Aogashima Volcano
1: Introduction / Aogashima Volcano
2: Geology and geohistory of Aogashima Volcano / Activities in historical times before the years of Tenmei / Eruptions from 1781 to 1785
3: Rocks
4: Subsurface structures / Surveillance and observation of volcanic activities / Prediction of future activities and hazard
5: Volcanoes accompanying submarine calderas on island arc setting
6: Volcanoes within back-arc rift setting / Surveillance, observation, and prediction of future activities and hazard of submarine volcanoes
7: References
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Subsurface structures / Surveillance and observation of volcanic activities / Prediction of future activities and hazard
Subsurface structures
In 1984, the Meteorological Agency carried out seismic study when blasting took place at Ochiyo Harbor and Seiho Tunnel. Also in 1987, Tokyo Council of Disaster Prevention had blasting at five points within the island to do the same. These were to study velocity structure at shallow depths in Ikenosawa crater. Japan Coast Guard carried out air-borne magnetic survey in 1987 and pointed out possible magnetic anomaly ( another volcano? ) to the south of Aogashima. In 1988, Tokyo Council of Disaster Prevention took part in electrical survey by which specific resistance of the rocks were found to be several hundred Ωm, quite a normal figure, down to 100 m deep while it suddenly decreases to several Ωm deeper than that. The low figures are one order smaller than those for terrestrial water suggesting existence of an aquifer mixed with sea water. Before the Tenmei eruptions, the water of Oike and Koike ponds in Ikenosawa was fresh. It suggests that lenses of fresh and salt water aquifers existed at that time. Presently the aquifer in Ikenosawa is almost sea water. On the other hand, near the geothermal sauna house the specific resistance shows high values. This is interpreted that as the temperature in the ground is too high preventing permeation of sea water.
Surveillance and observation of volcanic activities
A seismograph was installed in June 1984 at Mukaizawa Water Intake facility and engaged in continuous observation. Frequency of preliminary tremors with shorter than 10 seconds is only several per month. In 1973 and 1987, Tokyo Council of Disaster Prevention measured ground temperature at depth 1 m, and Meteorological Agency in 1984 measured the same at 20 cm depth. The distribution of ground temperatures in both studies did not show any significant anomalies. The regions with temperatures in 20-80°C range are distributed at Maruyama crater, western slope of Maruyama, and western and NE parts of Ikenosawa crater bottom. Those over 80°C is restricted only within around geothermal sauna house. After 1987, temperature monitoring at fixed points have been carried out with the cooperation of Aogashima local government. It shows very little change. Other than Ikenosawa , comparatively high geothermal temperature areas are located at NW of the heliport and northern slope of Kompira Shrine areas with 50-60°C. Fumarolic activity has been low and only located at inner wall of Ikenosawa crater and around Maruyama. Fumarolic gas carries no irritating odor of hydrogen chloride or sulfur dioxide. Smell of hydrogen sulfide is rare. According to a survey of Tokyo Council of Disaster Prevention in 1988, the volcanic gas consists of 99 % water and 0.5 % hydrogen sulfide. Considering the island 's geographical situation of being very remote from anywhere, we would like to point out the importance of discovery of early indications of eruption such as anomalous geothermal phenomena and opening of fissures by residents. It would be very important for prediction of volcanic activities and fortification of observational facilities.
Prediction of future activities and hazard
While main part of the stratovolcano was growing, the volcano erupted rather regularly, but after the eruption of aphyric basalt, different types of eruptions became the norm such as irregular intervals and mode of activities including surge type eruption covering most of the island and formation of Ikenosawa crater with debris avalanche. Therefore, it is not easy to predict when and where and what type of eruption would occur. In recent times the eruption is mostly 0.001 to 0.1 km3 in scale.
The eruptions might occur at Ikenosawa considering those in Tenmei , but possibility exists for fissure type eruption extending to the sea floor. The mode may be phreatomagmatic. The scale may be restricted to be comparable to that of Tenmei , but it could be explosive as the one at Torishima in 1902. Even if eruption occurs in Ikenosawa, ponds such as Oike and Koike which changed the water levels and also became salty just before the eruption no longer exist. The aquifer at shallow depths is different from that at the time of Tenmei. As the prevailing wind direction is westerly, ash fall deposits may accumulate mainly in the eastern side of the volcano. This has to be taken into account for considering countermeasures. If the volcano becomes active the harbor may become unusable.