Combined approach to estimate the depth of the magma surface in a shallow conduit at Aso volcano, Japan

Abstract

Open-vent volcanoes provide opportunities to perform various methods of observation that can be used to study shallow plumbing systems. The depth of the magma–air interface in the shallow portion of the conduit can be used as an indicator of the volcanic activity of open-vent volcanoes. Although there are many methods used to estimate the depth, most of them cannot constrain the depth to a narrow range due to other unknown parameters. To constrain the depth more accurately, we combine two methods commonly used for estimating the depth of the magma–air interface. They consider the acoustic resonant frequency and the time delay of arrivals between the seismic and infrasound signals of explosions. Both methods have the same unknown parameters: the depth of the magma–air interface and the sound velocity inside the vent. Therefore, these unknowns are constrained so that both the observed resonant frequency and time delay can be explained simultaneously. We use seismo-acoustic data of Strombolian explosions recorded in the vicinity of Aso volcano, Japan, in 2015. The estimated depths and the sound velocities are 40–200 m and 300–680 m/s, respectively. The depth range is narrower than that of a previous study using only the time delay of arrivals. However, only a small amount of the observed data can be used for the estimation, as the rest of the data cannot provide realistic depths or sound velocities. In particular, a wide distribution of the observed time delay data cannot be explained by our simple assumptions. By considering a more complicated environment of explosions, such as source positions of explosions distributed across the whole surface of a lava pond in the conduit, most of the observed data can be used for estimation. This suggests that the factor controlling the observed time delay is not as simple as generally expected. Graphic abstract

Infrasonic gliding reflects a rising magma column at Mount Etna (Italy)

“…Hence other studies have used expressions accounting for non-narrow pipes and/or have utilized the analytical solutions for exponential or Bessel horns, to model frequency as a function of crater size 14 , 29 . More recently, numerical models, both 1D and 3D, have been used to account for more complex geometries 15 , 20 , 30 , 31 . The SEC geometry at Etna is not easily characterized by an analytical expression or geometric approximation.…”

Heat transport process associated with the 2021 eruption of Aso volcano revealed by thermal and gas monitoring

“…During quiescent periods, the crater hosts a hot acidic lake called 'Yudamari' , of which water level changes in response to volcanic activity (Ikebe and Watanabe 1990;Terada et al 2008). Recent cycles of magmatic activity began in 2014; Strombolian eruptions and continuous magmatic ash emissions, locally called ' Ash eruption' at Aso volcano (Ono et al 1995), continued from November 2014 to May 2015 (Miyabuchi and Hara 2019;Ishii and Yokoo 2021). The activity then shifted to phreatomagmatic eruptions in September 2015 and October 2016 (Miyabuchi et al 2018).…”

Volcano infrasound: progress and future directions

“…Estimates of eruption mass and mass flow rate using infrasound appear possible ), and will likely become a reality in the next decade, potentially in nearreal-time for large eruptions. Open-vent volcanoes present a compelling opportunity for eruption forecasting using infrasound, and the theoretical groundwork has been laid to track changes in lava lake level (e.g., Johnson et al 2018;Watson et al 2019Watson et al , 2020Ishii & Yokoo 2021) and degassing intensity (e.g., Ripepe et al 2002Ripepe et al , 2010aPetersen & McNutt 2007). Future work will implement these tools in near-real-time.…”