Phreatic Eruptions Research Articles

Simulation of lahar flow potensial hazards in seulawah agam volcano eruption-aceh

Abstract Seulawah Agam volcano is an active stratovolcanic volcano located on Sumatra Island. During the decade following the phreatic eruption in 1839 up to now, there have only been a few small-scale increases in activity. The hazards associated with volcanic eruptions can be divided into primary hazards and secondary hazards. One type of significant secondary hazard is lahar flow. The consequences of this volcanic eruption must be overcome because the impact is the same as the main hazard, especially if efforts to overcome it are not taken seriously. Therefore, simulations are needed to understand the lahar flow patterns that can occur due to the eruption of the Seulawah Agam volcano. This lahar flow simulation is based on Digital Elevation Model (DEM) data, namely DEMNAS, and uses the LAHARZ application, which is based on Geographic Information Systems (GIS). The LAHARZ software is designed to calculate potential hazard zones exposed to lahar flows based on volume parameters. Lahar flow simulations were made in hydrological channels with several lahar volume scenarios, such as 1 x 105 m3, 1 x 106 m3, and 5 x 106 m3. The lahar volume estimate used is based on information from historical data on volcanic eruptions whose characteristics are almost the same as the Seulawah Agam volcano. This volume scenario shows differences in the extent of lahar flows and the size of the affected area. The results of this lahar flow simulation can be one of the first steps in efforts to mitigate the hazards of volcanic eruptions in the future.

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

Magma intrusion process during pre-magmatic period (2010−2013) of Sinabung volcano as revealed by seismicity of volcano-tectonic and hybrid earthquakes

Sinabung volcano in Indonesia reactivated, with a series of phreatic eruptions, in August and September 2010. These eruptions were followed by various types of magmatic eruption at the summit, including lava dome growth, pyroclastic density currents, lava flow, and vulcanian eruptions, starting in December 2013. Prior to the magmatic eruptions, Sinabung exhibited two sequences of phreatic eruption and an increase in the seismicity of volcanic earthquakes. This study clarifies the progress of magma intrusion during the pre-magmatic period based on the hypocenter distribution, waveform similarity, seismic moment, rupture length, and stress drop for volcano-tectonic (VT) and hybrid earthquakes. It was found that the hypocenters of VT earthquakes are distributed north and northwest of the summit at a depth range of 1–10 km below sea level. Deep seismicity (4–10 km) alternated with shallow seismicity (2–4 km). The last shallow seismicity, from July to mid-December 2013, was different from previous seismicity in terms of higher intensity, as determined from the seismic moment, a temporary increase in the rupture length, and the migration of hypocenters towards the summit. The seismicity of VT earthquakes was altered by a swarm of hybrid earthquakes in mid-December. The hybrid earthquakes were smaller, in terms of seismic moment (mostly <3 × 1010 Nm), than the VT earthquakes. Their hypocenters were concentrated in the shallowest depth range (−0.5 to 1.5 km below sea level) directly below the summit, their source process was repeatable (they could be grouped into six earthquake families), their dominant peak was in a low-frequency range (2.5–4.5 Hz), and they had a relatively low stress drop (<0.15 MPa). This suggests that the swarm of hybrid earthquakes was induced by a frequent repeated fracture of fluid-filled cracks due to the intrusion of magma up to directly below the summit. The transition of the earthquake family and the change in its source parameters, namely an increase in the stress drop prior to the appearance of a lava dome and then a slight decrease, may reflect a gradual change in internal pressure in the hypocentral zone through the magma intrusion process.

Ground deformations associated with an overpressurized hydrothermal systems at Azuma volcano (Japan) revealed by InSAR data

