10:15 AM - 10:30 AM
[SVC29-06] Testing Volcanic Unrest Index (VUI) in Hakone volcano
Keywords:Monitoring of volcano, Volcanic unrest index, Hakone volcano
In the Hot Springs Research Institute of Kanagawa prefecture, many kinds of observations are used for monitoring of the activity of Hakone volcano. We have the observations of earthquakes, GNSS, tilt, underground water, gas flux and chemical composition of volcanic gas. As the kinds of observation data are increased, it has been difficult to interpret multiple kinds of the data and condition of volcanic activity. It is important to summarize multiple observation data in order to explain volcanic activity for non-scientists such as residents and governmental offices. In this study, we discuss how to introduce the Volcanic Unrest Index (VUI; Potter et al., 2015) and attempt to arrange unrest parameters and criteria.
In the VUI framework, we categorize many unrest parameters in three unrest categories "Local earthquakes", "Local deformation " and "Geothermal systems and degassing". In the case of Hakone volcano, it is reasonable to use nine parameters calculated from raw observation data or determined from on-site monitoring, referring to Potter et al. (2015) and the observational condition of Hakone volcano. We calculate VUI with the determination of an index of 0 to 4 for each parameter, based on set criteria. However, fumarolic activity and volcanic seismicity are constantly observed at Hakone volcano, and the activity level is clearly higher than that of volcanoes with no such activity at all. Potter et al. (2015) recommended that such volcanoes be referred to as a "warmer volcano" and that VUI=1 should be set for normal conditions in the volcano. Therefore, the final index, which is the integer value from 1 to 4, is obtained by taking the average of the nine parameters' indexes. The nine parameters used in this study are bellow.
In the category of "Local earthquake", the three unrest parameters are the duration of swarm earthquakes, the number (or rate) of volcanic earthquakes, and occurrences of low-frequency earthquakes or volcanic tremor. Three unrest parameters of the category "Local deformation" are change of long baseline, change of short baseline or tilt, which show shallow deformation, and flow and temperature of groundwater or hot springs. Three unrest parameters of the category " Geothermal systems and degassing" are blowout of steam well, flux of sulfur dioxide observed by differential optical absorption spectroscopy (DOAS), and chemical composition of gas in fumarole area.
The 2015 eruption of Hakone was considered as a magmatic-hydrothermal eruption which is equivalent to VUI=3 in the VUI framework, therefore, we set criteria for each parameter to have a VUI=3 around June 2015. We calculate the VUI value for each month between January 2001 to December 2023. When there is no observation in the target time window, for example the observation did not be started, we treat the parameter as a blank and do not use it for the calculation of VUI.
In the results, we got the VUI=2 in April 2015, VUI=3 in May and June 2015, and VUI=2 in July and August 2015, in other words, the VUI values gradually increased before the eruption and gradually decreased after the eruption. The VUI values were 2 for the time of the unrests of 2013 and 2019 and VUI=3 for the unrest of 2001. These values are well consistent with the feelings of scientists. Therefore, VUI is a useful tool to judge volcanic unrest based on multiple observation data. We will consider how to input the observational data for real-time calculation of VUI and what parameters, for example low-frequency earthquakes or weak volcanic tremor with background level amplitude, can be added to calculate the VUI in the future.
In the VUI framework, we categorize many unrest parameters in three unrest categories "Local earthquakes", "Local deformation " and "Geothermal systems and degassing". In the case of Hakone volcano, it is reasonable to use nine parameters calculated from raw observation data or determined from on-site monitoring, referring to Potter et al. (2015) and the observational condition of Hakone volcano. We calculate VUI with the determination of an index of 0 to 4 for each parameter, based on set criteria. However, fumarolic activity and volcanic seismicity are constantly observed at Hakone volcano, and the activity level is clearly higher than that of volcanoes with no such activity at all. Potter et al. (2015) recommended that such volcanoes be referred to as a "warmer volcano" and that VUI=1 should be set for normal conditions in the volcano. Therefore, the final index, which is the integer value from 1 to 4, is obtained by taking the average of the nine parameters' indexes. The nine parameters used in this study are bellow.
In the category of "Local earthquake", the three unrest parameters are the duration of swarm earthquakes, the number (or rate) of volcanic earthquakes, and occurrences of low-frequency earthquakes or volcanic tremor. Three unrest parameters of the category "Local deformation" are change of long baseline, change of short baseline or tilt, which show shallow deformation, and flow and temperature of groundwater or hot springs. Three unrest parameters of the category " Geothermal systems and degassing" are blowout of steam well, flux of sulfur dioxide observed by differential optical absorption spectroscopy (DOAS), and chemical composition of gas in fumarole area.
The 2015 eruption of Hakone was considered as a magmatic-hydrothermal eruption which is equivalent to VUI=3 in the VUI framework, therefore, we set criteria for each parameter to have a VUI=3 around June 2015. We calculate the VUI value for each month between January 2001 to December 2023. When there is no observation in the target time window, for example the observation did not be started, we treat the parameter as a blank and do not use it for the calculation of VUI.
In the results, we got the VUI=2 in April 2015, VUI=3 in May and June 2015, and VUI=2 in July and August 2015, in other words, the VUI values gradually increased before the eruption and gradually decreased after the eruption. The VUI values were 2 for the time of the unrests of 2013 and 2019 and VUI=3 for the unrest of 2001. These values are well consistent with the feelings of scientists. Therefore, VUI is a useful tool to judge volcanic unrest based on multiple observation data. We will consider how to input the observational data for real-time calculation of VUI and what parameters, for example low-frequency earthquakes or weak volcanic tremor with background level amplitude, can be added to calculate the VUI in the future.