西之島火山の地球物理学的研究

抄録

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.