Andesitic and dacitic lavas form a certain kind of autobrecciated structure upon entering into the water instead of forming the pillow structure common in basaltic lavas. Such lavas consist of angular, polyhedral blocks ranging in size from a few meters to several centimeters in diameter cemented by comminuted fragments of the same material or by sediments. Matrix of palagonite is sometimes present. Individual blocks are compact and are often traversed by joints which divide them into still smaller polyhedral blocks. In blocks larger than 0.5m., which are usually angular in outline and sometimes spheroidal, closely spaced joints roughly perpendicular to the surface of the blocks are developed, indicating their quenching against the water. In the Pleistocene volcanoes of Izu Peninsula and Hakone, no such subaqueous autobrecciated lavas have been found even along the present sea coast. This fact implies that they have never been uplifted since the beginning of the Pleistocene or have been partly submerged because of the elevation of the sea level since the end of the glacial period.

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Gravity survey was made over the Aso and Kuju volcanic region occupying central part of Kyushu Island of Japan by use of Worden and LaCoste and Romberg geodetic gravimeters from 1965 to 1966. In this paper, the gravimetric feature of the Kuju volcanic region was discussed. About 20km north-east of the Aso caldera, is situated Kuju volcano group consisting of more than ten cones or domes and covering an area of 20km (EW)×15 km (NS). The number of the observed gravity stations is 146 over the Kuju volcanic region and an average station density was one station per 9.6 km². The Bouguer anomalies are characterized by a circular negative anomaly over the Kuju volcanic region. The negative anomaly extends over the Kuju volcanic group. The contours show concentric circular patterns. The Bouguer anomaly at the center relatively lowers to 26 mgal as compared with surrounding area. Gravimetric view thus favors to suppose the “low anomaly type caldera” in this region such as Aso and Aira though topography does not show it up clearly. From the detailed geological survey in this area, T. Matumoto proposed the buried caldera which he named Kuju caldera, formed by gigantic eruptions of tremendous amount of pumice. The eruption of large amount of pyroclastic materials are usually supposed to be related with the formation of calderas. The above mentioned proposal of T. Matumoto is based mainly on the distribution of the Kuju ignimbrites found in the surrounding area of the Kuju volcano group. From above comparative study of gravity and geology, the existence of buried Kuju caldera is reasonably supported. The underground structure over the Kuju volcanic region was computed from the gravity data by use of sin x/x method under the several reasonable assumptions.

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The air vibration, caused by the explosion of Volcano Kuchinoerabujima on November 22 in 1966, was recorded with electromagnetic seismograph in Volcano Sakurajima. The figure of the record at Station B in Sakurajima, 134 km in distance from explosion source, was apparently similar to that of seismic wave (Fig. 2). Therefore, it was necessary to investigate this record in detail. The problem of air vibration was generally studied on the path and arrival time of wave propagation in the atmosphere, using ‘ray theory’. In this paper, we studied on the energy of wave as well as path and arrival time in moving air. The results were summarized as follows. 1) The air vibration was dispersed in 4 phases, and arrived at Station B through different paths (Fig. 5). 2) Waves of each phase had different values on energy (E_n) and arrival time (t_n). Theoretical results, E₁ : E₂ : E₃ : E₄=1/2 : 1/0.9 : 1/0.7 : 1/40 and t₁ : t₂ : t₃ : t₄=404 : 414 : 426 : 436 (sec), showed good agreements to observed results of the record of seismograph. 3) Such dispersions of air vibration become more distinct where it is farther from the source.

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