Monochromatic infrasound waves observed during the 2014\u20132015 eruption of Aso volcano, Japan

Akihiko Yokoo,Takahiro Ohkura,Kyoka Ishii,Keehoon Kim

doi:10.1186/s40623-019-0993-y

Akihiko Yokoo, Takahiro Ohkura + Show 2 more

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https://doi.org/10.1186/s40623-019-0993-y

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Abstract

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.

Highlights

Erupting volcanoes are principal sources of infrasound waves
At τ = 1.55 s, Constraint on conduit size from observed infrasound waveforms Based on our finite-difference time-domain (FDTD) calculations described in the previous section, we invoke a simple model of the monochromatic pressure perturbation of the infrasound wave records during the 2014–2015 eruption at Aso volcano
Based on the results of the FDTD calculation including the actual topography, in which f0 showed a ∼ 15% decrease from its original value on a simple pipe resonance predicted by Eq (3) (Fig. 9d), the conduit length L could be estimated as ∼ 118 m using the peak frequency

Summary

Introduction

Erupting volcanoes are principal sources of infrasound waves. Previous work has shown that infrasound waves can be used to study and monitor volcanoes (Fee and Matoza 2013; Johnson and Ripepe 2011). We report the infrasound signals observed during the 2014–2015 eruption of Aso volcano in Japan (Fig. 1). 01-Nov 01-Dec 01-Jan 01-Feb 01-Mar 01-Apr 01-May 01-Jun date

Results

Conclusion