The seismic activity in and near the Ebino area is discussed based on the data obtained at the Kirisima Volcano Observatory (KVO), a branch station of Earthquake Research Institute, between May of 1964 and December 1971. A test for the goodness of fit to a Poisson distribution is tried with χ²-distribution for the daily frequency of earthquakes in each year. The occurrence of earthquakes during the period of 1964 to 1967 prior to the Ebino earthquakes in 1968 does not fit to a Poisson distribution. But earthquakes during the period of 1969 to 1971 was found to fit to a Poisson distribution. The variation of Ishimoto-Iida’s coefficient m was examined. The m values were found to be 1.8-2.0 which are not so large as the m values for shallow volcanic earthquakes. A significant difference of a values with time was not recognized. The energy release was calculated. The order of annual energy release before and after the Ebino earthquke swarm in 1968 are 10¹³ erg and 10¹² erg respectively. Two earthquake swarms in March, 1967 and in November, 1970 were occurred near Mt. Kurino-dake which is located at the west side of the Ebino area. The relation between the energy of an earthquake and the strain energy stored within a confined volume is given by the equation E=1/2 pμ x²ν. If this formla is applied to a case of earthquake swarm, the strain stored before the swarm could be roughly obtained provided that the conversion factor p, and the strained volume ν being equal to the both earthquake swarms. The order of magnitude of strain which was calculated along the above method was 10^-6 for the swarm in 1967 and was 10^-7 for that in 1971. These values of strain are reasonable as the strain stored up in low seismic acitivity field like southern Kyushu.

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The Miocene volcaniclastic rocks which contain abundant subaqueous lava flows, pillow breccias and hyaloclastites, are widely distributed in the southwestern Hokkaido. An example of them, the subaqueous lava flows exposed along the Oshoro shore, Otaru city, is dealt with in this paper. Form and structure of these subaqueous lavas are briefly described. The pahoehoe ropy structure which is characterized by presence of wrinkles and tensional cracks on the pillow surface, is shown.

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During the period from August of 1970 to January of 1972, four earthquake swarms occurred at the western part of the Aso Caldera. The 1st swarm occurred on 9 and 10 of August, 1970, outside of the southwestern caldera-rim, the 2nd swarm on 24 and 25 of June, 1971, at the Tateno Valley, the 3rd swarm on 5, 6 and 7 of July, 1971, at the region about 10 km northwards from the Tateno Valley and the 4th swarm on 12 and 13 of January, 1972, outside of the northwestern caldera-rim. Analyzing these four swarms, the results obtained are as follows : (1) Epicenters of these swarms appear to be migrated successively from the south to the north within a narrow straight zone along the western rim of the Aso Caldera. The foci of the swarms occurred under the outside of caldera distribute more or less horizontally, and on the other hand, the foci of the swarms occurred under the collapsed part of the caldera-rim, the Tateno Valley, distribute rather vertically. (2) The constituent materials within the spaces occupied by the foci are highly fractured in comparison with the normal one of the earth’s crust. Particularly the state under the Tateno Valley, a part of the Oita-Kumamoto tectonic zone which is running from the east to the west in the central Kyushu, seems to be more fractured than any other. (3) In most cases, the initial phases of earthquakes show the opposite sense at the seismic stations situated symmetrically with respect to the zone along the western calderarim. (4) A new tectonic zone, or a fracture zone, will be inferred from the distributions of foci and initial phases and the strained state within the spaces occupied by foci as above mentioned.

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