Inflations at active volcanoes are indicators of overpressure in the subsurface, which is known to be a phenomenon that precedes eruptions. Volcanic overpressure is induced by the injection of magmatic fluids, accumulated magma, or heat supply from greater depths. Azuma volcano (Japan) has experienced several episodes of volcanic unrest with increases in seismicity at the depth of the hydrothermal system, implying a potential increase in phreatic eruptions. The time series of interferometric synthetic aperture radar data, associated with the unrest episodes occurring in 2014–2015 and 2018–2019, revealed spatiotemporal variations of inflation episodes, centered around Oana crater, the most active fumarole of Azuma volcano. The modeled best-fit geometry of the elongated pressure source for the local deformation has the same dip as the overlying topographic slope direction and angle around Oana crater, suggesting the existence of topography-correlated layered structures within the hydrothermal system. In contrast, the broader deformation associated with the 2014–2015 unrest was explained by the overpressure of a horizontal flat source at 360–1500 m below sea level, showing the similar depth of the top as the conductive low-resistivity or low-viscosity body suggested by previous studies. The unrest episodes were thus interpreted as resulting mainly from the supply of magmatic fluids, or the transfer of heat from greater depths. Our study helps in understanding the shallow structure of this volcanic system and contributes to evaluating the potential for forthcoming eruptions in Azuma volcano.Graphical

Accurate hindcasting of explosive eruptions at Whakaari, New Zealand

Phreatic eruptions are small, sudden events, commonly with few precursory signals. They are driven by interactions between magmatic and hydrothermal processes at shallow levels beneath the surface. Here we show that a sequence of banded tremor events, which occurred several weeks before the 9 December 2019 eruption of Whakaari (White Island), New Zealand, can be used to hindcast this eruption. The banded tremor sequence reveals a progressively decreasing time interval between tremor bands. Extrapolating the tremor bands to a time interval of zero provides an accurate estimate, at least one week prior to the eruption, to within 10.2 hours of when the eruption would occur, with a 2.8-day range between 95 % confidence intervals. A similar set of tremor signals appeared before the 27 April 2016 eruption, and these signals provide a very accurate hindcast of this eruption to within 2.61 hours, with a 2.2-day range between 95 % confidence intervals. Our analysis indicates that this potential forecasting approach may prove useful for successfully and accurately forecasting future eruptions at Whakaari. The approach also may be applicable to other volcanoes similar to Whakaari which experience sudden phreatic and phreatomagmatic eruptions.

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.

Bacteria-Driven Fossil Ecosystems as Paleoindicators of Active Continental Margins and the Role of Carbonate Sediment-Hosted Vents in Geodynamic Reconstructions

Continental rifting of the Tisza microplate started during the Late Jurassic and resulted in phreatic eruptions, peperite, and the construction of a volcanic edifice in the Early Cretaceous in the Mecsek Mountains (South Hungary). In the SE direction from the volcanic edifice at Zengővárkony, a shallow marine (depth 100–200 m) carbonate sediment hosted a vent environment, and iron ore deposition occurred at the end of the Valanginian to early Hauterivian, hosting a diverse, endemic fauna of approximately 60 species. The detailed mineralogical analysis of the transport conduits included Fe oxides (ferrihydrite, goethite, hematite, and magnetite), quartz, mixed carbonate, pyrite, feldspar, Fe-bearing clay minerals, apatite, sulfates (barite, gypsum, and jarosite), and native sulfur. Filamentous, microbially mediated microtextures with inner sequented, necklace-like spheric forms (diameter of 1 μm) and bacterial laminae are also observed inside decapod crustacean coprolites (Palaxius tetraochetarius) and in the rock matrix. This complex ecological and mineralogical analysis provided direct evidence for the presence of bacteria in fossil sediment-hosted vent (SHV) environments on the one hand and for the intimate connection between bacteria and decapod crustaceans in hydrothermal environments 135 Ma before. This observation completes the fossil food chain of chemosynthesis-based ecosystems, from primary producers to the top carnivores reported for the first time from this locality.

Magmatic priming of a phreatic eruption sequence: the 2012 Te Maari eruptions at Mt Tongariro (New Zealand) imaged by magnetotellurics and seismicity

SUMMARY Magnetotelluric data from Mount Tongariro have been analysed using an unstructured tetrahedral finite-element inversion code that incorporates topography, which was not included in previous analysis of these data. Incorporating topography adds information, which stabilizes the resistivity inversion modelling, and for the first time allows details of the shallow hydrothermal system and its relationship with the underlying magmatic system to be resolved. Specifically, an electrically conductive zone between 4 and 12.5 km depth marks the underlying magmatic system, which is shown to directly connect via conductive pathways to the area where the most recent phreatic eruptions at Tongariro occurred in 2012. The resultant phreatic eruptions in 2012 August and November showed no new magmatic component to the eruption deposits. Nevertheless, by combining the magnetotelluric resistivity image with relocated seismicity, we can see that seismicity (a proxy for magma ascent) migrated from the top of the magmatic system into the hydrothermal system in the months preceding these eruptions. Magmatic interaction with the extant hydrothermal system likely caused the over pressurization for the phreatic eruption. This work highlights the utility of combining geophysical methods and petrological data to constrain phreatic eruption processes.

