Discolored seawater is generated along the seashore of Satsuma Iwojima as a result of mixing of acidic hot spring water with seawater. Variation of chemical composition of the discolored seawater due to the change of the mixing ratio at Higashi Hot Spring to seawater was examined. Chemical compositions in Si-Fe-Al 3-components system and color tone of the precipitates depended on the pH-value of the discolored seawater. Chemical compositions of the mixtures of the precipitates and the coexistent solutions reflected that of the hot spring water. It indicates that the discolored seawater is composed of a simple mixture of hot spring water and seawater without any separation processes such as removal of precipitate from discolored seawater.

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Small pyroclastic flows have been generated by collapses of lava domes at Unzendake volcano, Japan. Seismic and acoustic waves were analyzed that were excited by the pyroclastic flows. Minor seismic waves are excited at the time of lava dome collapse and low frequency seismic signals with larger amplitude follow them as the blocks of lava fall onto the slope. The low frequency seismic waves are interpreted as a result of the dome collapses and their amplitudes are correlative to the volumes of the collapsed lava blocks. While the pyroclastic flows descend along the mountain slope, high frequency seismic waves are emitted and they die out as the pyrociastics cease to flow. The variation of amplitude ratio among the seismic waves recorded at different stations indicates that the source of seismic signals runs down along the slope. By analyzing the acoustic waves observed by low frequency microphones, the precise locations of the dome collapses were determined. Acoustic signals intermittently observed during the flow of pyroclastics are inferred to be excited from sources moving with the pyroclastic flow front. Excitation of seismic and acoustic waves are sensitive to topographic singularities and their amplitudes and frequencies vary with time.

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