火山ハザードマップと噴火シナリオが火山防災対策に果たす役割

抄録

In the 1970s, a volcanic hazards program was started in Hokkaido, Japan. The first hazard map was published for Hokkaido-Komagatake volcano in 1983. The effects of the eruption on Izu-Oshima volcano in 1986 led to the preparation of nine volcanic hazard maps. From the late 1990s, many hazard maps were published, after the 1990-1995 eruption of Unzen volcano and the 2000 eruption of Usu volcano. The turning point for the preparation of the hazard maps came about during the 2000s when the Japan Meteorological Agency established the volcanic-alert-level system. Subsequently, the Japan Meteorological Agency changed the policies of the system after it was evaluated in 2003, based on the intensity of eruptions, and in 2007, based on the distances of the eruptions from populated areas. Afterwards, social factors were also taken into account, resulting in more complex planning scenarios for volcanic hazards programs that involved linking multiple administrative authorities in Japan. For such situations, various scenarios for mitigating volcanic hazards have been introduced for many volcanoes.

Two significant eruptions occurred after the introduction of the volcanic-alert-level system:the 2011 eruption of Shinmoedake in Kirisima volcanoes, and the 2014 eruption of Ontake volcano. The sub-Plinian eruptions on Shinmoedake caused some confusion in the implementation of the evacuations, but no one was killed in this incident, which was assigned an alert level of 2. The phreatic eruption of Ontake volcano, which claimed 63 lives, was assigned an alert level of 1. This article was prepared according to the list of hazard programs for each of these two volcanoes before their respective eruptions.

The hazard maps showing the precise locations of vents or areas of volcanic hazards in the cases of Usu volcano and Shinmoedake volcano likely helped facilitate evacuation during their respective eruption. Therefore, volcanic hazard mapping requires the accurate location of vents and accurate estimates of volumes of volcanic materials that can be set in motion during volcanic crises. Additionally, local administrations need to prepare case-study hazard maps based on volcanic crisis scenarios that consider the intensities and possible occurrences of volcanic eruption phenomena similar to those that occurred during the Kirishima eruptions. It is desirable that such maps conform to the volcanic alert levels assigned by the Japan Meteorological Agency. In 2015, the Act on Special Measures Concerning Active Volcanoes was amended taking into account the tragic outcome of the Ontake eruption in 2014. This article provides recommendations for volcanic hazards mitigation programs that can be implemented at locations near volcanic vents for the safety of hikers and tourists. This article also discusses the tasks and prospects of the implementation of volcanic hazard programs in Japan.