Aso Volcano Research Articles

Buccal Swab Samples from Japanese Brown Cattle Fed with Limonite Reveal Altered Rumen Microbiome.

The areas of the Mount Aso grasslands in Kumamoto, Japan, are the primary location for the breeding of the Kumamoto strain of Japanese Brown cattle (JBRK). Although Aso limonite, deposited by volcanic ash and magma, has been commonly fed to pregnant JBRK in this area, the mechanisms of its salutary effects on pregnant JBRK have not yet been elucidated. Approximately 100 days before the expected day of calf delivery, seven JBRK (four supplemented with limonite and three controls without limonite) were assigned to this study, from which a buccal swab was collected at the highest rumination every 30 days for 90 days. DNA extracted from these swabs was then analyzed using a 16S rRNA gene amplicon sequence analysis. Statistically significant differences between the two groups were discovered through beta-diversity analysis, though results from alpha-diversity analysis were inconclusive. The microbiota identified were classified into six clusters, and three of the main clusters were core-rumen bacteria, primarily cellulose digestion in cluster 1, oral bacteria in cluster 2, and non-core-rumen bacteria in cluster 3. In the limonite group, core-rumen bacteria decreased while non-core-rumen bacteria increased, suggesting that limonite feeding alters rumen microbiota, particularly activation of non-core-rumen microbiota.

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

The thermal activity of a magmatic–hydrothermal system commonly changes at various stages of volcanic activity. Few studies have provided an entire picture of the thermal activity of such a system over an eruptive cycle, which is essential for understanding the subsurface heat transport process that culminates in an eruption. This study quantitatively evaluated a sequence of thermal activity associated with two phreatic eruptions in 2021 at Aso volcano. We estimated plume-laden heat discharge rates and corresponding H2O flux during 2020–2022 by using two simple methods. We then validated the estimated H2O flux by comparison with volcanic gas monitoring results. Our results showed that the heat discharge rate varied substantially throughout the eruptive cycle. During the pre-eruptive quiescent period (June 2020–May 2021), anomalously large heat discharge (300–800 MW) were observed that were likely due to enhanced magma convection degassing. During the run-up period (June–October 2021), there was no evident change in heat discharge (300–500 MW), but this was accompanied by simultaneous pressurization and heating of an underlying hydrothermal system. These signals imply progress of partial sealing of the hydrothermal system. In the co-eruptive period, the subsequent heat supply from a magmatic region resulted in additional pressurization, which led to the first eruption (October 14, 2021). The heat discharge rates peaked (2000–4000 MW) the day before the second eruption (October 19, 2021), which was accompanied by sustained pressurization of the magma chamber that eventually resulted in a more explosive eruption. In the post-eruptive period, enhanced heat discharge (~ 1000 MW) continued for four months, and finally returned to the background level of the quiescent period (< 300 MW) in early March 2022. Despite using simple models, we quantitatively tracked transient thermal activity and revealed the underlying heat transport processes throughout the Aso 2021 eruptive activity.Graphical abstract

Vulnerability of Aso Volcano’s Disaster Mitigation System, as Revealed by the Phreatic Eruption of October 20, 2021

At Aso volcano, phreatic and phreatomagmatic eruptions have repeatedly caused volcanic disasters with fatalities near the crater. While it is important to conduct research on eruption forecasting for reducing the risk to tourists and climbers of this volcano, it is also important to improve disaster prevention literacy regarding the volcanic eruptions. To improve disaster prevention literacy, it is important to elucidate the mechanisms of volcanic disasters based on case studies. In this study, the author chronologically summarizes the incident of the phreatic eruption that occurred in 2021 at Aso volcano. Although the Japan Meteorological Agency issued volcanic activity information sequentially as the volcanic activity increased, this information did not lead to prompt and effective closure of mountain trails. There was also a problem with the collection of volcanic information by climbers. It became clear that information on volcanic activity was not used effectively by either those regulating the mountain trails or those using them. Efforts should be made to resolve these issues in the future.

