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[en] 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. .
[en] Imaging the shallow velocity structures beneath Aso caldera is necessary to further understand volcanism at the volcano and in the region. The network for monitoring Aso volcano has been progressively renewed and upgraded with denser and more modern instruments. We used approximately 4 years of seismic data recorded by a network of 25 seismic stations to image S-wave velocity (Vs) structures beneath Aso caldera with seismic noise interferometry. We calculated daily cross-correlation functions (CCFs) of broadband and short-period station pairs separately and then stacked CCFs monthly, to obtain the time-domain empirical Green’s functions and corresponding Rayleigh-wave phase-velocity dispersion curves. Finally, we constructed 1–5-s phase-velocity maps interpolated from nodes spaced 0.05° grid. The maps allow investigating crustal Vs structures between the surface and a depth of 6 km, likely related to shallow volcanic reservoirs and pathways. High velocities are found within the first kilometer of the crust beneath post-caldera central cones. Low velocities in the center of the post-caldera central cones extend from the surface to a depth of 1–2.5 km; we infer that the anomalies mark shallow hydrothermal reservoirs likely replenished by precipitation and hydrographic networks. The prevalence of high velocities below 3 km can be considered as consolidated igneous rock. Low-velocity anomalies identified at depths of 5–6 km beneath the post-caldera central cones could be a manifestation of magma accumulation. The low-velocity belts situated at 2.5–5 km depths are likely pathways for the transfer of hydrothermal fluids, volcanic gases, or melting magma to the surface. The northern part of the caldera shows relevant lateral velocity variations, with low velocities and high velocities predominant in the east and west, respectively. Other low-Vs anomalies appear near the surface to the west and northwest of Aso caldera. .
[en] A hot and acid crater lake is located in the Nakadake crater, Aso volcano, Japan. The volume of water in the lake decreases with increasing activity, drying out prior to the magmatic eruptions. Salt-rich materials of various shapes were observed, falling from the volcanic plume during the active periods. In May 2011, salt flakes fell from the gas plume emitted from an intense fumarole when the acid crater lake was almost dry. The chemical composition of these salt flakes was similar to those of the salts formed by the drying of the crater lake waters, suggesting that they originated from the crater lake water. The salt flakes are likely formed by the drying up of the crater lake water droplets sprayed into the plume by the fumarolic gas jet. In late 2014, the crater lake dried completely, followed by the magmatic eruptions with continuous ash eruptions and intermittent Strombolian explosions. Spherical hollow salt shells were observed on several occasions during and shortly after the weak ash eruptions. The chemical composition of the salt shells was similar to the salts formed by the drying of the crater lake water. The hollow structure of the shells suggests that they were formed by the heating of hydrothermal solution droplets suspended by a mixed stream of gas and ash in the plume. The salt shells suggest the existence of a hydrothermal system beneath the crater floor, even during the course of magmatic eruptions. Instability of the magmatic–hydrothermal interface can cause phreatomagmatic explosions, which often occur at the end of the eruptive phase of this volcano. .
[en] Strombolian explosions are one of the most studied eruptive styles and are characterized by intermittent explosions. The mechanism of a Strombolian explosion is modeled as a large gas pocket (slug) migrating through the magma conduit and then bursting at the air–magma interface. These ascending and bursting processes of the slug induce characteristic seismo-acoustic signals during each explosion: very-long-period (VLP) seismic signals, eruption earthquake signals, and infrasound signals. However, at Stromboli volcano, it has been reported that the ascent velocity estimated from the time differences between observed signals is nearly an order of magnitude higher than that expected from laboratory experiments simulating slug ascent. This discrepancy between observation-based and experiment-based velocities has not yet been fully explained and strongly suggests that the conventional model of Strombolian explosions should be partially revised. In this study, we attempted to validate the model of Strombolian explosions by estimating the gas phase velocity in the conduit in the case of Aso volcano. We recorded seismo-acoustic signals accompanying Strombolian events at Aso volcano, Japan, in late April 2015 via our monitoring network, and the ascent velocity of the gas phase was determined from the difference in arrival times between the VLP signals and the infrasound signals. Our estimated velocity exceeded 100 m/s, which is much faster than the experimental value of 7.5 m/s predicted for Aso volcano. To explain this rapid ascent velocity, we propose a revised model describing the migration of the gas phase via a more complicated mechanism, such as annular flow. In this model, we assumed that the gas phase ascends in the conduit at high velocity while making a pathway leading to the magma surface, most likely due to a temporary increase in the gas flux. Our model will help to deepen the understanding of the complicated dynamics in the magma conduit during a Strombolian explosion. .
[en] Continuous measurements of soil CO2 flux are useful for understanding degassing processes and for monitoring volcanic activities. Recent studies at many volcanoes have revealed that soil CO2 flux variations are significantly influenced by environmental parameters as well as volcanic processes. In this study, we conducted continuous monitoring of soil CO2 flux in the flank of Nakadake cone, Aso volcano, Japan, from January 2016 to November 2017. The results of our observations during an active period before and after a large phreatomagmatic eruption on 8 October 2016 and during a calm period from 2017 showed variations in soil CO2 flux due to oscillations in environmental parameters. Excluding these variations from the raw time series by multivariate linear regression analysis, the time series of soil CO2 flux presented some anomalous peaks in both the active and calm periods. Careful comparison of the anomalous peaks with the environmental parameters revealed that most of the anomalous peaks were likely due to an increase in wind speed and/or a decrease in barometric pressure. However, the anomaly after the 8 October 2016 eruption was not completely explicable by the variations in the environmental parameters and coincided with increases in seismic amplitude and plume SO2 flux. This anomaly was possibly attributed to an increase in magmatic CO2 flux. These findings emphasized the importance of careful statistical treatment of the soil CO2 flux data after excluding the influences of the environmental parameters at each measurement site. These statistical treatments will contribute to a better understanding of the degassing processes and monitoring of volcanic activities, including phreatic or phreatomagmatic eruptions. .
[en] We herein estimated the end of the ongoing deflation at Sinabung volcano, Indonesia, which began in 2014 under the assumption that the magma is incompressible and behaves as a Bingham fluid. At first, we estimated the temporal volume change of the deformation sources for deflation periods from the end of 2013 until December 2016. The deflation rate is decreasing gradually, and the deflation is expected to cease in the future. Next, we obtained the absolute pressure in the reservoir as a function of time by modeling the magma as an incompressible fluid inside a spherical reservoir with a cylindrical vent and applying the estimated volume change function to the model. We then estimated the yield stress of the Sinabung magma to be 0.1–3 MPa from a previously derived linear relationship between the silica content and yield stress of lava and the measured silica content of Sinabung ashfall from 2010 to 2014, which was reported to be 58–60%. The absolute pressure in the reservoir is expected to fall below the yield stress between August and September 2018, which means that the magma would stop migrating from the reservoir toward the summit under the assumption that it behaves as a Bingham fluid. As a result, the deflation of Sinabung is estimated to end between August and September 2018. In comparison with the 1991–1995 deflation at Unzendake, the total deflation volume of Sinabung is comparable, whereas the duration of the deflation of Sinabung is approximately 1 year longer. .