Temporal seismic observation at Kamikuga (KKG) in Tochigi Prefecture was carried out from December 18, 1982 to February 11, 1983. Four temporal stations have been installed and more than 300 seismic events have been observed. Cross-spectral analysis method (CSAM) are applied for the precise determination of relative hypocenters of microearthquakes. An earthquake swarm, which consists of events with very similar waveforms, was analyzed by CSAM. For some groups of similar earthquakes, it has been demonstrated that relative hypocenters are determined, with an accuracy of about 50 m. We found two kinds of such groups of similar earthquakes. The first kind is characterized by similar earthquakes, occurring in a very short time interval within a very small area. Characteristic separation among the hypocenters of similar events is smaller than a wavelength of P-waves. The second kind is characterized by similar events occurring rather intermittently in a region with a linear dimension much larger than the wavelength. Although the former would correspond to stick-slip similar events studied by many seismologists, the latter will be similar events controlled by frequency characteristics of propagation paths.

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Experiments on frictional sliding of rock samples, sandstone and granite, were performed in an attempt to investigate the fracturing process and the energy conservation during sliding. In the experiments, we observed rises in temperature due to frictional sliding, in addition to other basic physical parameters such as displacement, and normal and shear stresses.
Four sliding modes are distinguished: stable sliding, continuous stick-slip with small stress drop, episodic sliding, and violent stick-slip. The sliding mode is strongly affected by the presence of gouges, which tends to make sliding stable and stress drop small.
The temperature rise is remarkably related to the sliding mode which is characterized by the magnitude of stress drop. The temperature data shows that there is non-uniform distribution of heat sources and that the average heat source magnitude varies with the fractional stress drop, which is the ratio of stress drop Δτ to initial stress τ₁ before sliding. An empirical relation between the average heat source magnitude a and the fractional stress drop Δτ/τ₁ is given by a=1-Δτ/τ₁.
The temperature measurement during frictional sliding of rocks in laboratory experiment provides an effective method for investigating the fracturing process and the energy balance in earthquake source mechanism.

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This study investigates the generation of waves, due to the incidence of Rayleigh waves upon the corner of an elastic three-quarter space. Incident Rayleigh waves travel along one surface of the elastic three-quarter space. Integral equations are derived by use of the Fourier transform technique, and are solved by deforming the integration paths into paths along which the integrands vary smoothly in magnitude. Expressions for the energy flux of Rayleigh waves along two free surfaces and for scattered body waves, were obtained. Partition of energy flux and directivity of the scattered P and S waves are discussed. The agreement between our theory and the experiment made by another investigator is considerably good. Substantial amounts of energy of incident Rayleigh waves are scattered away in the form of S waves, in the same direction as incident Rayleigh waves. Another interesting and important feature is the fact that, just around the corner of the elastic medium, the dilatational and distortional parts of waves are very large in magnitude. In other words, a kind of trapping of these waves occurs around the corner. These two kinds of waves are out of phase and the resultant waves are small in magnitude, due to the result of cancelling.

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Phase velocities of Rayleigh waves at periods between 60 and 200 s are measured by using IDA network data, and are inverted to the phase velocity distribution in the Pacific. We apply ART (Algebraic Reconstruction Technique) for the inversion without any a priori regionalizations. In order to estimate the resolving power of our data for the lateral heterogeneity, we use the Backus-Gilbert method. At 60-120 s the lateral variations of the velocities are strongly correlated with the sea floor age or surface tectonics, while such a correlation is weak at 150-200 s. As the period increases, the amplitude of the heterogeneity decreases in most parts of the Pacific. The marginal sea regions have more complicated features than the normal Pacific Ocean. In the Philippine Sea region, the velocities are lower at short periods than the average of the Pacific, but the sign of the heterogeneity is reversed at long periods. This feature may suggest that a cold slab descending beneath this region shows up at long periods. The Aleutian Sea and the Tasman Sea are slow, at 60-200 s. This regional difference in velocities may be due to the difference in geometries of the descending slabs.

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The absolute magnitudes of a variety of driving forces that could contribute to the plate motion are evaluated, on the condition that all lithospheric plates are in dynamic equilibrium. The method adopted here is to solve the equations of torque balance of these forces for all plates, after having estimated the magnitudes of the ridge push and slab pull forces from known quantities. The former has been estimated from the age of ocean floors, the depth and thickness of oceanic plates and hence lateral density variations, and the latter from the density contrast between the downgoing slab and the surrounding mantle, and the thickness and length of the slab.
The results from the present calculations show that the magnitude of the slab pull forces is about five times larger than that of the ridge push forces, while the North American and South American plates, which have short and shallow slabs but long oceanic ridges, appear to be driven by the ridge push force. The magnitude of the slab pull force exerted on the Pacific plate exceeds to 40 % of the total slab pull forces, and that of the ridge push force working on the Pacific plate is the largest among the ridge push forces exerted on the plates. The high correlation that exists between the mantle drag force and the sum of the slab pull and ridge push forces makes it difficult to evaluate the absolute net driving forces. However, the slab resistances appear to contribute more to cancelling the driving forces than the mantle drag force. From stress estimation, it was found that high stresses are concentrated around the leading edge of the downgoing slab.

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