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| | Abstract | | Characteristic changes of gravity at Akita-Komagatake Volcano after the eruption of 1970 are proposed to model with thermal activity in the area. After the eruption, ground temperature in surrounding area of the crater increased and reached the maximum peak during 1977-80 and then decreased and decayed almost completely until 1998. The observed gravities considerably increased in the period from 1977 to 1998. This fact is very likely to be connected with thermal activity in the area. In this paper a model is proposed to explain the decrease of gravity by the thermal expansion of the ground. The model consists of a semi-infinite isotropic homogeneous medium with a particular zone of (1) semi-sphere or (2) semi-infinite cylinder immediately under the surface. Effects of thermal expansion by increase of the temperature in the zone are mathematically evaluated assuming the constant temperature in the zone. The results show that the thermal expansion causes rise of the ground surface and decrease of density. Gravity on the ground surface is decreased mainly by the former and partly by the latter. Observed phenomena are considered as explained almost by this model. |
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