1983 年三宅島噴火の熱的調査

概要

論文の詳細を見る
The volcano Miyake is a volcanic island located at 200 km south of Tokyo. On October 3 in 1983, a fissure eruption took place at the southwestern flank after a quiescence of 21 years since 1962. Several kinds of thermal surveys were carried out before and after the eruption, and the following three items were examined. 1) Geothermal activity at the summit area before and after the eruption : In general, a flank eruption is considered to be caused by a dike intrusion from the central conduit, which connects the summit crater and the magma reservoir at depth. The height of magma head in the conduit is expected to reflect the volcanic activity and the regional stress and to be elevated gradually associated with a supply of magma before the eruption. At the summit area, fumarolic and steaming activities continued discharging thermal energy along the rim of the old crater, which was buried by the lava flow and the scoria cone of the eruption of 1940. It is also known that the thermal activity increased from November, 1963, one year after the eruption of 1962. Therefore, a precursory and/or post eruptive thermal anomaly is expected to occur at the summit area. 2) Cooling of the eruption fissure : The present fissure is about 5 km long, and lava fountains took place at the northern part, while magmatophreatic explosions took place at the southern part. The fissure also intercepts the wall of old caldera. Cooling rates at those areas are expected to reflect the amount of heat source, which remains at shallow part after the eruption, and the difference of geological settings. 3) Thermal anomaly at the area between the northern tip of the eruption fissure and the summit : If the present fissure eruption was caused by a dike intrusion from the central conduit beneath the summit crater, magma or some high temperature matter must have existed and most possibly remains under those areas and thermal energy of those matter may be released through the subsurface cracks possibly created around them. The results are as follows. 1) Geothermal activity of the summit area increased from 1963 to 1970, and turned to decrease since 1975. No precursory thermal anomaly was detected through the surveys before and after the eruption ; at the end of the August in 1983, one month before the eruption, and on Oct., 6, 3 days after the eruption. The summit thermal activity began to increase about 10 days after the eruption and the steepest increase took place during one month after the end of October. Thermal energy release rate was estimated to be at the order of megawatt at the end of November and found to have increased as twice as that before the eruption. These facts are consistent with the results of apparent electrical resistivity surveys and chemical analysis of fumarolic gas. These facts suggest that a hydrothermal system supporting the summit thermal activity is located at shallow depth and the height of magma head was sufficiently deeper than that of the hydrothermal system just before the eruption. 2) Fumarolic temperature decreased rapidly at the northern tip of the eruption fissure and more and more gradually at the southern sites, while that of caldera wall decreased extremely rapidly. It is also indicated that the radius of crater near the caldera wall is significantly larger than that inside the caldera. These facts may reflect the difference of mechanical structure of volcanic body. The difference of cooling rates were not clear between the areas which caused lava fountains and magmatophreatic explosions. 3) Measurement of ground temperature at 70 cm depth was carried out repeatedly along the survey lines perpendicular to the fissure direction at the area between the summit and the northern tip of the eruption fissure. It was found that each profile has two peaks of temperature, and that thermal anomaly elongates about 130 m from the northern tip toward the summit.
特定非営利活動法人日本火山学会の論文
1984-12-28

1983 年三宅島噴火の熱的調査

スポンサーリンク

概要

著者

関連論文

スポンサーリンク