Monochromatic infrasound waves are scarcely reported volcanic infrasound signals. These waves have the potential to provide constraints on the conduit geometry of a volcano. However, to further investigate the waves scientifically, such as how the conduit shape modulates the waveforms, we still need to examine many more examples. In this paper, we provide the most detailed descriptions of these monochromatic infrasound waves observed at Aso volcano in Japan. At Aso volcano, a 160-day-long magmatic eruption occurred in 2014–2015 after a 20-year quiescent period. This eruption was the first event that we could monitor well using our infrasound network deployed around the crater. Throughout the entire eruption period, when both ash venting and Strombolian explosions occurred, monochromatic infrasound waves were observed nearly every day. Although the peak frequency of the signals (0.4–0.7 Hz) changed over time, the frequency exhibited no reasonable correlation with the eruption style. The source location of the signals estimated by considering topographic effects and atmospheric conditions was highly stable at the active vent. Based on the findings, we speculate that these signals were related to the resonant frequencies of an open space in the conduit: the uppermost part inside the vent. Based on finite-difference time-domain modeling using 3-D topographic data of the crater during the eruption (March 2015), we calculated the propagation of infrasound waves from the conduit. Assuming that the shape of the conduit was a simple pipe, the peak frequency of the observed waveforms was well reproduced by the calculation. The length of the pipe markedly defined the peak frequency. By replicating the observed waveform, we concluded that the gas exhalation with a gas velocity of 18 m/s occurred at 120 m depth in the conduit. However, further analysis from a different perspective, such as an analysis of the time difference between the arrivals of infrasound and seismic waves, is required to more accurately determine the conduit parameters based on observational data.
This paper presents a simple method to distinguish infrasonic signals from wind noise using a cross‐correlation function of signals from a microphone and a collocated seismometer. The method makes use of a particular feature of the cross‐correlation function of vertical ground motion generated by infrasound, and the infrasound itself. Contribution of wind noise to the correlation function is effectively suppressed by separating the microphone and the seismometer by several meters because the correlation length of wind noise is much shorter than wavelengths of infrasound. The method is applied to data from two recent eruptions of Asama and Shinmoe‐dake volcanoes, Japan, and demonstrates that the method effectively detects not only the main eruptions, but also minor activity generating weak infrasound hardly visible in the wave traces. In addition, the correlation function gives more information about volcanic activity than infrasound alone, because it reflects both features of incident infrasonic and seismic waves. Therefore, a graphical presentation of temporal variation in the cross‐correlation function enables one to see qualitative changes of eruptive activity at a glance. This method is particularly useful when available sensors are limited, and will extend the utility of a single microphone and seismometer in monitoring volcanic activity.
Both chemical and isotopic compositions of concentrated volcanic plumes are highly useful in evaluating the present status of active volcanoes. Monitoring their temporal changes is useful for forecasting volcanic eruptions as well. Recently, we developed a drone-borne automatic volcanic plume sampler, called SelPS, wherein an output signal from a sulfur dioxide (SO 2 ) sensor triggered a pump to collect plume samples when the SO 2 concentration exceeded a predefined threshold. In this study, we added a radio transmission function to the sampler, which enabled our operator to monitor real-time SO 2 concentration during flights and thus obtain more concentrated volcanic plume samples through precise adjustment of the hovering position. We attached the improved SelPS to a drone at Nakadake crater, Aso volcano (Japan), and successfully obtained volcanic plume samples ejected from the crater more concentrated than those obtained by using previous version of SelPS in 2019. Additionally, we found a significant linear correlation between the reciprocal of the concentration and isotopic ratios for the 2 H/ 1 H ratios of H 2 , 18 O/ 16 O ratios of CO 2 , and 13 C/ 12 C ratios of CO 2 within the plume samples. Based on the isotopic ratios of fumarolic H 2 ( δ 2 H = −239 ± 6‰) and fumarolic CO 2 ( δ 13 C = −3.58 ± 0.85‰ and δ 18 O = +22.01 ± 0.68‰) determined from the linear correlations, we estimated the apparent equilibrium temperatures (AETs) with magmatic H 2 O simultaneously and precisely for the first time in erupting volcanoes, assuming hydrogen isotope exchange equilibrium between H 2 and H 2 O (AET D = 629 ± 32°C) and oxygen isotope exchange equilibrium between CO 2 and H 2 O (AET 18O = 266 ± 65°C). We found that the AET 18O was significantly lower than the AET D in the crater. While the temperature of the magmatic gases was originally 600°C or more, most of the gases cooled just beneath the crater to temperatures around the boiling point of water. The improved SelPS enable us to determine both AET D and AET 18O in eruptive volcanoes, wherein fumaroles are inaccessible. Simultaneous and precise determination of both the AET 18O and AET D can provide novel information on each volcano, such as the physicochemical conditions of magma degassing and the development of fluid circulation systems beneath each volcano.