Dependence of resistivity of natural rock samples on humidity

Rock resistivity measurements under non-wet conditions are essential for interpreting subsurface shallow resistivity structure. This study investigated the resistivity of rock samples collected from several locations around the Nakadake craters of Aso Volcano, Japan. The resistivity of rock samples exhibited a logarithmically linear relationship with changes in absolute humidity, corresponding to possible seasonal variations and correlating with the porosity of rocks. As the samples maintained an identical cylindrical shape and size, the observed variations in humidity were unrelated to the external surface area but were caused by the internal pore structure of rocks. Furthermore, the external surface area of homogeneous granite was artificially varied using slits and its measured resistivity did not vary in the presence of slits. The measured distribution of electrical potential was well explained by a homogeneous model, and less so by a two-layer model, indicating that the region of resistivity change was not restricted to the surface of the sample, but rather covered the entire sample. Further, changes in resistivity in response to absolute humidity were caused by the adsorption of water molecules onto minerals at internal pore surfaces, as evidenced by the measured temporal scale of resistivity change based on a simple concentration–diffusion process of gaseous water molecules caused by humidity changes in the external environment. These findings imply significant variations in the resistivity of shallow subsurfaces owing to humidity changes; thus, the consideration of humidity is important for the interpretation of subsurface shallow resistivity structures.

Hydrothermal signature on episodic deflation/inflation ground tilt at Aso Volcano

Ground deformation in volcanic areas induced by geothermal fluid circulation can reveal useful information about the dynamical processes occurring in the subsurface hydrothermal system. In the present work, we investigate tiltmeter time-series recorded at Aso Volcano during 2011–2016, a time interval during which different phases of volcanic activity occurred. We performed polarization analysis of the data and identified peculiar long-lasting (hours) transients, defined as Very-Long-period Tilt Pulses. The transients were further characterized in terms of waveform cross-correlation, particle tilt pattern, energy, and time distributions. The analyses indicate that such signals, which appear like deflation–inflation (DI) events, are associated with a Poissonian process whose underlying dynamics evolves over time always driven by a Poissonian mechanism. The obtained results have been interpreted in light of the available geophysical, geochemical and volcanological information. In this framework, the Very-Long-period Tilt Pulses may be ascribed to the depressurization/pressurization of the shallow hydrothermal system according to a fault-valve mechanism, which was active with different efficiency throughout eruptive and inter-eruptive phases.Graphical

Aso volcano, Japan: assessing the 100-year probability of a new caldera-forming eruption based on expert judgements with Bayes Net and Importance Sampling uncertainty analysis

The Aso-4 explosive eruption on Kyushu, Japan, 89,500 years ago was one of the biggest eruptions in the last one hundred millennia, with a magnitude of approximately M8. Modern society requires the likelihood of natural events with potentially disastrous consequences to be evaluated, even if probabilities of occurrence are diminishingly small. For some situations, it is not satisfactory to assert an event scenario probability is “negligible” or can be “ignored”. Judicial hearings or litigation may require risk levels to be quantified, in which case, statements of scientific confidence could be decisive. Internationally, e.g., for nuclear site safety evaluations, event likelihoods on order of 10–7/year are often considered for quantitative assessment. At such hazard levels, this might include evaluating the proposition that a particular volcano can deliver a future super-eruption, a supposition that could be attached to Aso volcano. But, simplistically taking the average recurrence interval between past caldera-forming eruptions at a given volcano is an unreliable guide to the likelihood of a future repeat: each past event represented a unique set of tectonic and magmatic conditions within a continually evolving volcanic system. Such processes are not temporally stationary nor statistically uniform. To evaluate the probability of a new M8 event at Aso, within the next 100 years, we performed a comprehensive stochastic probability uncertainty analysis using a model implemented with advanced computational Bayes Net (BN) software. Our eruption process model is informed by multiple strands of evidence from volcanology, petrology, geochemistry and geophysics, together with estimates of epistemic (knowledge) uncertainty, adduced from reviews of published data, modelling and from expert judgement elicitation. Several lines of evidence characterise the likely structure, magmatic composition and eruptive state of the present-day Aso volcano, which has had numerous smaller eruptions since Aso-4. To calculate the probability of another M8 eruption of Aso, we implemented probabilistic ‘Importance Sampling’ in our model. With this approach, we find the chance of an Aso-4 scale eruption (characterised by mean volume 500 km3 DRE and approximate 90% credible interval [210 ‥ 1200] km3 DRE) is less than 1–in–1 billion in the next 100 years (i.e., < 10–9 probability). Based on current volcanological understanding and evidence, we believe this probability estimate is robust to within an order of magnitude.

