A small eruption occurred at the Mihara Crater of Oshima Volcano at midnight, Feb. 28/Mar. 1, 1974. The ejecta dispersed around the crater are mostly composed of highly vesiculated basaltic scoria (Fig. 1) with accessory lithic fragments and a small amount of pele’s hair. Under the microscope, phenocrysts of the 1974 scoria are plagioclase, orthopyroxene, pigeonite and rare augite. The groundmass consists of fresh brown glass with a few crystallites. The chemical and modal compositions of the 1974 scoria are shown in Tables 1 and 2, together with those of the 1950-51 lava, 1954 lava, 1964 scoria and 1972 scoria. The amount of phenocrystic plagioclase in the 1950-51 lava is only 5.3%. but it increases from 17.7 to 25.3% in the later ejecta. Phenocrystic pyroxenes also tend to increase in volume in the later ejecta, though not to the same extent as plagioclase. The phenocrystic pyroxene assemblage consists of augite and orthopyroxene, in the 1950-51 and 1954 lavas, while it consists of orthopyroxene, pigeonite and augite in the ejecta of 1964, 1972 and 1974 eruptions. The chemical compositions of phenocrystic minerals and groundmass were determined by EPMA analysis. The results are shown in Tables 3, 4 and 5, and in Figs. 2, 3 and 4. The change of bulk chemical compositions of the lavas and scoria from 1950-51 to 1974 is mainly controlled by the volume of modal phenocrystic plagioclase, as is clearly demonstrated in an addition-subtraction diagram (Fig. 2). Hence the “groundmass compositions” obtained by subtracting the composition of phenocrystic plagioclase (An₉₀) from each bulk composition are nearly the same as the glass composition of the 1974 scoria. The temperature of magma in the 1950-51 eruption was calculated as ca. 1200℃ using Wood and Banno’s method (1973) based on opx-cpx pairs. Although this value is considerably higher than the direct measurement results (1125℃, Minakami, 1955), it is in fair agreement with the estimated temperature of the magma. Due to the paucity of clinopyroxene phenocrysts in the 1974 scoria its composition could not be determined. However, by inference of the orthopyroxene composition, the temperature of the magma in the 1974 eruption was probably not significantly different from that of the 1950-51 eruption. In view of the activity of the Mihara Crater from 1950-51 to 1974 the following features are apparent: 1) After ten years dormancy, eruption of a considerable amount of nearly aphyric lavas (0.03 km²) occurred, which was intermittently followed by small eruptions of lavas and scoria. 2) The liquid composition of the products is fairly uniform throughout the duration of this activity. 3) The temperature of the magma seem not to have changed significantly. These features suggests that a compositionally zoned magma chamber was formed by gravitational sinking of plagioclase prior to the 1950-51 activity.

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Among the various type of volcanic earthquake and volcanic tremor, there are few type of volcanic earthquakes which including the coda wave vibration. The coda waves show slowly decaying for amplitude and have stationary period that the forms of them are resembled the damping free motion of seismometer pendulum. In this paper, writers call them convincingly "T-type earthquake". There are about sixty active volcanoes in Japan, but T-type earthquake has only been observed in the following six volcanoes namely Mt. Asamayama, Sakurajima, Mt. Tarumaeyama, Mt. Kusatsu-Shiranesan and Kuchinoerabujima. T-type earthquakes have a tendency to swarm, but its occurence is very rare. For example at Mt. Asamayama number of T-type earthquakes don’t exceed 0.1% of the total number of volcanic earthquakes in recent 10 years. The form of T-type earthquake differs a little from volcano to volcano but it generally resembles to the B-type earthquake except the part of coda waves. Common feartures of coda wave of T-type earthquake are as follows: 1) Periods of coda waves are distributed among 0.2～1.5 sec. 2)Damping coefficients “h” of coda waves are distributed among 0.003～0.03. Coda wave has predominant amplitude in the horizontal component of perpendicular direction from the crater. Attenuation of amplitude of the coda wave with distance from the crater is larger than the case of the B-type earthquakes which is mainly composed of the seismic surface wave. From the above mentioned result, writers suggest that the coda wave of T-type earthquake has a character of seismic SH wave. Comparison of maximum amplitude of T-type earthquake which recorded at different point on the same volcano suggests that origin of these earthquakes are located at shallow beneath the crater, similar to the case of the B-type volcanic earthquake. The coda waves of T-type earthquake are considered to generate by the free oscillation of some source beneath the crater. It is also a worthy of note that T-type earthquakes is only observed at the andesitic volcano which has erupted the Phreatic type eruption or the Vulcanian type eruption in recent years and T-type earthquakes sometime swarmed when abnormal volcanic activity was observed.

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The authors carried out temperature measurement of the ground surface at the Nakadake Crater of Asosan, used an infrared radiation thermometer, and compared the twice observation results at the calm period and the eruption time of the volcano. High temperature zones in the crater bottom and the crater wall were situated at the north half part of the Nakadake Crater, and anomalous high temperature zones at the eruption time extended about ten times as large as that of the calm period. Furthermore, these anomalous high temperature values at the eruption time were 10℃ to 15℃ higher than that of the calm period. These suggest that some thermally anomalous areas in the crater may change their thermal characteristics before or at the future eruptions. The thermal infrared surveys are very useful to volcano observation for the monitoring of some potentially hazardous crater, and these surveys can be done with safety and quickness.

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