Remarkable Temperature Rising of Groundwater Observed in Izu Oshima Island

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  • 伊豆大島における地下水温の高温化
  • イズ オオシマ ニ オケル チカ スイオン ノ コウオンカ

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Abstract

Periodic temperature observations have been carried out on the Ghyben-Herzberg type of groundwater at 11 sites in Izu Oshima Volcano since November, 1986; immediately before the 1986 eruption. The results lend us to conclude the followings; (1) The temperature of the Ghyben-Herzberg type of groundwater is about 19℃ in Izu Oshima Island usually. Rather high-temperature groundwater layers exist in some areas where related to ancient volcanic activites. (2) The change of groundwater temperature that has been observed in central and southern part of the Motomachi area since the 1986 eruption of Izu Oshima Volcano, is caused by the followings. 1) The change of the rate of the intrusion of deep-seated hot spring water into a shallow groundwater layer; relating to the increment of fluid pressure in the volcano caused by the rising of magmas (magmatic emanations). 2) The intrusion of relatively high-temperature saline water into a groundwater layer; relating to the increment of the difference of water temperature between a saline water layer and a groundwater layer caused by the heat supply from magmas (magmatic emanations). 3) The intrusion of high-temperature saline water; relating to the increment of the movability of water caused by the formation of groundcracks resulted from the change of pressure in the volcano. (3) The rising of groundwater temperature that has been observed in northern part of the Motomachi area, since about half a year after the 1986 eruption of Izu Oshima Volcano, is caused by the rising of thermal water through groundcracks and the spreading of it over a groundwater layer. This thermal water was formed in the 1986 eruption of Izu Oshima Volcano under the great influence of magmas (magmatic emanations) and volcanic gases.

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