Evolution of shallow volcanic seismicity in the hydrothermal system of La Soufrière de Guadeloupe following the April 2018 Mlv 4.1 earthquake

La Soufrière de Guadeloupe volcano is characterized by seismo-volcanic activity dominantly linked to an active hydrothermal system. This microseismicity is shallow, mainly triggered in swarms and characterized by repeating earthquakes. Four recurrent families of Volcano-Tectonic (VT) repeaters have been identified and the main repeater accounts for 80% of detections. Stacking repeating seismic waveforms produces MASTER events with a high Signal-to-Noise Ratio (SNR) and thanks to the deployment of a temporary seismic nodal array, their absolute locations can be better constrained. Because we observe systematic positive residuals of P-wave arrival times at almost all stations, we investigate potential bias in the velocity model in the shallow part of the dome. By increasing the P-wave velocity and decreasing the VP/VS ratio from 1.8 to 1.69, we reduce these residuals and improve the local velocity model. We use this new velocity model to locate each VT event relatively to is own MASTER hypocenter. This procedure offers a new image of the hydrothermal seismic activity that we locate under the acid lake of the summit Tarissan crater, along a sub-vertical conduit. We also define a linear relationship between the logarithm of the peak amplitude of seismic events and their duration to obtain a pseudo local magnitude. The approach enables to accurately and automatically estimate the magnitude during the event detection. We show that after the occurrence in April 2018 of the largest earthquake (Mlv 4.1) since the last phreatic eruption in 1976–1977, the swarm frequency and amplitude increased significantly, and a new family of VT repeater emerges, accounting for up to 14% of detections. We show a quiescence in the main repeaters activity in the month following the April 2018 earthquake, suggesting a significant stress release within the volcanic system, which is likely related to dynamic stress changes rather than static stress variations caused by the earthquake. Moreover, we detect the emergence of a new family of repeaters 3 months after the event, located ∼100 m above the main repeater family, that could be related to a fluid pressure increase induced by the regional hydrological cycle, superimposed to the internal forcing driven by the hydrothermal system. Finally, using a statistical approach, we highlight seasonal periodicities in the number of events and the released seismic moment at La Soufrière de Guadeloupe, with a dominant peak of seismic activity in October–November and a second, less significant, peak of in April.

Reappraisal of volcanic seismicity at the Kirishima volcano using machine learning

Volcanic earthquakes provide essential information for evaluating volcanic activity. Because volcanic earthquakes are often characterized by swarm-like features, conventional methods using manual picking require considerable time to construct seismic catalogs. In this study, using a machine learning framework and a trained model from a volcanic earthquake catalog, we obtained a detailed picture of volcanic earthquakes during the past 12 years at the Kirishima volcano, southwestern Japan. We detected ~ 6.2 times as many earthquakes as a conventional seismic catalog and obtained a high-resolution hypocenter distribution through waveform correlation analysis. Earthquake clusters were estimated below the craters, where magmatic or phreatic eruptions occurred in recent years. Increases in seismic activities, b values, and the number low-frequency earthquakes were detected before the eruptions. The process can be conducted in real time, and monitoring volcanic earthquakes through machine learning methods contributes to understanding the changes in volcanic activity and improving eruption predictions.Graphical

Modeling staged and simultaneous evacuation during a volcanic crisis of La Soufrière of Guadeloupe (France)

