11:00 AM - 11:15 AM
[SVC30-08] Tephra emission density evaluation by seismo-acoustic observation at Sakurajima volcano (Minamidake)
Keywords:Volcanic ash, Volcanic volatiles, Sakurajima volcano
Pyroclastic mass and its emission rate from volcanic eruptions are critical for evaluating eruption magnitude and potential hazards at active volcanoes worldwide. Regarding geophysical observations, ground deformation observations can estimate the tephra emission amount from eruptive activity at Sakurajima volcano, especially in underground tunnels where precise measurement is possible (Iguchi, 2016). However, each eruption emits pyroclasts and volcanic gas simultaneously, and the pyroclast-gas density at the vent ρ (kg/m3) is not necessarily a constant. In the case of Minamidake crater activity at Sakurajima volcano from 2017, the volatile emission is more dominant than that of the previous Showa crater (Iguchi et al., 2022). According to theoretical considerations with the two-phase equation of state, the ρ value is predicted to be up to 10 (Koyaguchi, 2008). The pyroclast discharge rate (kg/m3) is expressed as the sum of ρ, the exit velocity v, and vent cross-section area S. The v sequence can be constrained by infrasound observation near the vent. The ρ changes may be represented by seismic and infrasound power ratio, η (Johnson and Aster, 2005). If the flow in the conduit has a high ρ value, we expect an increase of η due to a relatively high drag between the flow and the conduit wall. Since η can change several orders of magnitudes referring cases at other volcanoes, we assume a relation of ρ=A/log(η). If the above idea is valid, we can determine A with observations and the amount of pyroclastic emission. To validate the idea, we examine the eruptive activity of the Minamidake crater at Sakurajima volcano.
We mainly focus on infrasound records at GON, 2 km from the Minamidake summit area. Since infrasound measurement started in April 2024, our target time window is from April 2021 to November 2024. Considering the active vent dimensions with several dozen meters, this study focuses on infrasound signals of 1-7 Hz, which has almost the same wavelength as the vent as the signal source. Vertical short-period seismic records at Arimura observation tunnel are also examined, focusing on the 2-3 Hz band (Iguchi, 2016). Since wind noise can considerably affect infrasound signals, we search volcanic ash emission time windows based on the seismic signals. Following the dipole model (e.g., Delle Donne and Ripepe, 2012) with a vent radius assumption of 10 m, the inferred v value in identified time windows has typically about up to 50 m/s. Several time windows with v of greater than 100 m/s are excluded to avoid signals of shock-wave.
We first examine the relationship between emission volume, converted from v and S, and the monthly tephra discharge amount reported by Kagoshima prefecture. Inferred discharge density (kg/m3) averages 1.5 and ranges from 0.02 to 8.7. The inferred density has a decreasing trend from April 2023, with a minimum peak of 0.02 on July 2023. This lowest density value mainly represents continuous infrasound tremor activity from July 10 to July 13, which suggests transient volatile emission increases. Although the density has an increasing trend after July 2023, we see a transient density decreasing on July 2024, representing infrasound pulses activity (suggesting a successive Strombolian eruptions activity) on July 14. In the target period, such infrasound pulse activities occurred on September 23, 2022, and October 11, 2023. Next, we examine the inferred monthly density and the total sum of log(η) for all events in the corresponding month. The relation can be mainly explained with A=5-50. Although coefficient A has a broad range of an order of magnitude, the results support our hypothesis. Further examination of the previous Minamidake and Showa crater eruptive episodes will validate the above results.
We mainly focus on infrasound records at GON, 2 km from the Minamidake summit area. Since infrasound measurement started in April 2024, our target time window is from April 2021 to November 2024. Considering the active vent dimensions with several dozen meters, this study focuses on infrasound signals of 1-7 Hz, which has almost the same wavelength as the vent as the signal source. Vertical short-period seismic records at Arimura observation tunnel are also examined, focusing on the 2-3 Hz band (Iguchi, 2016). Since wind noise can considerably affect infrasound signals, we search volcanic ash emission time windows based on the seismic signals. Following the dipole model (e.g., Delle Donne and Ripepe, 2012) with a vent radius assumption of 10 m, the inferred v value in identified time windows has typically about up to 50 m/s. Several time windows with v of greater than 100 m/s are excluded to avoid signals of shock-wave.
We first examine the relationship between emission volume, converted from v and S, and the monthly tephra discharge amount reported by Kagoshima prefecture. Inferred discharge density (kg/m3) averages 1.5 and ranges from 0.02 to 8.7. The inferred density has a decreasing trend from April 2023, with a minimum peak of 0.02 on July 2023. This lowest density value mainly represents continuous infrasound tremor activity from July 10 to July 13, which suggests transient volatile emission increases. Although the density has an increasing trend after July 2023, we see a transient density decreasing on July 2024, representing infrasound pulses activity (suggesting a successive Strombolian eruptions activity) on July 14. In the target period, such infrasound pulse activities occurred on September 23, 2022, and October 11, 2023. Next, we examine the inferred monthly density and the total sum of log(η) for all events in the corresponding month. The relation can be mainly explained with A=5-50. Although coefficient A has a broad range of an order of magnitude, the results support our hypothesis. Further examination of the previous Minamidake and Showa crater eruptive episodes will validate the above results.