三宅島火山活動の発端となった岩脈貫入のシナリオ

概要

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The initial stage of the 2000 Miyakejima volcano activities started at about 1800 LT, Jun. 26 with seismic swarms and crustal deformation, and continued until June 27. These phenomena are interpreted to be due to dike intrusions and their migrations from the south-eastern to the western parts of the volcano. Our analyses of tilt data, which were observed at five bore-hole stations, focused on their temporal and spatial changes, and revealed that the magma began to intrude toward the south of the summit area from a magma reservoir beneath the south-eastern part of the volcano. The head of the intruded magma was as shallow as 200 m beneath sea level, but the volcano did not erupt at that time. After that the magma moved to the western region and intruded toward a depth of 500 m beneath sea-level, causing an extraordinary tilt change as large as 170,≪rad at MKA by 0100, Jun. 27. The magma system established during this initial stage is a key to recognizing the many following phenomena such as spasmodic eruptions, tilt-step, and volcanic gas emissions. Volcanic activities continued for more than one year.