The seismic and fumarollic activity of La Soufrière de Gaudeloupe increased in 1992. Continuing unrest led the Observatoire volocanologique et sismologique of Guadeloupe (OVSG-IPGP) to recommend in July 1999 to the authorities that the volcano alert be set to “Vigilance” (yellow). The OVSG-IPGP has been particularly vigilant and reinforced its monitoring following another significant increase of unrest in 2017 that culminated in magnitude 4.1 felt earthquake and a probable failed phreatic eruption. Volcanic activity remains difficult to forecast precisely, so the only way to stay safe, in case of an impending eruption, is to move away from the threatened area. This can be a major problem for the authorities and the population. In the French overseas departments, despite the presence of several volcanoes, there is limited experience in managing volcanic emergencies, especially in areas with a high population density and strategic assets, such as the Basse-Terre region of Guadeloupe. Therefore, it is crucial to devise and assess an emergency management strategy to identify potential problems and dangers that may arise during a mass evacuation. Crisis exercises can be planned to prepare the authorities and the population, but they are rarely carried out due to the human and resource costs involved. A series of evacuation scenarios are evaluated through simulations. The scenarios model staged and simultaneous evacuations with different speeds of individual response times. The aim of this research is to evaluate the two main evacuation strategies defined in the current volcano emergency response plan for La Soufrière of Guadeloupe, revised in 2018 by the authorities. This paper describes a calibrated agent-based model of mass evacuation and its exploration focusing on the potential staged evacuations of the southern Basse-Terre area. The overall objectives of this research are to: (1) test the evacuation strategy of the current emergency plan, and (2) provide relevant information to stakeholders. The results of these simulations suggest that there is no significant difference between the two evacuation strategies. It is estimated that 95% of the population will be evacuated within 20 h with a simultaneous or a staged evacuation. Whatever the scenario, the simulation results show high levels of road congestion. However, the staged evacuation will significantly reduce the number of vehicles on the network during the peak time of the evacuation and therefore reduce dangerous situations and the potential for adding crises within a volcanic crisis.

The Onset, Middle, and Climax of Precursory Hydrothermal Intrusion of the 2018 Phreatic Eruption at Kusatsu‐Shirane Volcano

AbstractTriggering intrusions of phreatic eruptions are often observed as seismic and ground deformation signals on a time scale of minutes. The current understanding of hydrothermal intrusions still needs improvement to obtain insight into the eruption scale from the observables. We examine local geophysical data from the precursory hydrothermal intrusion of the 2018 phreatic eruption of Kusatsu‐Shirane volcano. To achieve an integrated intrusion model, we divide analyzing time window into the onset, middle, and climax. Focusing on the transient response of tilt data for the sudden pressurization, we estimate a vertical tensile opening (1.7 × 103 m3/s in 40 s) at 1.1 km depth for the intrusion onset. Pressurization can represent the start of vapourization. Very long period (VLP, 0.033–0.1 Hz) seismic signals are adopted to constrain the middle and climax phases. We obtained two sequential semi‐horizontal tensile crack oscillation sources with peak volume changes of 3.6 × 104–1.9 × 105 m3 at 0.3–0.6 km depths. The second VLP source acted as a final trigger of the eruption to cause depressurization in the shallow portion of the intruded region, which is constrained as having reached 0.1 km depth by surface deformation. Simultaneously, we find another depressurization originated from depth in the climax due to a decrease in the hydrothermal intrusion rate. Through comparison with the 2014 Ontake phreatic eruption, the total inflation volume may correlate with eruption scales. Intruded hydrothermal fluid and local structure characteristics also may have to be considered to evaluate the eruptions scales from inferred signal source intensity.

Chemical and thermal changes in hot spring waters and fumarolic gases related to volcanic unrest at Meakandake volcano, Japan: Results of long-term geochemical monitoring from 1986 to 2022

