MODIS夜間赤外画像による浅間山2004-2005年活動の熱観測

抄録

Asama volcano, located in the central part of Japan, repeated medium to small scale Vulcanian eruptions, from September through December 2004. We analysed the activity between January 2004 and April 2005 using nighttime infrared data from MODIS -Moderate Resolution Imaging Spectroradiometer -onboard Terra and Aqua satellites, in conjunction with the data from ground-based instruments obtained simultaneously and chemical composition of the ejecta. The observed period is divided into four stages, S-I, S-II, S-III and S-IV, based on the relationship between thermal anomalies observed by MODIS and eruptive activity. S-I (second half of August) is the thermally active period preceding to the series of eruptions. This may have been caused by a magmatic supply to the shallow level of the conduit, resulted from dyke intrusion at a deep level (1km bellow sea level) in July, as suggested by the ground deformation monitoring. This magmatic supply probably resulted in initiating the eruptive activity of S-II. S-II (1 September -mid-December) is the thermally active period during the series of eruptions, which is sub-divided into two stages, S-IIa (1 September -10 October) and S-IIb (10 October -mid-December), by the two thermally active pulses in this period. Similar pulses are also recognized in the time-series variation of eruptive amount of S-II, as well as seismicity and SO₂ discharge rate. A deep dyke intrusion observed in the late S-IIa may have resulted in a new supply of magma to the shallow level, which caused the IIb activity. After two months of inactive period of S-III (late December 2004 -February 2005), S-IV (March-at least April 2005), the post eruptive thermally active period, started. Although this stage did not involve eruptive activities, SO₂ discharge rate, level of volcanic glow and height of plume raised. This may have been caused by the third magmatic supply to the shallow level in late January. At the volcanoes possessing open to semi-open passages between the summit crater and shallow level of the conduit like Asama, MODIS may detect pre-eruptive thermal anomaly, which can be utilized for monitoring those distributing in remote areas, such as east Asia.