Dynamics between earthquakes, volcanic eruptions, and geothermal energy exploitation in Japan

Intruding magma brings high temperatures close to the surface, thus offering possibilities for harnessing large amounts of heat for geothermal exploitation. Mount Aso in southern Japan showed frequent volcanic activity during 2016, accompanied by significant earthquake activities with tens of thousands of aftershocks (Kumamoto sequence). Here we investigate the influence of earthquake/volcanic activity on the future productivity of nearby geothermal power plants to determine whether the activity is detrimental or beneficial to energy exploitation. Model results show an increase in {text {CO}}_2 pressure and temperature with a spatio-temporal correlation between modeled earthquake locations and aftershock decay rates along the entire sequence, showing that seismic activity opened pre-existing vertical cracks providing pathways for the ascending magma. Interestingly, the minor but still significant eruption of Mount Aso in October 2021 may have enhanced future geothermal power generation, indicating a vigorous and active system, possibly increasing the future geothermal power production.

Temporal variation in the depth of the magma surface at Aso volcano in 2014–2015

Monitoring the depth of the magma surface at open-vent volcanoes can be a practical tool to infer temporal variations in the magma supply during an eruption. We focus on the magmatic eruption of Aso volcano in 2014–2015 to estimate the temporal change in the depth of the magma surface, and show that this needs to be coupled with an understanding of the shallow conduit geometry if it is to be done in a representative manner. The eruption lasted 5 months from November 2014 and ending with a crater floor collapse in May 2015. During the eruption, we recorded seismo-acoustic waveforms related to frequent Strombolian explosions. The infrasound signals show several distinct peak frequencies derived from acoustic resonance inside the vent. We estimate the depth of the magma surface using the time delay of seismo-acoustic signals and the peak frequency of infrasound signals. In addition, the temporal variation in the shape of the conduit is constrained by the overtone frequency of the acoustic resonance. From the beginning of the eruption to early-January 2015, the magma surface was located at a depth of sim 200 m, and the conduit was a cylindrical pipe. Later, between January and February, the magma surface rose to sim 120 m, and the shape of the conduit changed to a conical frustum flaring inside. This finding indicates that the magma was injected into the shallow conduit and that it heated and weakened the conduit wall near the magma surface. Before the cessation of the magmatic eruption, the magma surface dropped by approximately 70 m. This magma drainage and, primarily, the instability of the conduit shape caused the crater floor to collapse. We show the possibility of tracking and assessing the depth of the magma surface and the shallow conduit geometry even with limited seismo-acoustic observations.

Groundwaters and deep-seated fluid circulation around Aso Volcano, Southwest Japan, revealed by multivariate statistical analysis of the geochemical data

To understand the fluid circulation under volcanoes and their surrounding area, geochemical compositions of 590 groundwater and spring/river water samples from the central Kyusyu in SW Japan, including hot and mineral spring waters around Aso and Kuju Volcanoes, have been analyzed for 12 major solutes as well as oxygen‑hydrogen and helium isotopic ratios. To extract independent processes and sources involved in formation of the groundwaters, whitened data-based k-means cluster analysis (KCA) has been applied to the data set of 12 major solutes. Eight clusters have been identified by KCA, which were mapped geographically and were compared with the isotopic compositions, temperature, and pH that were not included in KCA. By the geographical mapping, a concentric zonal structure of the clusters emerges in the Aso caldera, which resulted broadly from two fluid cycling systems from caldera rim and central cones. In the entire study area including Aso and Kuju Volcanoes, the mapping also shows geographical provenance, and the distribution of each cluster shows characteristics related to geological features, such as upwelling of deep-seated fluids along the prominent tectonic lines. On the other hand, the Piper diagram-based classification is significantly different from the KCA results and exhibits neither a concentric zonal structure within the Aso caldera nor a regional geographical provenance. These results suggest that multivariate statistical analysis such as KCA is potentially useful for investigating the origins of groundwaters and shows the advantages over classical Piper diagram-based evaluations.

Millennial-scale coseismic landslide history inferred from topographic and stratigraphic features of a post-caldera cone of Aso Volcano in southwestern Japan