Phreatic eruptions at Nakamachineshiri and Ponmachineshiri reflect the vigorous fumarolic and hydrothermal activities of Meakandake volcano. Thus, phreatic eruptions are likely to reoccur in the future, and hydrothermal systems should be understood to mitigate the risks of potential future hazards. Here, we reveal the results of long-term geochemical monitoring of hot spring waters and fumarolic gases since 1986 and discuss the chemical and temperature changes in hot spring waters, discharging at the foot, related to volcanic unrest. In Nakamachineshiri, related to the increase in seismic and fumarolic activity since 2019, the HCl concentration of the fumarolic gas has increased, and δD and δ18O values have also increased to approach those of the andesitic water. Simultaneously, with the increase in fumarolic and hydrothermal activity, the temperature and Cl concentration of the Meakan Onsen (MO) hot spring water discharged at the western foot of Nakamachineshiri have increased. The chemical characteristics of the fumarolic gas and MO water indicate that they were derived from a hydrothermal system in the shallow subsurface of Nakamachineshiri. Thus, we can consider that the temperature and chemical increases in the MO water represent activation of the system, and monitoring the MO water provides important information for understanding the present state of Nakamachineshiri. At Ponmachineshiri, phreatic eruptions occurred in 1988, 1996, 1998, 2006, and 2008, and seismic and fumarolic activity has sometimes increased since 1986 when we began our monitoring program. At the western foot of Ponmachineshiri, hot spring water is discharged at Yunotaki (YNT_U and YNT_L) and Lake Onneto (ONT). The results of our monitoring suggest that the YNT_U water is formed by mixing thermal water with a high HCO3 molar ratio (HCO3-type) and thermal water containing Cl and SO4 (Cl-SO4-type), which causes temperature and chemical changes related to the mixing ratio of these thermal waters related to volcanic unrest, especially increases in seismic activity. The Cl and SO4 concentrations in the ONT water began to increase approximately two years before the phreatic eruptions and increase in fumarolic activity. Such chemical increases can be explained by the increase in the supply of Cl-SO4-type thermal water and are likely to indicate precursor changes in volcanic unrest. Therefore, we expect to utilize these hot spring waters to understand the present state of Ponmachineshiri and detect precursor changes to future volcanic unrest.

Magma and hydrothermal sources below the northern part of Paramushir Island (Kuril Arc) inferred from ambient noise tomography

Paramushir is the northernmost island of the Kuril Arc comprising several Holocene volcanoes, of which two are presently active: Ebeko and Chikurachki. We present the first crustal-scale three-dimensional seismic shear-wave velocity model of the northern part of Paramushir derived with the use of data of 20 temporary stations operated in 2021–2022 and one permanent station. The continuous seismic data were used to obtain the Rayleigh wave dispersion curves for periods ranging from 0.5 s to 12 s through computing ambient seismic noise cross-correlation functions. Then the 3D shear wave velocity distribution was constructed by seismic tomography. The synthetic tests demonstrate that the model has sufficient resolution down to 7–10 km depth. We observe a series of low-velocity anomalies at the depths of 4–6 km below the volcano-origin Vernadsky Ridge. They may likely represent magma reservoirs that are responsible for Holocene eruptions along the ridge. These anomalies are overlain by the high-velocity anomalies associated with the rigid cover consisting of an upper-crustal granite-granodiorite body and consolidated magma intrusions. Along the eastern flank of Ebeko we reveal a thin low-velocity anomaly at a depth of ∼1 km that can be interpreted as a water-saturated layer. The contact of this layer with hot magma intrusions leads to the formation of a large amount of steam that is ejected during the frequent phreatic eruptions of Ebeko and from numerous fumaroles in the summit area. On the other hand, this layer, which is following down to the Pacific Coast, might be promising for the geothermal energy exploitation.

Fluid-mineral dynamics at the Rincón de la Vieja volcano—hydrothermal system (Costa Rica) inferred by the study of major, minor and rare earth elements in the hyperacid crater lake

