Airborne magnetic surveys of Suwanose-jima Island were conducted on August, 24 and 25, 1999 at the altitude of 1066m by a proton magnetometer installed on a craft of YS-11. Magnetic anomalies of total force are calculated by subtracting IGRF-10. The negative peak (-326nT) is observed over the north to northeastern coastal zone and a positive peak (242nT) is over the southeastern slope. Another positive peak is also recognized over the southern tip of Island. The volcanic edifice is approximated by the assemblage of the prismatic bodies for the 3D magnetic analyses. A 3D model analysis based on an uniform magnetization assumption shows the magnetization intensity of 3.14A/m corresponding to a magnetization direction polarized in the direction defined by declination of 0° and inclination of 42.5°. A 3D non-uniform magnetization analysis, where each block is magnetized in the same direction as in the uniform model with a variable magnetization intensity, is conducted to derive an internal magnetic structure of the edifice. This result makes it apparent that the volume weighted mean magnetization intensity is ranging from 2.87 to 3.12A/m, and a non-uniformity of magnetization from 40% to 14%, by taking into consideration of the base layer underlain by the volcanic edifice. Besides, the following features are also made apparent. (1) Relatively low magnetization zones (-0.5～-1.5A/m) are recognized in the southwest of Mt Ontake. This low magnetization zone extends to the base layer between 0m and 700m below sea level. This zone is thought to be linked with the conduit of Suwanose-jima Volcano. (2) Relatively strong magnetization zones (0.5～1.5A/m) are recognized in the southeast of the summit area and they further extend northeastward to the east offing of Suwanose-jima Island. (3) The base layer between 0m and 700m b.s.l. is generally small in magnetization intensity (-0.5～-1.5A/m) except the high magnetization zone denoted in 2). The above features may imply that the base layer is composed of more siliceous rocks than pyroxene andesites forming the present volcanic edifice.

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This paper presents an overview of diverse computer simulations, such as those based on the finite difference method, lattice Boltzmann method and molecular dynamics, to discuss their potential contribution to volcanology. The general features of several numerical methods are described to provide information for finding appropriate approaches to investigate various volcanic processes on a wide range of time and space scales. This paper also outlines previous numerical studies of some volcanic phenomena, including the generation of magma deep underground, the ground deformation due to magma injection into the Earth’s crust, and the evolution of giant eruption columns penetrating into the stratosphere. Fundamental issues on conducting computer simulations of volcanic phenomena are then discussed with the aim of promoting effective numerical studies in volcanology.

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フィリピン共和国,ルソン島南東部に位置するイロシンカルデラは,イロシン火砕流の噴出によって形成された.この研究では,この火砕流堆積物中の炭化木片を採取し,加速器質量分析(AMS)法によって¹⁴C年代を測定した.得られた¹⁴C年代は35,930±250BP(NUTA2-10795)であり,41,329±169calBPの暦年に予察的に較正される.この結果は,イロシンカルデラの噴火史研究だけでなく,フィリピンにおける広域テフラのデータベース構築にも貢献することが期待される.

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