To understand the detailed process in the initial stage of an explosion earthquake, we studied short-period seismic waveforms of three explosive eruptions of Sakurajima volcano in November 1987. A single force directed downward or spherical pressure source were conventionally used as equivalent body forces for the explosion earthquakes. In the present study, to obtain a more generalized description of the source of the explosion earthquakes, we adopted two kinds of source models, namely a moment tensor M_xx, M_xy, M_xz, M_yy, M_yz, M_zz, or a combination of a vertical single force and five components of moment tensor F_z, M_xx, M_xy, M_xz, M_yy, M_yz. The source time functions for the force F_z, or couples M_ij, were determined by waveform inversion. The observed waveforms can be explained satisfactorily by the moment tensor model. The estimated source time function indicates that a rapid expansion and subsequent contraction occurred in the source region of the explosion earthquakes. The alternate model including the vertical single force, however, can not explain the compressional first motion that is one of the notable features of the explosion earthquakes at Sakurajima. These findings suggest that the moment tensor provides a more appropriate system of equivalent body forces in the initial stage of the explosion earthquakes. A vertical force dipole M_zz was found to be the largest of the diagonal elements of the moment tensor in the initial expansion of the source. This is consistent with the result obtained by Iguchi (1994). We hence consider that the explosion earthquake begins with a vertically-directed volumetric expansion of the source region.

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Various types of low-frequency seismic events with predominant periods longer than 2 s have been observed at Unzen volcano since 1991. These events are, in chronological order, low-frequency tremors, A-type volcanic earthquakes, seismic events triggered by pyroclastic flows, and explosion earthquakes. In this study, the source processes of a volcanic tremor episode on May 10 and an explosive eruption on June 11 were investigated by waveform inversion. A vertical single force or a moment tensor were used as equivalent body forces for those events. To determine the appropriate source model for the events, we performed two kinds of inversions : the first is based on assumed source time functions, and the second estimates the shape of the source time functions. The tremor, which is composed mainly of Rayleigh waves, was better explained by a moment tensor representing contraction of the source than a vertical single force. The explosive eruption was also better explained by the moment source. The source time functions we derived indicate that the source first contracted and subsequently expanded about 3-4 s later. This functional dependence may be related to the mass flux of the explosion which culminated about 4 s after the event onset. The volume change of the source region of the explosion earthquake represents only about 1 % of that estimated from measurements of tilt associated with magmatic activity.

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The Iki island comprises many monogenetic volcanoes of alkalic basalt, and some volcanoes of various kinds of acidic alkalic and subalkalic rocks. They are composed mainly of scoria cones and lava flows. Excluding some unclassified lava flows that comprise the lower part of the island, 52 lava flows can be recognized. Based on the detailed field observations and the K-Ar age determination of 41 samples, stratigraphic relationship of volcanic rocks and the volcanic history of the monogenetic volcano were established. The volcano on the Iki island has grown over a period of 15 Ma to 0.6 Ma. At least after 3.5 Ma, volcanic activity was intermittent, separated by three quiescent periods. Each active period is 0.3-0.7 million years long and the quiescent period is 0.3-0.4 million years. Eruption volume during the each active period is almost the same (0.012-0.059 km³). Eruption rate of each active period is also almost the same (47-84 m³/year).

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Analytical method by secondary ion mass spectrometry, SIMS, is developed for water micro analysis of hydrous silicate glasses. Standard hydrous glasses are synthesized and carefully examined their homogeneities. “Matrix effect” for the determination of hydrogen analysis by the SIMS method is corrected using their SiO₂ content (wt. %). Using the SIMS, water contents of andesitic to rhyolitic glasses can be estimated precisely (〓±0.5wt. % H₂O). As an application of the SIMS analysis, water contents of melt inclusions and matrix glasses from the Aira eruptive products are examined. The water contents of the melt inclusions are 5-7 wt. % H₂O. This is consistent with the water contents calculated by the relationship between water solubility and water pressure from the data of Aramaki (1971).

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