Volcanic lakes are complex natural systems and their chemical composition is related to a myriad of processes. The chemical composition of major, minor, Rare Earth Elements (REE) and physico-chemical parameters at the hyperacid crater lake of Rincón de la Vieja volcano (Costa Rica) are here investigated during February 2013–August 2014. The study of the lake chemical composition allows to identify the main geochemical processes occurring in the lake and to track the changes in the volcanic activity, both important for active volcanoes monitoring. The total REE concentration (∑REE) dissolved in the crater lake varies from 2.7 to 3.6 mg kg−1 during the period of observation. REE in the water lake samples normalized to the average volcanic local rock (REEN-local rock) are depleted in light REE (LREE). On the contrary REEN-local rock in the solids precipitated (mainly gypsum/anhydrite), from lake water samples in laboratory at 22°C, are enriched in LREE. The low variability of (La/Pr)N-local rock and (LREE/HREE)N-local rock ratios (0.92–1.07 and 0.66–0.81, respectively) in crater lake waters is consistent with the low phreatic activity (less than 10 phreatic eruptions in 2 years) observed during the period of observation. This period of low activity precedes the unrest started in 2015, thus, it could be considered as a pre-unrest, characterized by infrequent phreatic eruptions. No clear changes in the REE chemistry are associated with the phreatic eruption occurred at mid-2013. The results obtained investigating water-rock interaction processes at the Rincón de la Vieja crater lake show that rock dissolution and mineral precipitation/dissolution are the main processes that control the variability of cations composition over time. In particular, precipitation and dissolution of gypsum and alunite are responsible for the variations of REE in the waters. Despite the low variations of (La/Pr)N-local rock and (LREE/HREE)N-local rock ratios, this study allows to suggest that REE can be used, together with major elements, as practical tracers of water-rock interaction processes and mineral precipitation/dissolution at active hyperacid crater lakes over time, also during periods of quiescence and low phreatic activity.

The Seismic Activity of the Nasudake Volcano Using Hypocenter Data (1998–2022) from the National Research Institute for Earth Science and Disaster Resilience

The seismic activity of the Nasudake volcano is summarized based on hypocenter data from the National Research Institute for Earth Science and Disaster Resilience (NIED). The observation period spans 24 years, from October 1998 to December 2022, and the number of earthquakes during the period is 12,064. Of these, 80.1% occurred between depths of 0.7 and 2.6 km; 94% registered between -1.6 and 0.9 on the Richter scale. The lower limit of the detectable magnitude (magnitude of completeness, Mc) for the transition of the volcano-observation system was examined and found to be -0.5 for Mc before 2005 and -1.4 for Mc after 2012. Focusing on zones where seismic activity was concentrated, we divided the active zones into four areas. The greatest concentration of seismic activity, 76.4% of the total number of earthquakes, was between Chausu-dake and Asahi-dake. The depth of the earthquakes in this active area was shallower than 2 km, and most were micro-earthquakes (1 ≤ M &lt; 3) or smaller. Earthquakes have been occurring regularly for 24 years. This active area includes the Chausu-dake lava dome formed by activities in 1408–1410 and two craters where phreatic eruptions have occurred since the Meiji era (1868–1912). Therefore, in considering volcano disaster-prevention measures, the relationship between this seismic area and volcanic activity should continue to be closely monitored.

The sequence of the 2017–2018 eruptions and seismo-acoustic activity at Kirishima volcano group

Kirishima volcano consists of more than 20 eruptive centers. Among them, Shinmoe-dake had magmatic eruptions in October 2017 and March 2018. Subsequently, another active cone, Iwo-yama, had phreatic eruptions in April 2018. These events were unique in that the 2018 eruption was the first effusion-dominated eruption of Shinmoe-dake and the first simultaneous activity of two cones of the Kirishima volcanic group ever documented. We report the detailed sequence of the events by combining areal photos, satellite images, and seismo-acoustic data analyses with the other published information. The seismo-acoustic data clarify the eruption onset and the transitions of the behaviors in three stages for each of the 2017 and 2018 eruptions. For both eruptions, we present regularly repeated tremors or ’drumbeat’ earthquakes in the second stage, which interpret as gas separation from magma, leading to the ash-poor plume in the 2017 eruption or the effusive eruption in the 2018 event. We also propose that the 2017 and 2018 eruptions of Shinmoe-dake and the 2018 eruption of Iwo-yama are sequential events linked by the degassing of magma beneath Shinmoe-dake. An eruption like the 2017–2018 eruptions of Shinmoe-dake would leave few geological records and could be captured only by modern techniques. Although Shinmoe-dake has been believed to be an example of less-frequent eruptions, effusive eruptions like the 2018 case might have occurred more frequently in the past , but the following eruptions had obscured their records. The timelines summarized in this study will be useful in future studies of Kirishima volcanoes and world equivalences.Graphical

