Boron contents of condensed water samples from fumaroles (orifice temperature : 90～850℃) at Satsuma-iwo-jima Volcano were determined by using a spectrophotometric method based on the extraction of methylene blue-tetrafluoroborate ion pair. Condensed water samples were collected with special caution to avoid the contamination of boron from the sampling apparatus and from volcanic incrustations deposited around fumaroles. Boron contents of the high temperature fumarolic gases (＞700℃) were rather constants as shown in Table 1. On the other hand, those of low temperature fumaroles vary considerably. The F/B atomic ratios in condensates increase almost linearly with the orifice temperatures of fumaroles above 200℃. However, they show considerable scattering around 100℃. The B/Cl ratios are almost constant above 200℃. But, they also vary very widely around 100℃. These results indicate that boron in the volcanic gas behaves similarly to chlorine above 200℃. Below 200℃, fractionation of boron is very much complicated, because of the partial condensation of water in the gas and of the deposition of sassolite as volcanic sublimate.

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Geological and geophysical explorations of Nishinoshima (NO) and Nishinoshima-shinto (NN), a newly born volcanic island in the western Pacific Ocean (27°14.6′N, 140°52.6′E) and its adjacent area were carried out in March and July of 1974. The results of the geophysical investigations such as geomagnetic survey around NO and NN, magnetic properties of volcanic rocks, and observations of volcanic microtremors, are as follows ; two anomalies of total intensity normally magnetized, were found near the top of the volcano (Fig. 3). The measurements of Z-component of geomagnetic field by the fluxgate type magnetometer was made tentatively on the southern part of NN (Fig. 4). Based upon the geomagnetic anomaly distribution, geomagnetic model of Nishinoshima volcano was proposed as in Fig. 9. Ten specimens of Nishinoshima volcanic rocks (andesite) were used for the measurements of magnetic properties including the direction of polarity and the intensity of NRM, magnetic susceptibility, and the grain size of magnetic minerals. The volcanic rocks in NO and NN have the normal magnetization and rather high intensity, 10^-2～10^-4 emu/g, than the average intensity of andesitic rocks (Fig. 6 and Table 1). It is infered that the small grain size of magnetic minerals in the NN volcanic rocks is the main reason of the high NRM intensity. Observations of volcanic microtremors were made about 3.5 hours, but no A-type event of volcanic earthquakes was detected during this short periods. Spectrum analysis of microtremors shows that the predominant period is about 0.4～0.5 sec in NN. According to the results of chemical analysis of NN lava, SiO₂ content is about 58%, which is fairly high value compared with the estimated value from the relation between SiO₂ contents and the predominant period of microtremors (Fig. 13), consequently it may be concluded from this fact that the viscosity of NN lava flow was unusually low.

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At Sakurazima Volcano, the tremors such as a sinusoidal wave of which amplitude is modulated are frequently observed. This type of tremor is called as “C type”. The relatively simple type of the tremors is as a Gaussian wave packet. In the present paper, the characteristics of oscillation, spectral characteristics, etc. are investigated for relatively simple type tremors. At Stations B and A, the particle motions of the tremors are in both longitudial and transverse directions to the Minamidake Crater. The oscillation characteristic at Station C shows the Rayleigh type wave passing through from the northeast to the southwest. The mean periods of the tremors are relatively longer for waves with small amplitudes, and shorter for larger amplitude waves. It is suggested that relatively complicated or continuous tremors are consisted of the numerous events taking place one after another.

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