The preferred orientations of dike swarms of 12 areas that formed in the Early and Middle Miocene are investigated in order to know the azimuths of horizontal maximum compressional principal tectonic stress (σH_max) in the Southwest Japan. The most concentrated trends of dikes are parallel to the Southwest Japan Arc in each region to the north of the Butsuzo tectonic line and are perpendicular to that in each region to the south. Therefore, the directions of σH_max obtained from the preferred orientation of dike swarms are parallel to the Southwest Japan Arc on the northern side of the Butsuzo tectonic line and nearly perpendicular to it on the southern side, respectively. This results coincide with the directions obtained from the strike and the sense of motion of faults and folds formed in the same age.

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Spectral features of seismograms are investigated for the events closely related to four eruptions on February 26, 27 and 28, 1975, based upon an assumption that variations in spectral compositions can be interpreted as the changes in focal depths of events. A swarm of volcanic earthquakes started about 17 minutes before the eruption of 03h55m (JST) on February 26. The generation of these events is correlated to the explosion mechanism because the highest activity of the swarm occurred at the eruption and because focal depths of the events were as deep as that of the explosion earthquake. Two eruptions occurred at 03h35m and 04h48m on February 27. Events related to the latter eruption are readily distinguished from those following the former by the predominant frequency of power spectrum, spectral composition, running spectrum or seismic activity. Energy sources of the two eruptions were at small depth under the crater. Precursory activities of strong B-type volcanic earthquakes occurred 8 minutes before the violent eruption of 00h16m on February 28. Running spectra of Ct-type, low predominant frequency in the spectrum and little emission of volcanic smoke imply that a great deal of energy might be interrupted at a depth from flowing up.

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The ejected ashes were collected from the points 0 to 4.5 km from the sources during the whole period of the pumice and phreatic eruptions in the 1977-1978 activity of Usu Volcano, Hokkaido, Japan. The dry ash samples (＜2 mm) of pumice eruptions (Aug. 7-14, 1977) were weakly alkaline (pH=7.3-8.2), and rich in water-soluble and exchangeable Ca²⁺ and Na⁺, but poor in water-soluble Cl^- (69-580 ppm) and SO₄^2- (282-1231 ppm). By contrast, the ejected ashes of phreatic-phreatomagmatic eruptions (at the Craters B, H and I) were strongly acidic (pH=3.4-5.9), and rich in water-soluble Cl^- (495-17260 ppm) and SO₄^2- (160-4830 ppm), as shown in Table 1 and Fig.4. Whereas the ejected ashes from the Craters K, L and M were neutral or weakly alkaline (pH=6.8-8.5), and showed a slight decrease in the concentration of the water-soluble Cl^- (421-1064 ppm) and SO₄^2- (256-1031 ppm). When the Cl/S molar ratios of the water-soluble components of the ash (1.9-19.2) ejected from the Craters A, B, H and I were also taken into consideration, it was suggested that the temperatures of the coexisting gas phase with these ashes were very high. As shown in Table 1, the ash samples contained 2.5-6.2% under 2μ clay. The clay fractions (either under 2μ or under 0.2μ) separated from the samples were characterized by the presence of abundant montmorillonite and subordinate chlorite and metahalloysite (Figs. 6, 7, 11 and 12). These clay minerals are considered to have derived from the atrio-deposits on the summit of Usu Volcano. In this connection, late in August, 1977, it was confirmed that two clay flows consisting mainly of montmorillonite were squeezed out on the surface of the atrio due to crustal movement (Fig. 3). The ejected ash had a high adsorptivity and viscosity, and also showed high dispersion in air and water. Such properties are probably due to the presence of the montmorillonitic clay in the ash. The presence of such clay minerals in the ash gave rise to severe disaster to the urban, forest and agricultural lands near the volcano during the eruptions on rainy days. The silica contents of the 1977-1978 ashes, pumices and essential blocks are plotted in a variation diagram together with those of the 1944 tephra (Showa-Shinzan) (Fig. 13). The silica contents of the 1977-1978 ashes showed a wide range from 53.91 (Crater A) to 68.00 wt% (Crater K). The difference in the color of dry ash may depend on the chemical and mineralogical compositions. In this connection an interesting relationship was found between the variation in silica contents of the ash and the sequence of the Second Stage eruption. As shown in Table 1 and Fig. 13, the ashes ejected in the Substage I (typical phreatic eruptions) were greyish brown or grey in color and poor in silica (SiO₂=53.91-57.55 wt%), whereas in the Substages II and III (phreatomagmatic-magmatic eruptions) the grey or greyish white and silica-rich ashes (SiO₂=61.63-68.00 wt%) became abundant. It was noticed that the ashes produced by the magmatic eruptions were mostly consisted of the essential dacite, such as the greyish white ash (SiO₂=68.00 wt%) which was ejected from the Crater K on August 24, 1978.

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