Frequency distribution of magnitude of the ultramicro-earthquakes was studied by using the data of the Matsushiro earthquake swarm observed in December 1966.
As a result of analyses, the Gutenberg-Richter's formula concerning the frequency distribution of magnitude was found to hold well for the magnitude range down to about -0.9. The number of shocks then decreased very sharply with decreasing magnitude and no shocks of magnitude less than -1.3 were registered, though still within the limit of detection capability of the sensors. The seismograms of smaller events, if any, should be identified as earthquake motions, because the signal to noise ratio for a minimum shock was yet about 3, so that smaller events could not be masked by disturbances.
A crude estimation showed that the linear dimension of the focal region for the minimum shock was of the order of a couple of meters. The size of focus for the shock of magnitude -0.85, where a peak in the frequency distribution occurred, amounted roughly to double the size for the minimum one.

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Two methods are proposed to infer the seismicity. They are worse than the usual procedures in accuracy, but exceed in taking up all the earthquakes including the ones which have been neglected hitherto. However, they need, in principle, the observations under the same conditions. The methods are as follows.
a) The seismicities at the observation points during the observation periods can be expressed by the total numbers of earthquakes observed.
b) If a, in Ishimoto-Iida's statistical relation n(a)=k·a^-m, is taken as the maximum amplitude of ground motion and measured in any properly defined common unit (e. g.μkine), then, k expresses the bulk of seismicity.
It was studied whether the above methods were really practical or not, making use of the seismological bulletins of the co-operative observations of ultramicro-earthquakes. The results show that the effectiveness of these methods depend on the objects and the circumstances of the studies.
Unfortunately, the co-operative observations were not carried out under the same observation condition with use of the instruments of the same chracteristics. Then, when the effectiveness of the above methods were studied, various conditions under which the observations had been carried out were normalized numerically. In the course of this normalizatian, some conclusions have been obtained on the maximum amplitudes reported in the bulletin of the co-operative observations. They are as follows.
Ishimoto-Iida's statistical relation shows that the frequency distribution of the maximum amplitudes is linear on a bilogarithmic paper. But it was able only at about half of the number of stations to find out the linear portions of Ishimoto-Iida's distributions of maximum amplitudes. This result shows the inferior feature of the observations.
The reports from the stations which have the linear portions can be divided into two groups. The one has the linear portion below the maximum amplitude less than 100μkine and the other greater than that. This fact indicates that the characteristics of the instruments used were different among the observation parties.
In general, it may be said that the amplitude of the ground motion at the station near the epicenter is larger than the one of the distant station. There is, however, a lot of pairs of stations which disobey this generality. This fact shows that it is meaningless to compare the maximum amplitudes reported from each station and to discuss something (e. g. magnitude) in detail.

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The Kakuto caldera is located in the northern part of Kirisima Volcanoes. The Ebino earthquake swarm in 1968 and other two earthquake swarms in 1913-1915 and 1961 respectively were recorded in the caldera. The microseismic activity in the Kakuto caldera is discussed in this paper using the seismological data of Kirisima Volcano Observatory (KVO), a branch station of Earthquake Research Institute. The relation between the seismic activities in the Kakuto caldera and the Ebino plateau is also studied.
The location of earthquakes which are recorded during the period of September 1971 to February 1972 and which have magnitude larger than about -0.5 were determined and the seismic activities from 1964 to 1971 in the both area were discussed.
The summary of the results is as follows:
1) The earthquakes under the temporal stationary state activity are located in all over the caldera. Particularly even in stationary state the activities are more dense in the area where the Ebino earthquake swarm occurred in 1968 as compared to the outside.
2) One micro or ultramicro earthquake of magnitude 1 at largest occurred in every two or three days, as the activities in the stationary state. In addition to the activities, the small anomarous activities, such as a mainshock-aftershocks type and a small earthquake swarm type were recorded. It was only few times in a year that such anomarous activities were recorded.
3) There seems to be an inverse correlation between the seismic activities of the Kakuto caldera and the Ebion plateau, the south part of the Kakuto caldera.
4) A process which will explain the above inverse relation is as follows:
i) The accumlation of the strain in and around the Kirisima Volcanoes starts.
ii) Destructions of the crust in the structuary weak domain (the Ebino plateau) start and progress (KVO was opened in 1964. The destructions already started at that time).
iii) The destructions in the less weak domain (the Kakuto caldera; Novemver 1967) start and the destructions in the weak domain end.
iv) The destructions in the less weak domain progress (the Ebino earthquake swarm occured on February 1968).
v) The destructions in the less weak domain and the strain release end (around 1970).
5) In order to confirm the above process, a series of earthquake swarms was recorded. Two swarm occurred in the Ebino plateau on April 1966 and near Mt. Kurino-dake, the west side of the Ebino plateau on March 1967. The energy release of the swarms is about 1×10¹⁴ erg. One year after the second swarm, the Ebino earthquake swarm occurred in the Kakuto caldera on February 1968. The total energy release of the Ebino earthquake is 1.6×10²¹ erg.