The 2016 Mw 7.0 Kumamoto earthquake triggered numerous landslides of fallout tephra deposits around Aso Volcano in southwestern Japan. Although rainfall-induced landslides have been repeatedly observed on the tephra-mantled slopes of this region, the spatial patterns and frequency of coseismic landslides remain unclear because of the scarce landslide evidence associated with palaeoseismicity. To examine the distribution of landslides induced by the 2016 Kumamoto earthquake and possibly by prior large earthquakes, we conducted geomorphological analyses using airborne light detection and ranging (LiDAR) data, together with geological investigations of tephra deposits in the Takanoobane lava dome (TLD), a post-caldera cone of Aso Volcano. Repeated LiDAR surveys performed before and after the earthquake allowed us to quantify the volume and geometry of five coseismic landslides (K1–K5), as well as three component blocks of the K1 landslide (K1a–c). The surveys also allowed identification of 12 older landslides (O1–O12) fully covered by vegetation. Source areas of recent coseismic landslides ranged from 1050 to 14,920 m2; those of older landslides were estimated to be 1110–15,730 m2. The H/L ratios of the K1 landslide and its component blocks K1a–c ranged from 0.139 to 0.165, despite their volumes ranging from 104 to 105 m3. The K1 landslide occurred on the southwest-facing slope, where older landslides were not recognised before the earthquake. Its slip surface was formed at the base of the 30 ka pumice layer. The largest one of the older landslides (O1) is located on the south-facing slope, adjacent to the K1 landslide. Stratigraphic investigations revealed that most of the tephra sequence, including the 30 ka pumice layer, has already been eroded at the centre of the O1 landslide. Radiocarbon dating further constrained the landslide age to ca. 7.0 ka. Because its topographic and stratigraphic features are similar to the features of the K1 landslide, the O1 landslide may have been triggered by a palaeoearthquake approximately 7000 years ago, which was previously unknown around the Aso caldera region.

Impact on the Electric Infrastructure Due to Volcanic Ash from a Hydrovolcanic Eruption of Aso Volcano in 2016

A hydrovolcanic eruption in the early hours of October 8, 2016 triggered a power outage of about 29,000 houses (∼50,000 habitants) in the Aso caldera area, Kyushu, Japan. Although it was recovered naturally by the early morning of that day, the power outage occurred again in the evening following rainfall that began in the afternoon. The power supply in the caldera area had been temporarily maintained despite the heavy landslide damage due to strong earthquakes in April 2016, without the supplemental transmission lines for the normal standard. The power outage is considered to have been due to the insulator flashover in the power substation, Ichinomiya, Aso, around which the mass of volcanic ash was about 3,000 g/m2 or less (≤3 mm thick). This outage resulted from high electric conductivity of the volcanic ash that was wetted with strongly acidic water dammed in the crater before the eruption. This may be one example of the power outage incident due to thin ash fall.

Tourists’ behavior for volcanic disaster risk reduction: A case study of Mount Aso in Japan

Despite preceding research on the general public's knowledge and perceptions of natural hazards, there remains a lack of information specifically on volcanoes in Japan. This study seeks to determine whether people who had visited Mount Aso—one of the active volcanoes in Japan—had taken any self-protective measures. We asked people who visited the area around the crater of Mount Aso about their thoughts and actions related to volcanic disaster risk reduction. Internet-based questionnaires were disseminated nationally via the Rakuten Research, Inc. platform. The total sample size was 400. The logistic regression analysis shows that on-site information provided near the craters—such as knowledge of past damage, bulletin boards, and broadcasts—had a relatively greater impact on safety-seeking actions than prior checking of websites and the Volcanic Alert Level by tourists. Based on our findings, future evidence-based policies regarding volcanic disaster risk reduction should emphasize on-site efforts together with wide dissemination of publicly available information, including the Volcanic Alert Level.

Validation of Repetitive Volcanoseismic Signals in Aso Volcano, Japan With Distant Stations: Implications of Source Characterization and Remote Sensing in Uninstrumented Volcanoes

AbstractRepetitive volcanoseismic signals, including very‐long‐period signals (VLP) and long‐period signals (LP), provide a unique probe of fluid transport processes inside magmatic‐plumbing system. While syneruptive signals are often detected and analyzed with regional or/and global seismic networks to retrieve eruption location and mechanism, repetitive noneruptive volcanoseismic signals are generally small, and they are typically detected with in situ stations near the volcanic edifices. Here, we show that repetitive VLP and synchronous deformation events in Aso volcano, Japan, can be detected in the high (15–30 s) and low (50–100 s) VLP bands, respectively, at seismic stations located ∼30–1,000 km away from their sources. Changes in the polarities, phases, and amplitudes of VLP and synchronous deformation events observed at the in situ stations can be verified by the seismic waves in the two VLP bands, respectively, at distant stations up to 150 km. Forward modeling of the amplitude decay in the two VLP bands against epicentral distance corroborates the source locations previously determined by the in situ data, whereas the joint data analysis of in situ and distant stations at high VLP band suggests the presence of single‐force component (i.e., force/moment ratio of 10−4 m−1) in the source of VLPs. We advocate that not only can systematic data mining against established global and regional seismic networks potentially expand the detection capability of repetitive volcanoseismic signals backward in time when in situ observations were unavailable, but it could also substantially improve the detection and monitoring capacity in otherwise uninstrumented volcanoes, complementary to remote sensing of ground deformation.

