[S-VC28] Active Volcanism

[SVC28-P13] The depth of the air-magma interface and the sound velocity in the shallow conduit of Aso volcano inferred from seismic and infrasound signals

*Kyoka Ishii¹, Akihiko Yokoo¹ (1.Graduate School of Science, Kyoto University)

Keywords:Shallow conduit, Infrasound, Aso volcano

The depth of the air-magma interface in the shallow conduit can be used as an indicator of the volcanic activity of open-vent volcanoes. Although there are many methods for the estimation of such depth, most of them are unable to constrain the depth into the narrow range due to multiple unknown parameters. In order to estimate the depth more accurately, we combine two methods; one is using the time delay (Δt) of arrivals between seismic and infrasound signals of explosions (Ishii et al., 2019), and another is using the resonant infrasound in the conduit observed as the peak frequency (f₀) (e.g., Johnson et al., 2018). The time delay of the signals is controlled by the depth of the air-magma interface (i.e., the depth of the explosive source) and the sound velocity in the conduit, and the peak frequency of infrasound is also controlled by the same two parameters. Therefore, this combined method provides both estimations of the depth and the sound velocity in the conduit simultaneously. The seismic and infrasound data were acquired at the vicinity of Nakadake 1st crater of Aso volcano, Japan in April 2015. A high-frequency seismic arrival is followed by an infrasound compressional phase accompanied by each Strombolian explosion at the vent. The infrasound signals are characterized by clear peaked frequency spectra (f₀, 0.5 Hz), which are derived from acoustic resonance in the gas-filled conduit (Yokoo et al., 2019). In this study, 318 events are used for analysis. Before combining the two methods, some effects have to be considered regarding the relationship between the infrasound peak frequency f₀ and the estimated depth, such as the conduit shape (cylinder or frustum), the sound velocity profile in the conduit, and the topography between a source and a station. Therefore, the estimation of the depth is performed by the following procedures. First, the conduit shape is determined by the infrasound frequency ratio of the fundamental resonant mode and overtone. The frequency ratio is theoretically related to the ratio of radii between open and closed ends of a pipe (Ayers et al., 1985). The observed frequency ratio of the peak frequency and overtone (2 Hz) suggests that the shape of the conduit wall is like a frustum of which the small end is open. Then, 3-D FDTD simulation is conducted to test how the sound velocity profile in the conduit and the topography of the propagation path affect the observed peak frequency of infrasound. High-resolution topographic data and 1-D profiles of sound velocity are assumed in the simulation. There is a transition zone from sound velocity in the conduit to that in the air at shallow depth within the 1-D profiles of sound velocity. The results of the 3-D simulation show the synthetic peak frequency of infrasound at a station is lower than the theoretical value (Ayers et al., 1985), especially in the case that sound velocity in the conduit is faster than that in the air. Based on this result, the calibrated relationship among the three parameters, the peak frequency of infrasound, the depth, and the sound velocity in the conduit, are acquired. Finally, two methods, using time delay of seismo-acoustic signals (Δt) and infrasound peak frequency (f₀), are combined. The depth and the sound velocity in the conduit estimated by the combined method are <100 m and 300-600 m/s, respectively. This estimated depth is constrained in the narrower range than the estimates by using only Δt (Ishii et al., 2019). However, only 10 % of all events are used for the estimation because the observed pair of Δt and f₀ of the other events are not explained by any realistic depths and sound velocities. This problem may be solved by considering the horizontal location of the explosive source.