio than the overlying inner low-V zone. The crust and the uppermost mantle in the northeastern part of Japan seem to have lower Poisson's ratios than those in the central and southwestern parts of Japan.

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A very accurate system to observe the temperature change of a deep ground-water has been developed. A specially made quartz crystal is adopted as a sensing unit which enables the temperature measurement to the accuracy of 1/1, 000°C. Other advantages of the system are the simplicity of the instrument, ease of the installation, and low cost, which make possible a number of installations in an area. The concentration of the observations is especially important since the natures of wells might be different from place to place.
More than 25 sets of the instruments were made and 15 sets have been already installed in Hokkaido and Honshu.
According to the preliminary results, a coseismic signal was observed at a well in eastern Hokkaido when an earthquake of magnitude 7.5 took place at the distance of 200km. Other than the coseismic signal, a number of unexpected temperature variations have been observed.

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ScS and sScS phases recorded by the long-period seismographs are analyzed to study the attenuation of shear waves in the upper mantle beneath the Sea of Japan. Data from the Vladivostok deep-focus earthquake of September 10, 1973 are used. The value of Q_β', the average Q_β for the mantle above the hypocenter (depth=552km), is about 70 for the frequency range of 0.02-0.07cps. This value is lower than those of some of previous estimates in and around the Japanese region, but is consistent with Q_R, Q of Rayleigh waves, for the trench and marginal sea region obtained from the "pure-path" analysis of long-period (T=200-300sec) Rayleigh waves. It is emphasized that the signal-to-noise ratios should be taken into account to obtain an unbiased estimate of Q.

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Thermal expansion of β-Mg₂SiO₄ (modified spinel; orthorhombic symmetry) was measured by the X-ray powder diffraction method in the temperature range between 20°C and 800°C. Grüneisen's theory of thermal expansion was applied to the linear and volume expansion data, and thermal expansion coefficients were obtained to be α_a=9.14, α_b=5.46, α_c=5.92, and α_v=20.6 in unit of 10^-6/K, at 293K. Gruneisen's parameter γ_G=1.26 was obtained by assuming the Debye temperature to be 952K. The magnitude of volume expansion coefficient of β-Mg₂SiO₄ is located between those of γ- and α-phases. β-Mg₂SiO₄ is found to be a significantly anisotropic material both in thermal expansion and elasticity.

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A very compact time code generator is designed for an OBS system and portable observing system with battery power supply. By use of a microprocessor it is constructed without any hardware counter or any shift register. All the process of a time code generator is realized by software, and the CPU clock is the sole timing unit. The program is stored in 256 byte memory.

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An analysis has been made of the rupture mechanism of two earthquakes which occurred in the vicinity of the oblique collision beneath the tip of the descending Philippine Sea plate and the Pacific plate beneath Chiba, Japan. The M=6.5 1956 Chiba earthquake occurred in a dense nest of seismicity at a depth of between 70 and 80km beneath Chiba city, and may represent deformation at the tip of the Philippine Sea plate. The mechanism of the M=5.5 1965 Chiba earthquake indicates that the stress field in the lower seismic plane of the Pacific plate beneath Chiba is undisturbed by the collision.
The M=6.1 1968 Saitama earthquake may be regarded as an interplate event occurring on the upper surface of and near the tip of the Philippine Sea plate. The high stress drop of several hundred bars of this earthquake and of the 1956 earthquake in the Chiba nest may reflect the penetration of the tip of the Philippine Sea plate into the Eurasian and Pacific plates, respectively.

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A possible mechanism for the occurrence of aseismic faultings is theoretically studied. Aseismic faultings are considered very slow fault motions which emit nearly no seismic wave. We analyse, as a mathematical model, the quasistatic extension of longitudinal shear cracks in a viscoelastic medium. A Dugdale model is adopted as a fracture model and small-scale yielding is assumed. Inhomogeneously distributed stress-drop and fracture strength are assumed on the crack surface. It is assumed as a fracture criterion that if the displacement discontinuity at the crack tip exceeds a certain value δ, the crack tip extends. The fracture strength is defined by the value δ in this paper.
Aseismic faultings preceding and following seismic faultings and those accompanied by no seismic faulting are explained, in a unified fashion, by our model.

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