Sampling Volcanic Plume Using a Drone-Borne SelPS for Remotely Determined Stable Isotopic Compositions of Fumarolic Carbon Dioxide

Both chemical and isotopic compositions of concentrated volcanic plumes are highly useful in evaluating the present status of active volcanoes. Monitoring their temporal changes is useful for forecasting volcanic eruptions as well. Recently, we developed a drone-borne automatic volcanic plume sampler, called SelPS, wherein an output signal from a sulfur dioxide (SO2) sensor triggered a pump to collect plume samples when the SO2 concentration exceeded a predefined threshold. In this study, we added a radio transmission function to the sampler, which enabled our operator to monitor real-time SO2 concentration during flights and thus obtain more concentrated volcanic plume samples through precise adjustment of the hovering position. We attached the improved SelPS to a drone at Nakadake crater, Aso volcano (Japan), and successfully obtained volcanic plume samples ejected from the crater more concentrated than those obtained by using previous version of SelPS in 2019. Additionally, we found a significant linear correlation between the reciprocal of the concentration and isotopic ratios for the 2H/1H ratios of H2, 18O/16O ratios of CO2, and 13C/12C ratios of CO2 within the plume samples. Based on the isotopic ratios of fumarolic H2 (δ2H = −239 ± 6‰) and fumarolic CO2 (δ13C = −3.58 ± 0.85‰ and δ18O = +22.01 ± 0.68‰) determined from the linear correlations, we estimated the apparent equilibrium temperatures (AETs) with magmatic H2O simultaneously and precisely for the first time in erupting volcanoes, assuming hydrogen isotope exchange equilibrium between H2 and H2O (AETD = 629 ± 32°C) and oxygen isotope exchange equilibrium between CO2 and H2O (AET18O = 266 ± 65°C). We found that the AET18O was significantly lower than the AETD in the crater. While the temperature of the magmatic gases was originally 600°C or more, most of the gases cooled just beneath the crater to temperatures around the boiling point of water. The improved SelPS enable us to determine both AETD and AET18O in eruptive volcanoes, wherein fumaroles are inaccessible. Simultaneous and precise determination of both the AET18O and AETD can provide novel information on each volcano, such as the physicochemical conditions of magma degassing and the development of fluid circulation systems beneath each volcano.

The October 14, 2021 Eruption of Nakadake First Crater, Aso Volcano (Japan): Insights from its Eruptive Deposits

The October 14, 2021 Eruption of Nakadake First Crater, Aso Volcano (Japan): Insights from its Eruptive Deposits

Tephrostratigraphy and Eruptive History of Aso-4/3 Tephra Group, Aso Volcano: Preparatory Process for Aso-4 Ignimbrite Eruption

Tephrostratigraphy and Eruptive History of Aso-4/3 Tephra Group, Aso Volcano: Preparatory Process for Aso-4 Ignimbrite Eruption

Rapid vegetation recovery at landslide scars detected by multitemporal high-resolution satellite imagery at Aso volcano, Japan

Vegetation cover is an important factor in hillslope stability and landslide susceptibility. The objective of this study is to explore vegetation recovery in rainfall-induced and coseismic landslide scars at the Aso volcano in Japan. We focus on the temporal changes and effect of topography on vegetation recovery using multitemporal satellite images of RapidEye from 2010 to 2019 and PlanetScope from 2016 to 2020 and finer spatial resolution images of unmanned aerial vehicles (UAVs). The study area is characterized by semi-natural grassland and frequent landslide events triggered by heavy rainfall in 2012 and the 2016 Kumamoto earthquake. This study validates grass vegetation recovery using the normalized difference vegetation index (NDVI) and NDVI ratio, which represents the relative difference in NDVI between the landslide scars and grass areas. The results show a distinct decrease in the NDVI ratio at landslide scars after the heavy rainfall and Kumamoto earthquake. The NDVI ratio rapidly increases in landslide scars after the events. The results indicate that NDVI at both landslide scars will reach the same level as the pre-landslide grass vegetation within 12 years. The slope aspect direction determines the topographic effects of grass vegetation recovery. The north- to west-facing landslide scars seemed to experience rapid grass vegetation recovery.

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

Victor(iou)’s myologists: snapshots of a legacy

Victor(iou)’s myologists: snapshots of a legacy

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.