Seismicity in and near the Japanese islands has been well investigated by many authors. However, only shallow earthquakes are treated in most of the previous papers. This is because there are not enough materials of magnitudes of intermediate and deep focus earthquakes.
First, the magnitudes of earthquakes whose focal depths are deeper than 70km are determined by the present author's method. By the aid of materials of magnitudes, the activities of shallow, intermediate and deep focus earthquakes in and near the Japanese islands are investigated and compared with the world wide seismic activity.
The main results of this investigation are summarized as follows:
1) The average number of shocks of magnitudes greater than 6 in and near the Japanese islands (28°-48°N, 128°-150°E) is 16-17 in a year, which is equivalent to about 6% of the total number in the whole world. The average energy released by these shocks is 2.7×10²³ erg per year, which is equivalent to 9 or 10% of the total energy released in the whole world. This result indicates the following fact; the number of large shocks in and near the Japanese islands is greater than the number expected from the magnitude frequency relation of earthquakes in the whole world.
2) The “b” value in the Gutenberg·Richter's magnitude-frequency relation logN=a-bM is roughly 1.0 for any of shallow, intermediate and deep focus earthquakes.
3) In and near the Japanese islands, the number of shocks of depths between 50 and 100km is greater than the number expected from the average ratio in the whole world, and the number of shocks of depths between 100 and 300km is smaller. The number of deep focus shocks, especially shocks of depths between 300 and 350km, is greater than the number expected from the average ratio in the whole world.
4) There is no marked correlation between the time rates of seismic activities in shallow, intermediate and deep focus earthquakes.

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The development of microcracks in the deformation of rocks was studied with microscopic method.
A coarse sandstone was deformed under the confining pressure of 500kg/cm², 1000kg/cm² and 1500kg/cm² at a strain rate 2.7×10^-6/sec. The locality of the sandstone is Oshima in Sakito-Matsushima coal field, northwestern Kyushu, Japan. The age is oligocene. Most of the phenocrysts are quartz and the matrix contains a lot of calcareous materials.
The mode of deformation is brittle under 500kg/cm² and transitional under 1500kg/cm². The experimentation was stopped at several stages of deformation as shown in fig. 4. The deformed specimens were cut into thin sections parallel to the axis of cylinder and perpendicular to the fractures. When the microcracks have opening, they are dark under crossed nicols and clear under uncrossed nicols.
Microcracks appear at very early stages. Most of microcracks have zigzag patterns. Average length of the straight parts of this zigzag lines ranges from 0.01 to 0.03mm. The density of microcracks increases approximately in proportion to strain. It's opening begins to spread at about the yield point. The preferred orientation of microcracks are visible in early stages. They concentrate to the direction parallel into the maximum compression and two conjugate directions which have an angle of 30°-40° with it. However under the confining pressure of 500kg/cm², they concentrate relatively more to the direction parallel to the maximum compression than under 1500kg/cm². Microcracks concentrate in some particular bands at early stages. The band is called “deformation band” in this paper. The direction of deformation band has an angle of 45° or a little less degrees with the direction of maximum compression. The deformation bands grow at later stages. The occurrence of macroscopic cracks are associated with the distribution of deformation bands.

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Four earthquakes of M=2.8, 2.9, 4.0, and 4.1 occurred at the north part of Osaka Pref. in Japan, which were accompanied by many aftershocks. The great part of energy of each aftershock sequence was released within a day after the respective main shocks. The focal mechanism of the aftershocks was similar to that of the respective main shocks, but different types were also found.
The “b” was examined on each aftershock sequence, and 0.53, 0.87 and 0.70 were obtained. It is supposed that those values are related to the focal depth.

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Using the data obtained from co-operative observations of microearthquakes in 1963 and 1964, some properties of microearthquakes in the vicinity of the Neo Valley Fault were investigated in comparison with large earthquakes occurring within the past 70 years in the area concerned.
As the result, we could find that the seismic activity of the microearthquakes was closely related to the earthquake fault on a large scale, though the microearthquakes were not always distributed uniformly over the fault zone. Many shocks were occurring on the southwest side of the fault and they decreased monotonously with distance from the fault. In the northeast side of the fault, however, there was an aseismic area where no shock was occurring. Moreover, it is noteworthy that large earthquakes which occurred since the outbreak of the Mino-Owari Earthquake in 1891 were also distributed in exactly the same manner.
According to these facts, it has become apparent that the seismic activity of microearthquakes may well represent the seismicity in the past on a somewhat great scale. From this point of view, the generation mechanism of the earthquake fault was considered briefly in connection with the manner of seismic activity in the vicinity of the Neo Valley Fault, though it was too early to discuss such a problem in more detail.

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Since August 1965, lots of earthquakes have been occurring at Matsushiro and its vicinity, northern part of central Honshu. In order to elucidate the cause of the peculiar phenomenon various kinds of observations and surveys have been done in the earthquake occurrence area.
On the basis of data obtained by the permanent seismological stations of the Japan Meteorological Agency and 5 temporary stations established in the area, the mechanism of earthquakes which occurred during the period from October 1965 to February 1967 was analysed.
The distribution of the initial motion of P waves for the earthquakes is separated by two straight nodal lines, which are perpendicular to each other, and lie in the directions of N35°-55°W and N35°-55°E. This suggests that the maximum pressure of stresses generating the earthquakes exists in the azimuth ranging from N80°W to N100°W. The number of nodal plane solutions obtained is more than 300.
Results of the statistical studies on the solutions and on the relationship between the focal mechanism and the geodetic surveys are summarized as follows:
1) The predominant directions of the pressures corresponding to the major earthquakes systematically changed by 10°to 20°on December 1965 and April 1966.
2) There exist systematic variations in the predominat directions of pressures in each earthquake occurrence area. This may be caused by the variation of local geology in the area.
3) The strike direction of N55°W for one of the two nodal lines agrees well with the strike of fault produced by the earthquake swarm in the central part of the earthquake occurrence area.
4) The predominant pressure directions are in good harmony with the axes of contraction of horizontal strains computed on the basis of data obtained by the geodetic survey carried out during the period from November 1965 to August 1966 at the central part of the earthquake occurrence area.

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According to the theory of free convection of viscous fluid heated from below by Rayleigh and others, free surface of the fluid is high or low where the convective motion is upward or downward. This together with the theory of mantle convection may explain the formation of guyots in the Pacific. A quantitative check of this idea is made in the present paper. Temperature perturvations required are of the order of 30°and 300°for 3000 and 300km thickness of the convective layer, respectively. For the 3000km thickness and the kinematical viscosity 3×10²¹ estimated by Haskell, we get the fluid velocity of the order of 20cm/year, a little larger than previously supposed. The discrepancy may be avoided by assuming a thinner convective layer or a larger kinematical viscosity.

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