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

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

Episodic transport of discrete magma batches beneath Aso volcano

Magma ascent, storage, and discharge in the trans-crustal magmatic system are keys to long-term volcanic output and short-term eruption dynamics. How a distinct magma batch transports from a deep reservoir(s) to a pre-eruptive storage pool with eruptible magma remains elusive. Here we show that repetitive very-long-period signals (VLPs) beneath the Aso volcano are preceded by a short-lived (~50–100 s), synchronous deformation event ~3 km apart from the VLP source. Source mechanism of a major volumetric component (~50–440 m3 per event) and a minor low-angle normal-fault component, together with petrological evidence, suggests episodic transport of discrete magma batches from an over-pressured chamber roof to a pre-eruptive storage pool near the brittle-ductile transition regime. Magma ascent velocity, decompression rate, and cumulative magma output deduced from recurrent deformation events before recent 2014 and 2016 eruptions reconcile retrospective observations of the eruption style, tephra fallouts, and plume heights, promising real-time evaluation of upcoming eruptions.

Episodic transport of discrete magma batches beneath Aso volcano

Magma ascent, storage, and discharge in the trans-crustal magma plumbing system are key to long-term volcanic output and short-term eruption dynamics. Petrological analytics, geodetic deformations and mechanical modeling have shaped the current understanding of magma transport. However, due to the lack of observations, how a distinct magma batch transports from a crystal-rich mush region to a crystal-poor pool with eruptible magma remains enigmatic. Through stacking of tilt and seismic waveform data, we find that episodic long-period tremors (LPTs) located near sea level beneath the Aso volcano are accompanied by a synchronous deformation event, which initiates ~50 seconds before individual LPT event and concludes seconds after. The episodic deformation source corresponds to either an inflation or a deflation event located ~3 km below sea level, with a major volumetric component (50-440 cubic meters per event) and a minor high-angle normal-fault component. We suggest that these deformation events likely represent short-lived, episodic upward transport of discrete magma batches accompanied by high-angle shear failure near the roof of the inferred magma chamber at relatively high temperature, whereas their recurrences, potentially composition dependent, are regulated by the brittle-to-ductile transition rheology under low differential stress and high strain rate due to the surge of magma from below, regulating long-term volcanic output rate. The magma ascent velocity, decompression rates, and cumulative magma output deduced from the episodic deformation events before recent eruptions in Aso volcano are compatible with retrospective observations of the eruption style, tephra fallouts, and plume heights, promising real-time evaluation of upcoming eruptions.

Identification of slip surfaces in shallow landslides at Aso volcano, Japan based on the fine fraction-plasticity index relationship

Shallow landslides have frequently occurred in the Aso volcano, Kyushu island, Japan. Yet, observations of the effects of the physical properties of the soil on the landslide stratigraphy have not been explained. In this study, we conducted field observations—at two landslide sites in the Takadake mountain (Aso volcano) area—to identify the slip surfaces. We found that slip surfaces (at both sites) were located in the lower part of the N3-4 Kuroboku soil layer. This was determined by characteristics of the physical properties of the soil, including particle size distribution and plasticity index. Furthermore, we identified the relationship between plasticity index and the fine fraction of the soil to help explain the identification of slip surfaces. Results showed that Kuroboku and Scoria layers have different characteristics according to the plasticity chart (liquid limit-plasticity index relationship) as well as plasticity index-fine fraction relationship.