11:15 AM - 11:30 AM
[SVC33-09] Efforts to improve the Volcanic Alert Levels criteria of Usu volcano
Keywords:Monitoring of volcanic activities, Volcanic alert levels, Precursor earthquake, Seismic energy release rate
Usu volcano, which have been repeatedly erupted at intervals of decades since 1663 eruption is one of the most active volcano in Hokkaido. The precursor phenomena immediately before eruption of Usu volcano is characterized by frequent occurrence of felt-earthquakes and large crustal deformation. Local government around Usu volcano mainly take early evacuation action using these clear precursors. Japan Meteorological Agency have applied volcanic alert level (VAL) for Usu volcano since 2008. The criteria of judgement of Usu volcano’s VAL which was published at March, 2020 is set mainly by the number of earthquakes immediately before eruption based on experience in past eruption cases. We will report the efforts to quantify the criteria of VAL by using geophysical observation data.
Iguchi et al. (2019) constructed the idea of empirical event branch logic based on magma intrusion rate at Sakurajima volcano. Fuwa and Miyamura (2022) applied this idea to interpret abnormal activities prior to eruptions or unrest events of Usu volcano after the 1977 eruption. As their result, the magma intrusion rate in the initial stage of each event, in which magma rises from a shallow magma chamber at several kilometers below sea level toward the summit crater to a depth of around 2 kilometers, varies with types of activity. The magma intrusion rates in summit eruption, flank eruption and unrest event correspond to high, medium and low, respectively.
It is difficult to monitor the magma intrusion rate by using geodetic observation date in real time. Based on the correlation between the intruded magma volume and the integrated seismic energy in Usu volcano showed by Fuwa and Miyamura(2022), we consider using the seismic energy release rate of volcanic earthquakes associated with magma intrusion as a quantitative index for monitoring volcanic activity. A comparative study of the energy release rate of 5 events revealed that the seismic energy release rate of 1977 summit eruption is two orders of magnitude higher than that of the 2000 flank eruption. It is revealed that seismic energy release rate of the 1977 summit eruption was two orders of magnitude higher than that of the 2000 flank eruption by comparative study of that of 5 events. The seismic energy release rate of 3 unrest events are even lower than that of 2 eruptive cases.
We propose that the empirical threshold of seismic energy release rate is added to the current criteria of VAL for level 4 and level 5. If any one of the multiple monitoring items reaches the criteria, the level will be raised immediately. In addition, we will also concider the use of earthquake magnitude to detect "felt earthquakes" in the current Level 4 and 5 criteria, as well as the use of tiltmeter records to detect "expansive crustal deformation" in the current Level 4 criteria.
Iguchi et al. (2019) constructed the idea of empirical event branch logic based on magma intrusion rate at Sakurajima volcano. Fuwa and Miyamura (2022) applied this idea to interpret abnormal activities prior to eruptions or unrest events of Usu volcano after the 1977 eruption. As their result, the magma intrusion rate in the initial stage of each event, in which magma rises from a shallow magma chamber at several kilometers below sea level toward the summit crater to a depth of around 2 kilometers, varies with types of activity. The magma intrusion rates in summit eruption, flank eruption and unrest event correspond to high, medium and low, respectively.
It is difficult to monitor the magma intrusion rate by using geodetic observation date in real time. Based on the correlation between the intruded magma volume and the integrated seismic energy in Usu volcano showed by Fuwa and Miyamura(2022), we consider using the seismic energy release rate of volcanic earthquakes associated with magma intrusion as a quantitative index for monitoring volcanic activity. A comparative study of the energy release rate of 5 events revealed that the seismic energy release rate of 1977 summit eruption is two orders of magnitude higher than that of the 2000 flank eruption. It is revealed that seismic energy release rate of the 1977 summit eruption was two orders of magnitude higher than that of the 2000 flank eruption by comparative study of that of 5 events. The seismic energy release rate of 3 unrest events are even lower than that of 2 eruptive cases.
We propose that the empirical threshold of seismic energy release rate is added to the current criteria of VAL for level 4 and level 5. If any one of the multiple monitoring items reaches the criteria, the level will be raised immediately. In addition, we will also concider the use of earthquake magnitude to detect "felt earthquakes" in the current Level 4 and 5 criteria, as well as the use of tiltmeter records to detect "expansive crustal deformation" in the current Level 4 criteria.