High frequency and low frequency earthquakes were observed associated with the intrusion of dacitic magma beneath the summit of Unzen volcano, Japan. The low frequency earthquakes were accompanied by small impulsive infrasonic signals, whereas the high frequency earthquakes were not. From the records of a low frequency microphone network, the infrasonic pulses are inferred to be excited at almost the same times as the earthquake occurrences. Source location of the infrasonic pulses could be accurately identified from the records of the low frequency microphone network. The sources were distributed around areas from which a new lava was emerging. The infrasonic pulse is inferred to result from an emission of volcanic gases through a fracture of an existing lava dome due to the intrusion of new magma.

抄録全体を表示

An electrical resistivity model of the Aso caldera was constructed using the magnetotelluric (MT) survey data obtained at 200 sites in the caldera area. The modeling reveals the following results : (1) the high-resistivity basement of Cretaceous rocks beneath the caldera floor slants gradually to the center of the caldera ; (2) the low-resistivity layers, of less than 50 ohm-m, are found commonly around the sea level beneath the caldera ; and (3) the majority of these low-resistivity layers are distributed along the ring-like belt surrounding the high-resistivity central cone. The most likely cause of the low-resistivity value at deep layers in the Aso caldera is high water content in high porosity rocks such as tuff and tuff breccia. As the Aso caldera was formed by violent eruptions of pyroclastic flow, these low-resistivity layers are interpreted as the “fall-back” materials in the caldera formation process. Therefore, this modeling result implies that the Aso caldera is the “funnel-shaped type” one formed by the pyroclastic volcanism.

抄録全体を表示

Tokachi-dake volcano in Central Hokkaido, Japan, began to erupt in Dec. 1988 from the 62-II crater after dormancy for 26 years, and the intermittent eruptions occurred 23 times until Mar. 1989. Volcanic ashes were ejected 14 times during the 1988-1989 eruption and water-soluble components on the ashes were analyzed. The ashes were rich in the water-soluble components and the great part of the volcanic ejecta was altered stale rocks in the vent. This feature suggest that the water-soluble components were derived not only from volcanic sublimates that formed during the 1988-1989 eruption but also from the sublimates that have formed in the vents since the last eruption in 1962. Cl/S molar ratio in the ash leachates came to be high as the volcanism became active. Although accumulated volcanic sublimates before eruption made the Cl/S ratio of the ash leachates different from those of eruptive gases, change of the ratio of the leachates agreed well with the observed change of the volcanic activity. It was shown that analysis of water leachates of volcanic ashes was a safety and effective method for estimation of volcanic activity. Volcanic sublimates ; halite, sylvite, thenardite, gypsum, alunogen, halotrichite, epsomite and melanterite were assumed to be sources of the water-soluble components and the normative mineral compositions of the samples were calculated. Proportion of halite + sylvite was changed corresponding to the eruption scale. The change of the mineral composition suggested that temperature of volcanic gases discharged went up in the middle of January, 1989 and down early in March, 1989.

抄録全体を表示