The Seismic Activity of the Nasudake Volcano Using Hypocenter Data (1998–2022) from the National Research Institute for Earth Science and Disaster Resilience

The seismic activity of the Nasudake volcano is summarized based on hypocenter data from the National Research Institute for Earth Science and Disaster Resilience (NIED). The observation period spans 24 years, from October 1998 to December 2022, and the number of earthquakes during the period is 12,064. Of these, 80.1% occurred between depths of 0.7 and 2.6 km; 94% registered between -1.6 and 0.9 on the Richter scale. The lower limit of the detectable magnitude (magnitude of completeness, Mc) for the transition of the volcano-observation system was examined and found to be -0.5 for Mc before 2005 and -1.4 for Mc after 2012. Focusing on zones where seismic activity was concentrated, we divided the active zones into four areas. The greatest concentration of seismic activity, 76.4% of the total number of earthquakes, was between Chausu-dake and Asahi-dake. The depth of the earthquakes in this active area was shallower than 2 km, and most were micro-earthquakes (1 ≤ M < 3) or smaller. Earthquakes have been occurring regularly for 24 years. This active area includes the Chausu-dake lava dome formed by activities in 1408–1410 and two craters where phreatic eruptions have occurred since the Meiji era (1868–1912). Therefore, in considering volcano disaster-prevention measures, the relationship between this seismic area and volcanic activity should continue to be closely monitored.