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[SVC32-08] Relation between volcanic ash emissions and volcanic tremor amplitude from the first crater of Nakadake, Aso Volcano.
Keywords:volcanic microtremor, ash-fall deposit, Aso Volcano, Nakadake first crater
Volcanic microtremors are thought to be excited by the movement and oscillation of magma and hydrothermal fluids. Sassa (1935) showed that various types of volcanic microtremors have occurred at Aso volcano since the start of seismic observations. In recent years, volcanic microtremors have become one of the indicators of volcanic activity at the volcano, as the increase or decrease in amplitude of volcanic microtremors is used as a criterion for determining the eruption alert level.
Volcanic microtremors may also occur in association with eruptions. Iguchi (2013) and Iguchi (2016) found a correlation between the amount of volcanic ash continuously ejected by non-explosive eruptive activity at Sakurajima and the amplitude of volcanic microtremors, and proposed a method for monitoring volcanic ash ejection using eruptive microtremors. Continuous magmatic eruptions occurred at the Nakadake first crater, Aso volcano from November 2014 to May 2015 and from July 2019 to June 2020. This study summarizes the characteristics of volcanic microtremors observed during these magmatic eruptions and examines whether the monitoring method of ash emissions using eruptive microtremors can be applied to Aso volcano.
Iguchi (2013) investigated the correlation between the monthly integrated power spectra of microtremors observed at Sakurajima volcano and the amount of volcanic ash emissions in various frequency bands and showed that the correlation was best in the 2-3 Hz frequency band. Iguchi (2016) then approximated the monthly integrated values of 2-3 Hz amplitude spectra and volcanic ash emissions with a linear equation. Therefore, this study also examines the relationship between the integrated values of 2-3 Hz microtremor amplitudes and volcanic ash emissions. Miyabuchi and Hara (2019) and Miyabuchi et al. (2025) determined volcanic ash discharge rates at Aso volcano as daily average values based on high frequency volcanic ash sampling. This study investigates the relationship between the average volcanic ash release and the integrated microtremor amplitude (2-3 Hz) for the same period as the ash sampling.
A positive correlation was found between the integrated amplitudes and volcanic ash emission. However, the whole period could not be approximated by a single linear equation, as in Iguchi (2016). There was a significant difference between the calculated and actual emitted amounts. When the period was divided according to the slope of the accumulated ash fall and the linear equation approximated for each period was changed, the calculated emitted amount was close to the actual emitted amount. The same combination of three linear equations could approximate volcanic ash emissions from the 2014-2015 and 2019-2020 eruptions.
It is not clear at present how these changes in the linear equation correspond to the phenomena below the crater. Because the 2014 active period division corresponds to the period when the magma head's position and the shape of the conduit change, as indicated by Ishii et al. (2023), these phenomena could include mechanisms of microtremor excitation.
In contrast, the timing for changing the parameters of the linear equation is currently determined by the temporal changes in the integrated emitted volume. Using the microtremor amplitude to monitor volcanic ash emission rate is still difficult. A future work is to investigate whether it is possible to determine when to change the linear equation using other information, such as the amplitude of infrasound.
Volcanic microtremors may also occur in association with eruptions. Iguchi (2013) and Iguchi (2016) found a correlation between the amount of volcanic ash continuously ejected by non-explosive eruptive activity at Sakurajima and the amplitude of volcanic microtremors, and proposed a method for monitoring volcanic ash ejection using eruptive microtremors. Continuous magmatic eruptions occurred at the Nakadake first crater, Aso volcano from November 2014 to May 2015 and from July 2019 to June 2020. This study summarizes the characteristics of volcanic microtremors observed during these magmatic eruptions and examines whether the monitoring method of ash emissions using eruptive microtremors can be applied to Aso volcano.
Iguchi (2013) investigated the correlation between the monthly integrated power spectra of microtremors observed at Sakurajima volcano and the amount of volcanic ash emissions in various frequency bands and showed that the correlation was best in the 2-3 Hz frequency band. Iguchi (2016) then approximated the monthly integrated values of 2-3 Hz amplitude spectra and volcanic ash emissions with a linear equation. Therefore, this study also examines the relationship between the integrated values of 2-3 Hz microtremor amplitudes and volcanic ash emissions. Miyabuchi and Hara (2019) and Miyabuchi et al. (2025) determined volcanic ash discharge rates at Aso volcano as daily average values based on high frequency volcanic ash sampling. This study investigates the relationship between the average volcanic ash release and the integrated microtremor amplitude (2-3 Hz) for the same period as the ash sampling.
A positive correlation was found between the integrated amplitudes and volcanic ash emission. However, the whole period could not be approximated by a single linear equation, as in Iguchi (2016). There was a significant difference between the calculated and actual emitted amounts. When the period was divided according to the slope of the accumulated ash fall and the linear equation approximated for each period was changed, the calculated emitted amount was close to the actual emitted amount. The same combination of three linear equations could approximate volcanic ash emissions from the 2014-2015 and 2019-2020 eruptions.
It is not clear at present how these changes in the linear equation correspond to the phenomena below the crater. Because the 2014 active period division corresponds to the period when the magma head's position and the shape of the conduit change, as indicated by Ishii et al. (2023), these phenomena could include mechanisms of microtremor excitation.
In contrast, the timing for changing the parameters of the linear equation is currently determined by the temporal changes in the integrated emitted volume. Using the microtremor amplitude to monitor volcanic ash emission rate is still difficult. A future work is to investigate whether it is possible to determine when to change the linear equation using other information, such as the amplitude of infrasound.