Volcanic activity at the snow-capped summit of Mt. Chokai was first noticed by the captain of scheduled airline on March 1, 1974. The activity began with swarm of volcanic earthquakes, succeeded by fumarolic activity and finally explosion took place and a few craters were formed at eastern (late February-early March) and western (late April) foot of 1801 lava dome (Shinzan). The ejecta were exclusively fine-grained air-fall ash and accidental blocks. The blocks, formed the mud flow mixed with melted snow, rushed down the slope of volcanic edifice. Essential materials, such as bombs, air-fall scoria, or lava flows were not erupted. Rapid melting of snow was supposed to have been triggered by the formation of fumaroles caused by ascent of hot magma and associated juvenile gas. Thermal energy consumed for melting and evaporating snow is calculated as 3×10²¹ ergs. Total volume of mud-flow deposit is around 3×10⁴ cubic meters, and that of air-fall ash is an order of 10⁵ cubic meters. The entire area which showed thermal activity is approximately 700×200 meters, elongated in east-west direction. Distribution of earthquake foci was also trending east-west just passing the prehistoric summit and parasitic vents. Direction of vent alignments is the same for the most volcanoes in northeastern Japan, and is supposed to reflect regional stress field.

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Observation of the level change of the bottom-floor of the central pit in the summit crater has been carried on at Mihara-yama, Oshima Island since 1968. Pictures of the bottom-floor were taken by observers went down into the central pit along its wall at the earlier stage of the study and by radiocontrolled model helicopters and an observation balloon later. The depth of the bottom-floor was measured with wire, ropes and transits. Rise of 95 meters of the bottom-floor of the central pit was observed after a minor eruption of Mihara-yama on February 28, 1974. Before the eruption, the bottom-floor had kept roughly constant height of about 360 meters above sea level during this decade. The bottom-floor may have become shallower by a pile of flooded lava during activity of the eruption. After that the surface of the bottom-floor may have been uplifted as a whole by rise of the head of the magmatic column accompanying formation of spatter cones by Strombolian eruptions at the central part and flood of molten lava from the margin of the bottom-floor in the central pit. The active stages of Mihara-yama and Miyakejima may be determined by relation between frequency of volcanic earthquakes in Oshima Island and those in Miyakejima Island as shown in Figure 7. In an active stage of each island, there exist three maxima of frequency of volcanic earthquakes and a major eruption occurrs at the second maximum of volcanic earthquakes. It is estimated that an active stage of Mihara-yama started at 1970 from Figure 7 and that the first maximum will finish at 1975. The minor eruption of Mihara-yama in 1974 occurred just during the first maximum of volcanic earthquakes. The second maximum of the volcanic earthquakes is expected to come before long, accompanying a major eruption. The rise of the bottom-floor of the summit crater of Mihara-yama represents that of magmatic head. Generation of volcanic earthquakes and elevation of the head of the magmatic coulmn can be explained as phenomena caused by compressive stress concentrated under the volcano, though the mechanism which joints those earthquakes and level change has not been known. Therefore, this remarkable uprise of the bottom-floor in the summit crater of Mihara-yama might be a precursor of a coming major eruption.

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A well-preserved and nearly rooted charred tree trunk with “fossil smoke” was found from the Kuro-fuji pyroclastic-flow deposit (Fig. 1). A flow-unit enclosing the trunk is 120 cm thick and consists of poorly vesiculated essential fragments and volcanic ash. Within the bed, the constituent fragments characteristically decrease their sizes downward (Fig. 3), and at the bottom a seam of volcanic ash covers directly the lower unit. The tree trunk is 7.5 m long and 16 cm across at most and lies nearly flat (Fig. 2). No branches are observed to be left intact. The lower end connects with a block of earth. Below this block of earth is a hole corresponding to it in size and shape, apparently indicating the former position of the root. The “fossil smoke”, here called, is a portion of the deposit which is characterized by brownish pigmentation and lack of finer, ashy fraction. It extends from the trunk upwards branching off many vertical pipes which are truncated at the suaface of the flow unit by overlying another unit. The fossil trunk located at 4.5 km NW from the eruption of outflow of the pyroclastic flow, and lies in a direction about 70 degrees deflected from that which joins this locality and the eruption center. The facts mentioned above suggest us that the trunk was pushed down, buried and charred by the pyroclastic flow at the site where it grew. The Infrared absorption analysis was applied to six samples collected from various parts of the trunk and each temperature of charring was estimated (Fig. 4). The outer parts of the trunk show temperatures between 350° and 400℃, whereas the inner part a rather lower temperature close to 350℃. The process of the pyroclastic-flow emplacement may be as follows (Fig. 6) : Although the pyroclastic-flow cloud started to rush down toward NW, the lower part of the cloud was deflected its course to SW along the valley, separated from the higher part which was going straight over the topographic relief. The first blast of the pyroclastic flow passed slashing the branches of the standing tree, and uninterruptedly the main part of the pyroclastic flow reached and pushed over the tree trunk. The trunk was buried in situ as the basal part of the pyroclastic flow was deposited, and was overlain by the upper part of the pyroclastic flow which immediately followed. Though the pyroclastic flow was deposited within a very short span of time, in more detail it was deposited layer by layer from base to top.

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Small eruptions have continuously taken place on the Miharayama Volcano from February 28 to the end of June 1974. An infrared radiation thermometer and a vertical type seismometer were set on the crater-rim of the Miharayama Volcano and series of eruptions were observed on June 12, 1974. Electrical signals of the radiation energy coming from the craterlet at the time of eruptions and seismic signals following the eruptions were simultaneously recorded in a data-recorder. An instant of eruptions of hot materials enable accurately to determine from the pulse which have recorded by the infrared radiation thermometer. From the analysis of the observational data the following results were obtained : The observed seismic signal which has been so called “isolated type eruption-tremor” is characterized by distinct phases : namely P₁-, P₂-, Q- and X-phases. Q-phase exactly corresponds to the time of eruption. P₁-phase precedes about 6-10 sec. and P₂-phase precedes within 5 sec., while, X-phase is delayed by 4 to 30 sec. from the time of eruption. P₁-and P₂-phases are always accompanied with the eruption, while, X-phase is not always followed to that. There are the periodicity of 40 sec. or 80 sec. between two succesive tremors or eruptions. Ishimoto-Iida’s statistical formula for the frequency distributions of maximum amplitude does not adapt for the observed eruption-tremors. Downward initial motions were observed in the every phases of the eruption-tremors, but there are some exceptional cases for the P₁, P₂, Q-phases.

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