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[SVC28-P06] Hazard evaluation of ballistics from Vulcanian eruptions using infrasound observations
Keywords:Vulcanian eruptions, Volcanic ballistics, Volcano infrasound
Volcanic ballistics are the main hazard factor from explosive eruptions worldwide. The hazard area of ballistics is defined by the maximum reaching distance. Although the energy of launching ballistics is determined by the total energy and energy partition ratio (Gerst et al., 2013), estimating these source parameters is considerably challenging except for some idealized situations. Sakurajima volcano in Japan has had a series of Vulcanian eruptions at the summit craters (Minamidake and Showa) since 1955. There is a positive relation between the maximum infrasound amplitude and the maximum ballistics reaching distance at the Minamidake crater (Iguchi and Yamada, 2021). This relation suggests a link between the initial launching velocity of ballistics and infrasound excitation. Based on this background, we attempt to evaluate the maximum reaching distance of ballistics by infrasound observations. We focus on the latest Minamidake activity period from October 2017.
Sakurajima Volcano Research Center, DPRI, Kyoto University, operates an infrasound microphone (SI104, Hakusan) at the Harutayama branch office, where 3 km away from the Minamidake crater. The maximum ballistics reaching distance data is obtained from Japan Meteorological Agency. We convert the original data with approximate distance to the exact distance from the vent with a random number. Following trajectory analysis by Iguchi et al. (1983), the reaching distance is converted to the initial vertical launching velocity Vmax (m/s).
The activity at Minamidake crater since October 2017 is characterized by a series of eruptive episodes (Iguchi et al., 2022). In particular, a climax event (June 4th, 2020) of an episode from September 2019 has a maximum reaching distance of 3.3 km. An essential feature of infrasound records of the period is the longer duration of the onset pulse. Since the feature suggests a longer duration of gas emission in the eruption onset, the gas thrust is expected to contribute to the initial velocity of ballistics. Therefore we focus on the peak amplitude of integrated infrasound waveform (Imax) (Pa・s) as a proxy for evaluating Vmax.
We examine the relation of Vmax and Imax of 518 events in the corresponding period (Figure a) with a representative range of 1.0×10-4–3.0×10-4 by standard deviation. However, the Vmax/Imax ratio is not constant throughout the range. Figure b shows that the upper limit of Vmax/Imax is inversely proportional to Vmax. We examine this feature based on the equation of motion of ballistics. Assuming gravity acceleration and air drug is negligible, the equation of motion is approximated as ma=sP(t), where mass m (kg), vertical acceleration a(m2/s), the bottom area of ballistics s (m2), and pressure at the crater bottom P(t) (Pa). If we integrate the relation in the time domain, and the integrated pressure can be approximated as Imax, we obtain Vmax/Imax =s/m. Assuming the ballistics as cuboid shape, the ratio of s/m is equal to 1/ρd, where ρ is the ballistics density and d is the explosion depth. Under the condition of ρ as a constant, a decrease of Vmax/Imax implies an increase of d.
The longer duration of the infrasound pulse at the eruption onset represents a gas emission rate increase, which links to an increase in volcanic activity. Therefore, the gas supply to accelerate ballistics is necessary for the maximum ballistic reaching distance longer, as well as the gas accumulation beneath the crater bottom. Our consideration suggests a positive relation between Vmax and d. This relation may be interpreted as increasing lithostatic pressure at the explosion source contributing to Vmax. The range of Vmax is also expected to have an upper limit since volcanic edifice and conduit have a finite rupture strength. The relation between the upper limit of Vmax/Imax and Vmax(Figure 1b) may be adopted for assessing the maximum possible range of ballistics reaching distance.
JMA is acknowledged for sharing the ballistics data.
Sakurajima Volcano Research Center, DPRI, Kyoto University, operates an infrasound microphone (SI104, Hakusan) at the Harutayama branch office, where 3 km away from the Minamidake crater. The maximum ballistics reaching distance data is obtained from Japan Meteorological Agency. We convert the original data with approximate distance to the exact distance from the vent with a random number. Following trajectory analysis by Iguchi et al. (1983), the reaching distance is converted to the initial vertical launching velocity Vmax (m/s).
The activity at Minamidake crater since October 2017 is characterized by a series of eruptive episodes (Iguchi et al., 2022). In particular, a climax event (June 4th, 2020) of an episode from September 2019 has a maximum reaching distance of 3.3 km. An essential feature of infrasound records of the period is the longer duration of the onset pulse. Since the feature suggests a longer duration of gas emission in the eruption onset, the gas thrust is expected to contribute to the initial velocity of ballistics. Therefore we focus on the peak amplitude of integrated infrasound waveform (Imax) (Pa・s) as a proxy for evaluating Vmax.
We examine the relation of Vmax and Imax of 518 events in the corresponding period (Figure a) with a representative range of 1.0×10-4–3.0×10-4 by standard deviation. However, the Vmax/Imax ratio is not constant throughout the range. Figure b shows that the upper limit of Vmax/Imax is inversely proportional to Vmax. We examine this feature based on the equation of motion of ballistics. Assuming gravity acceleration and air drug is negligible, the equation of motion is approximated as ma=sP(t), where mass m (kg), vertical acceleration a(m2/s), the bottom area of ballistics s (m2), and pressure at the crater bottom P(t) (Pa). If we integrate the relation in the time domain, and the integrated pressure can be approximated as Imax, we obtain Vmax/Imax =s/m. Assuming the ballistics as cuboid shape, the ratio of s/m is equal to 1/ρd, where ρ is the ballistics density and d is the explosion depth. Under the condition of ρ as a constant, a decrease of Vmax/Imax implies an increase of d.
The longer duration of the infrasound pulse at the eruption onset represents a gas emission rate increase, which links to an increase in volcanic activity. Therefore, the gas supply to accelerate ballistics is necessary for the maximum ballistic reaching distance longer, as well as the gas accumulation beneath the crater bottom. Our consideration suggests a positive relation between Vmax and d. This relation may be interpreted as increasing lithostatic pressure at the explosion source contributing to Vmax. The range of Vmax is also expected to have an upper limit since volcanic edifice and conduit have a finite rupture strength. The relation between the upper limit of Vmax/Imax and Vmax(Figure 1b) may be adopted for assessing the maximum possible range of ballistics reaching distance.
JMA is acknowledged for sharing the ballistics data.