The western region of the Aso Caldera is well known to display the considerable activity of earthquakes. Then, the seismic observation was carried out during three months from June to August of 1973. The purpose of this observation was to analyze the nature of earthquakes and to get information on the tectonic structure at the western caldera rim. The 21 hypocenters were located below the western region and the central cone of the Aso Caldera, and the focal mechanism solutions were determined from the P wave first motions. The distribution of epicenters was distinguished into two zones, one was the zone striking from the Tateno Valley to the west and another was the zone striking north to south along the western rim of caldera. These two zones intersect each other at the Tateno Valley where the caldera wall is breached. The hypocentral depths of earthquakes occurring in the former zone were deeper than in the latter zone. The focal mechanism solutions implied that the direction of maximum compressive stress was northwest-southeast for the earthquakes occurring in the former zone, on the other hand, for the earthquakes occurring in the latter zone the direction of maximum compressive stress was northeast-southwest. These appearances were suggesting that these zones were undoubtedly equal to the tectonic lines and that the right lateral strike slip was occurred at each zone.

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Toyoha geothermal field including Toyoha Mine of south-western Hokkaido is located in nearly the north end of Nasu volcanic zone. To make clear the subterranean thermal structure of this area, the writers tried following three steps of calculation. The first is the analysis of heat flow to seperate into the various heat sources, second comparison of observed temperature distribution and computed one by a thermal conduction model from large spherical heat source, and the third is calculation of stationary states of subterranean temperature distribution using cylindrical model of hydrothermal reservoir. For the first step, socalled “running mean method” is applied, especially taking the difference of two running mean, analysis of geothermal data (heat flow) in the southwestern part of Hokkaido can effectively be made. Namely, from the surface heat flow analysis, three subterranean heat sources are found with their sizes and ages. (ref. Fig. 3(b)). In the second step, surface temperature distribution at and adjacent Toyoha geothermal field is calculated by the thermal conductivity using the spherical heat sourc model, and this sesult is compared with the observed surface temperature distribution. Though generally speaking, the result of this comparison seems to be good, there appears a remarkable surface temperature anomaly at eastern part of Toyoha Mine. To explain this anomaly, as the third step, the calculation of thermal stationary states is made by using a cylindrical model, from which it becomes clear that the local surface temperature anomaly is due to the upward heat transportation by the liquid or steam. Consequently, the heat flow consists of thermal conduction and heat transportation at Toyoha geothermal field.

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On the southern foot of the Shikaribetsu volcano, central Hokkaido, many small hills scatter, which are composed mainly of hornblende-pyroxene andesite blocks derived from Higashi-nupukaushi lave dome with a small amount of ash and lapilli. Although they have been considered as mudflow deposits, there are some evidences that they have formed through high-temperature avalanche or nuee ardente, i.e. 1) oxidation phenomena on the surface, 2) remnants of secondary fumaroles, and 3) carbonization of included wood fragments. Twentyfour volcanic blocks of the deposits from 5 localities were collected for measurement of the N-pole orientation of natural remanent magnetism. The orientation of the samples from two localities shows good concentration, but the rest being random. Although stability tests have not been made, the temperature of the former two was probably near or slightly higher than the Curie point just after emplacement of the deposits, but that of the latter was lower. In reviewing the so-called “mudflow hills” in Japanese volcanic regions, it is noticed that many of the hills were formed either by high-temperature avalanche or by low-temperature dry avalanche, rather than by mudflow. Accordingly, the term “mudflow” should be used only for the “true” mudflow which flowed admixed with a large quantity of water.

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A remarkable seismic activity took place during the period from December 1964 to February 1965, near Oshima, one of the Izu Islands, south off Honshu, Japan. This seismic activity had three distinct stages. The first stage from 9 to 13 December, 1964, is of foreshock-mainshock-aftershock type in which the main shock is of magnitude 5.8. The second stage from 25 to 31 December 1964, is of swarm type, which includes three large shocks of magnitudes 5.2, 5.3 and 5.5. The third stage on 20 February, 1965, is of typical swarm type containing eight shocks of magnitudes larger than 4.0, of which the largest one is of magnitude 4.8. Volcanic tremors began to occur on 27 December, 1964, and the Volcano Mihara-yama had eruptions intermittently from 29 December, 1964, to 7 January, 1965. No intrinsic relation between seismic activity and volcanic eruptions is recognized in the present case, though a model implying indirect relation had been offered by Dr. K. Nakamura. Generally speaking, there is no significant correlation between volcanic eruptions of Mihara-yama and seismic activity near Oshima.

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