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Exact solution for head and reflected waves from an SH line pulse source in a dipping layer overlying an elastic medium is derived using the Cagniard-Garvin's method.
It is found that the number of branch points increases with decreasing dip angle in the case of a dipping structure, and finally it degenerates into twe in the case of a horizontal layer. Ray paths corresponding to the waves contributed by them are also revealed.
Exact solution for the horizontal layer case is obtained using the same procedure for comparison with the dipping layer case.
A few examples of theoretical seismograms for dip angles 3.0°, 2.0°, 1.5°, 1.0° and 0.0°(horizontal layer case) are computed by numerical integration. Dispersive Love waves are shown for the case of a horizontal layer and a dipping layer with small dip angle of 1.0°. It is also shown that the number of peaks and troughs of the whole wave train decreases with increasing dip angle.
The behavior of surface waves in a dipping layer using theoretical seismograms will be reported elsewhere.

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The relation between the total duration of seismic waves (F-P) and the earthquake magnitude (M) was investigated by using the data obtained at the Abuyama Seismological Observatory and its array stations. The well-known formula, M=αlog(F-P)+β, was compared among the observation stations and also in the cases from the seismographs with different constants. The coefficients in the formula, α and β were found to represent respectively the frequency-dependent and the frequency-independent characteristics of both the observation station and the seimograph.
Several attempts were made to relate the F-P time to the nature of seismic coda waves. A crude estimation then gave the inference that the above formula would be taken out from the AKI's (1969) analysis of the coda as scattered waves.

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The size of battery with its pressure case is one of important factors in the ocean bottom seismographic observation. The oil-filled battery is desirable because it is used without pressure case even in deep sea. A discharge test, of the oil-filled battery was carried out off Nozimazaki. And an ocean bottom seismographic observation was performed in Suruga Bay utilizing the oil-filled battery. The results are very good. The ocean bottom seismograph has detected several earthquakes near Suruga trough in about 3 days, in Feb., 1972. The seismic activity of microearthquakes near Suruga trough seems to be a little higher than that in Sagami Bay obtained by another ocean bottom seismographic observation.

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Seismic activity in Hiroshima and Shimane prefectures and surrounding areas was investigated on the basis of the observational data obtained by the Shiraki Microearthquake Observatory and its substations.
Since May 1968, locations of microearthquakes of magnitude down to 1.0 have been capable from data obtained by the Shiraki observation net only. Data from other sources, for example JMA stations and other microearthquake stations, are used if necessary and available.
About 1100 epicenters were determined during the period from May, 1968, to June, 1970. Epicentral distribution thus obtained shows features as follows: a seismic zone parallel to the coast line of the Sea of Japan in the direction NE-SW, a seismic zone perpendicular to the former one in the east of Hiroshima prefecture, several aseismic regions, one of which is adjacent to the most activee seismic region with a sharp boundary.
In the spring of 1970, there occurred a very active seismic swarm near Kutsugahara station. The rise and fall of the activity were observed and investigated in detail. Some 2500 earthquakes of magnitude down to 0.0 were recorded, of which about 260 larger shocks were located.
The maximum earthquake in this sequence was of magnitude 4.6 and after that a great many earthquakes including many felt shocks were observed. The activity declined slowly with several recurrences including two earthquakes of magnitude 4.3. On the contrary, microearthquake activity was not higher than usual immediately before the maximum earthquake.
Epicenters in this sequence are concentrated in a region 3km in width and 7km in length. Focal depths were about 15km at first and became gradually shallow to a few kilometers for following ten days.
Thirteen days before the maximum earthquake, there occurred a shock of magnitude 2.6 followed by many aftershocks. This aftershock activity was reduced to the normal level for twenty or thirty hours. Epicenters in this activity were located a few kilometers northwest to the epicentral area of the activity thirteen days later. The former activity may be regarded as foreshocks in a broad sense of the word.
Moreover several seismic activities were observed and most of them are of swarm type, that is, regarded as superpositions of swarms and/or foreshock-mainshock-aftershock sequences.

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