<特別寄稿>火山観測から火山防災へ

Abstract

This paper describes the characteristics of volcanic eruptions obtained from observations at Sakurajima, Satsuma-Iwojima, Kuchinoerabujima, and Suwanosejima in the Ryukyu Islands, and Guntur and Merapi volcanoes in Indonesia, and how the characteristics of volcanic activity revealed by the observation can be linked to forecasting eruption and hazard assessment. Since 1955, Sakurajima has been experiencing Vulcanian eruptions that cause damage due to falling volcanic bombs and air-shocks. Tiltmeters and strainmeters installed in underground tunnels have enabled us to detect the uplift and inflation of the volcanic body as a precursor to a Vulcanian eruption. B-type earthquakes that precede a Vulcanian eruption are accompanied by Strombolian eruptions and degassing. This results in the formation of a lava dome and underlying gas pocket. Volatile content is an important parameter in forecasting eruption style. As on Sakurajima, the segregation of hypocenters of A-type and B-type earthquakes is identified at Satsuma-Iwojima and ground uplift preceding eruptions is detected at Suwanosejima. The precursors to the 2015 eruption at Kuchinoerabujima were stronger than those of the 2014 eruption. It was necessary to forecast the hazard factors and their extent from the observed phenomena, however the alert zone based on the observation could not extend prior to the 2015 eruption. The energy of seismic activity preceding the 2010 eruption at Merapi Volcano allows to forecast the extent of pyroclastic flow that occurred after the seismicity. A support system for decision making was developed in Indonesia to utilize the forecasting for warning announcements. It is necessary to evaluate the types of hazards and their extent from observation data. Hazard assessment is important for Sakurajima, where a large-scale eruption is expected to occur in the future based on the amount of ground uplift in the Aira caldera, and should be reflected to alert zone of Level 5 (evacuation) warnings issued by the JMA.