Being disturbed by volcanic micro-tremors, it is difficult to find the initial P waves of micro-earthquakes observed near the crater. Tomoda’s method was applied to obtain the crosscorrelation function between P wave portions of micro-earthquakes recorded at two adjoining stations and hypocentres were determined. The energy spectra of P wave portions of micro-earthquakes were calculated making use of the same method. The frequency at which the maximum value of the spectrum is found varies regularly around the epicentre.

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Yokoate-jima is a guitar-shaped island measuring 10 km round in circumference. The island consists of two stratovolcanoes, To-ho (East peak) 512 m high and Sei-ho (West peak) 268 m high. Initial activity would have taken place during the late Pleistocene on the submarine ridge where the present island is situated. The activity of the To-ho is divided into three stages. During the first and second stages, were formed a stratovolcano composed of lavas and pyroclastic materials of two pyroxene andesite and olivine-bearing two pyroxene andesite. During the third stage, eruption of pyroclastic materials of olivine-bearing two pyroxene andesite took place. The activity of the Sei-ho is also divided into three stages. During the first stage, alternate eruption of lavas and pyroclastic materials of two pyroxene andesite built up a stratovolcano. During the second stage, intrusion of lava dome of two pyroxene andesite formed the Sei-ho volcano. Eruption of pyroclastic materials of two pyroxene andesite occurred during the third stage.

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In the vicinity of the active crater of the vulcanian type volcano, there occur B type (Minakami, 1960) volcanic micro-earthquakes. Many investigations about the nature of B type earthquake have been done, but the mechanism of which is left unsolved. To study the mechanism of B type earthquake, it is necessary to know the geographical distribution of initial motion. And judging from the nature of B type earthquake, high sensitive seismometric observation near the crater is indispensable so as to record its initial motion. From this view point, the author has begun a preliminary observation at the two stations near the crater of volcano Sakurajima. Maximum magnification of seismograph system is about 2×10⁵ times at 15 c/s. The results obtained through the observation are as follows. At No.1 station which is 1, 600 m away from the active crater, some B type earthquake inital moitions were recorded clearly, but at No.2 station which is 2, 700m away from the crater, the initial motions were observed vaguely. The direction of initial motion at No.1 station is not uniform, but some are upward and others are downward. This fact seems to suggest that the mechanism of B type earthquake and that of the explosion one are not of the same nature, because at Asama or Sakurajima volcano, when initial motion is clear, explosion earthquakes always begin with upward phase.

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Three types of metamorphic mineral parageneses are developed in basic volcanic rocks of the so-called “unmetamorphosed” Paleozoics of the Inner Zone of Southwest Japan. These rocks are characterized by the mineral associations of 1. Chlorite-calcite, 2. Prehnite-pumpellyite-chlorite, and 3. Epidote-actinolite-chlorite. In the former two clinopyroxene of the original rocks does not suffer the recrystallization, while in the latter the mineral is entirely replaced by actinolite. Olivine is always recrystallized into chlorite. The parageneses 1 and 2 may correspond to the prehnite-pumpellyite metagreywacke facies, and 3 to the greenschist facies. The absence of any critical Ca-Al silicates in type 1 may be due to the effects of variation of H₂O and CO₂ pressure in metamorphism or due to the lack in rocks of suitable chemical composition.

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