On Novemer 16, 1987 Izu-Oshima volcano erupted again after a 1 year quiescence. Beginning 20 seconds prior to the eruption, low-frequency waves with large amplitudes (1.6 cm/s, at most) were observed at Gojinka-chaya (GJK) which is located 1.8 km north-west of the central crater of the Izu-Oshima volcano. This station consists of a low gain velocity-type seismometer with wide frequency range and data have been recorded by a digital event recorder. The pre-emption seismogram was modeled as a set of point sources embedded below the central crater in order to obtain the mechanism of eruption or the generation process of low frequency waves. An inversion was performed to obtain 3 independent components, moment tensor (M_XX+M_YY), M_ZZ and single force F_Z. The results are as follows: there was an upward single force at shallow depth (0.5 km), a downward single force at middle depth (1.0-1.5 km), and an upward single force at the deepest (2.0 km). This set of forces is interpreted to represent the draining of magma (or hot dry rock) down through the vent. There was also a negative M_ZZ moment tensor component at shallow depth. This can be interpreted as a shrinking horizontal tensile crack caused by the initiation of magma migration. A remarkably large component of (M_XX+M_YY) was found at middle depth. This can be interpreted to mean that the vent was at a high pressure before the eruption.

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The vesiculation of dissolved volatiles in the magma could be a driving mechanism of magma ascent that might cause an episodic and abrupt occurrence of volcanic eruption. The bulk density of magma is significantly reduced by the formation and expansion of bubbles in the magma, and it is formulated as a function of pressure under the assumption that the solubility of the volatile component follows the Henry’s law and that bubbles change their volume like an ideal gas. A simple expression of ascent velocity associated with buoyancy force is applicable to various mechanisms of magma migration involving fracture, viscous resistance, pipe and permeable flows in the crust. Magma is assumed to stay first in a magma reservoir under a gravitationally stable condition. The vesiculation may be accidentally initiated in some part of magma by the decrease of magma pressure due to elevated mamga level, or crustal stress changes. Resultant bubble inclusion reduces the bulk magma density so that the magma begins to move upward subject to buoyancy. The ascent motion as well as the vesiculation process is accelerated at shallower depth with decreasing pressure. This process of magma ascent is accompanied by a significant expansion of the magma volume, and finally leads to a volcanic eruption on the crustal surface.

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A study of two piston cores from the Estase I cruise (R/V Jean Charcot), concerning volcanic ash layers interbedded with sediment from active continental margin off Japan, allows us to characterize a part of the late Quaternary explosive volcanic activity. It shows evidence of continuous acidic volcanic activity more or less potassic, with three basic ash layers (high-alkali-tholeiitic to low-alkali-tholeiitic affinity) in a East Honshu core, and two moderately potassic ash layers in a south-west Kyushu core. From geochemical studies and isotopic age determination of ash layers (¹⁸O/¹⁶O and ¹⁴C), it was possible to correlate the basic ones of the East Tohoku core with the Nasu volcanic area and an acidic one of the south-west Kyushu core with the Aira-Tn ash (AT), 22000 years BP.

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