Seismic structure and its implication on the hydrothermal system beneath Mt. Ontake, central Japan

Mt. Ontake is an active volcano in central Japan where phreatic eruption activity is prominent. A shallow subsurface structure in the summit region of this volcano has been scarcely studied despite its importance. Our study examines the structure from three seismic analyses: a typical P-wave velocity from the semblance of the vertical propagation of intermediate-depth earthquakes (IDEs), a layered velocity model from P- and S-wave arrival times of shallow volcanic earthquakes, and a pseudoreflection profile from the autocorrelation functions of the IDEs. Our results consistently indicate the presence of three layers, which are interpreted as younger (> 0.1 Ma) and older (0.39–0.78 Ma) eruptive deposits and the basement. A comparison of the structure with hypocentres and deformation sources suggests fluid migration controlled by the structure.Graphical

Study of geothermal and volcanic activity at Mount Awu, the deadliest active volcano in North Sulawesi Province, Indonesia using Optical Satellite Imagery

The history of the north part of Sangihe Island is the motivation for using the strengths of remote sensing methods to monitor the volcano activities and geothermal potential of Awu Volcano. We used the last 50 years of satellite image data composed by Landsat 1, Landsat 4, Landsat 5, Landsat 7, ASTER, and Landsat 8 to get the multi-temporal analysis and results. This study used the single-channel algorithm to derive land surface temperature (LST). We also used the band combination and ratio to understand the relationship between the Awu Volcano's geological condition and geothermal potential. The NDVI result as a component of the LST calculation shows that the vegetation is increasing when there is no eruption event. However, the amount of vegetation decreased after the 1994 and 2004 eruptions. The LST results show the 2004 eruption gave the highest surface temperature in the last 50 years, with a number of 318.69 °K as the mean value in the caldera. The multi-temporal nature of LST explained the activity of Awu Volcano, including the increased status announced by the Center for Volcanology and Geological Hazard Mitigation (CVGHM) in 2016 and 2021. The accuracy of LST results is seen by the same pattern in the LST provided from the Landsat and ASTER data and the R2 of time and LST with the values of 0.1529 and 0.2436 in the volcano slope and caldera, respectively. The long gap between the phreatic eruption in 1994 and the previous eruption in 1964 revealed that there was a caldera lake formed in Awu Volcano, which was detected by the natural color band combination from 1972 to 1996. The geothermal potential is centralized in the volcano slope around the caldera, with a value of almost 10 °K and some areas in the slope with a value of >5 °K. The first area that contains high geothermal potential is located in the basaltic andesite, with the alteration detected by the combination of bands 7/4, 6/3, and 5/7. Mount Awu's multiple eruptions have shown that multi-source optical satellites could be used to keep an eye on volcanic activity and could also be used as a first step in figuring out geothermal potential.

Shallow resistivity structure around the 2018 craters of Mt. Motoshirane of Kusatsu-Shirane Volcano, Japan, revealed by audio-frequency magnetotellurics

In 2018, a phreatic eruption occurred at Mt. Motoshirane, a pyroclastic cone group of the Kusatsu-Shirane Volcano in central Japan. The eruption was abrupt, and no signs of volcanic activity have been observed in recent years, in contrast to the other major pyroclastic cone group, Mt. Shirane, which hosts the active crater lake and has endured repeated phreatic eruptions. To understand the mechanism of the eruption at Mt. Motoshirane, information on the shallow hydrothermal system, which is thought to be the source region of phreatic eruptions, is required; however, few studies have been conducted on this particular cone group. In this study, we conducted an audio-frequency magnetotelluric survey in 2020 to reveal the shallow resistivity structure around the 2018 craters. A three-dimensional resistivity structure model showed generally two layers, with high resistivities at shallow depths overlain by low resistivities underneath. The boundary between the layers corresponded to the top boundary of the Neogene basement rocks. These low resistivities were not found beneath the 2018 craters; therefore, part of the Neogene basement rocks could have been lost by the eruption beneath the 2018 craters. This is consistent with the geochemical study on the mineral assemblage of the erupted deposits, which suggested that the explosions reached the depth of the basement.Graphical