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VII. Activity Report of the National Committee for

Volcanology and Chemistry of the Earth’s Interior

Setsuya NAKADA Chair of the National Committee for Volcanology and Chemistry of the Earth’s Interior Eisuke FUJITA Secretary General of the National Committee forVolcanology and Chemistry of the Earth’s Interior

T

here has been significant volcanic activity and progress in volcanology in the last four years (2003-2006).

1. Eruptive events The volcanic activity from 2003 to 2006 was more stable than that from 1999 to 2002, when there were major eruptions at Usu and Miyakejima. Small volcanic eruptions occurred at Tokachidake,

Meakandake, Suwanosejima.

Asama,

Miyakejima,

Sakurajima,

Satsuma-Iwojima,

Fukutoku-Okanoba,

and

At Asama volcano, small eruptions occurred from February to April 2003 for the first time since 1990. The most active period started on September 1, 2004, when four middle-scale eruptions occurred by November. Associated with these eruptions, volcanic bombs struck the flank of the volcano, and ash-falls were observed as far as 200km from Asama. A seismic swarm was also recorded and a lava pond was observed at the bottom of the summit crater. This was the most activity since 1983.

Sakurajima had some explosions from 2003 to 2005 within the summit crater, and a small eruption occurred outside the summit crater on June 4, 2006, the first since 1949.

High levels of volcanic activity without explosive eruption were observed at several volcanoes. Volcanic gas was still being emitted from Miyakejima, even though the activity was decreasing. Seismic activity and abnormal phreatic activities were episodically observed at Aso.

2. Volcanological Studies

There was specific progress in geophysics, geochemistry, and geology and much new knowledge and many new techniques were acquired, helping clarify the characteristics of Japanese volcanoes. Integrated observations contributed to improving comprehensive models for volcanic processes.

Dense broadband seismic observation and ground deformation observation revealed magma migration before and during observation. At Asama, this kind of observation also led to an interpretation that magma intrusion occurred shortly before the eruption. Specific long-period earthquakes, indicating the interaction between magma and hydrothermal system, ceased before the eruption. The analyses of explosion earthquakes suggested models of magma’s expansion just prior to the explosion, magma’s vesiculation due to pressure-release, and successive magma ascents. Absolute and relative gravitational observation revealed the location of the magma head. SAR analysis was able to estimate the lava effusion quantity and the variation of lava cake depth at the summit of Asama. Low electric resistivity areas correspond to the intruded magma, which is inferred from geodetic surveys. A semi-real-time thermal observation system using satellite data detected an abnormal increase of temperature preceding the eruption.

At Izu-Oshima, the seismicity in the summit caldera synchronizes with both expansion and contraction of the whole island area, and the expansion rate correlates with the subsurface CO2 content. A long-term electric survey at Izu-Oshima volcano clarified the detailed three-dimensional resistivity structure.

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