Precursors to the 2015 Eruption of Kuchinoerabujima Volcano and Decision Making on Evacuation

― ― Abstract 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 charac-terized 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 fuma-role. 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 SO 2 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, SO 2 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