2015年口永良部島噴火の火山活動推移と避難の意思決定

抄録

 On August 3, 2014 and May 29, 2015, eruptions occurred at the Shindake summit crater of Kuchinoerabujima volcano in the Ryukyu Islands, southwestern Japan. The Japan Meteorological Agency (JMA) upgraded the Volcanic Alert Level (VAL) to 3 (warned zone within 2 km from the crater) after the onset of the 2014 eruption and to 5 (evacuation) after the onset of the 2015 eruption. The possibility of implementing early warnings for eruptions and forecasting the area most likely to suffer damage from volcanic eruptions are examined based on monitoring data and disaster-affected areas of historic eruptions. The onset of the 2014 eruption was preceded by a 15-year prolonged increase in volcanic activity that started in July 1999. Only a short-term tilt change was observed immediately before the eruption. The prolonged volcanic activity is characterized by: 1) repeated bursts of seismicity; 2) ground inflation events around the crater associated with increases in seismicity; 3) increases in geothermal activity and 4) appearance of fumarole. The short-term process consisted only of a tilt change of crater-side up one hour before the onset of the 2014 eruption. The phenomena prior to the 2015 eruption were more intense than those prior to the 2014 eruption, as demonstrated by seismicity, which included a felt earthquake six days before the eruption; larger ground deformation; higher rate of discharge of SO₂ gas; and, higher temperature. Despite more intense activity, VAL remained at 3. VAL was upgraded from 3 to 5 immediately after the 2015 eruption and then all of the residents were evacuated from the volcanic island by ferry boat. Decreases in seismicity, SO₂ gas discharge rate, and geothermal activity led to an initial reduction of the alert zone radius to less than 2.5 km in October 2015. A further reduction to less than 2 km (VAL 3) was later implemented based on deflation around the summit area in June 2016. Problems related to evacuation decision-making in May 2015 are as follows: 1) JMA had no clear judgment criteria for VAL 4 and 5; 2) Volcanic hazards had not been evaluated based on monitoring data in the precursory period; 3) JMA did not indicate a clear hazardous zone in the warning information of VAL 5 even after the eruption; 4) Disaster measures, which assumed a pre-eruption VAL rise, were not appropriately implemented due to the post-eruptive VAL 5 declaration; and, 5) Return of evacuees to their homes was delayed due to management failure by the decision-making board. It is recommended to develop a method for constantly evaluating volcanic hazards from monitoring data with the progress of volcanic activity.