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Temporal variations of volume and mass in the magma chambers of Sakurajima Volcano were modeled using leveling and relative gravity data collected around the volcano during the eruptive period from 1975 to 1992, to reveal a physical mechanism for the excessive gravity increase observed at the volcano. The following two deflation sources were estimated from the leveling data: a deeper source of -4.2 ×106 m3/yr located 8000 m deep beneath Aira Caldera, and a shallower source of -7.25 × 105 m3/yr located 3600 m deep beneath the center of Sakurajima Volcano. These deflation sources cannot fully explain the gravity increase of up to 15.75 microGal/yr observed at the volcano, because a gravity increase of only 10 kg/yr, and the position of the point mass agreed with that of the shallower magma chamber within its error range. This result suggests that the shallower magma chamber gained mass despite the chamber deflation during the 1975-1992 eruptive period, and can be quantitatively explained by the accumulation of degassed magma in the shallower chamber. Our modeling results also suggest the importance of gravimetry in addition to crustal deformation observations in quantifying the rate of magma mass supply, because the magma supply to the deeper chamber was calculated to be +5.39 × 1010 kg/yr from the gravity and leveling data, which is six times greater than that calculated from the leveling data only.

Very-long-period (VLP) seismic events at Asama volcano in central Japan are characterized by a transient signal of 10–20 s duration. Associated with the transient motion, a high-frequency (HF) oscillation within the 5–10 Hz band is observed. We investigated the location and size of the oscillation source in the HF band (HF source) using an amplitude source location method and compared our results with those for the VLP band (VLP source) deduced in our previous waveform inversion study. We analysed 1437 VLP events recorded during an intense observation campaign during 2008–2009, and additional 571 events surrounding an eruptive activity in 2015 (including a VLP event that immediately preceded an eruption). The HF source locations of most events were deeper than the VLP source locations by ∼150 m, although there was almost no time lag between the signals. This suggests a strong connection between the two sources, with one source immediately responding to the other. The eruptive VLP event had an HF source location close to normal VLP events but had a greater event size. We surmise that the HF signal is caused by an inflow of volcanic gas from depth into a semi-vertical crack-like cavity (as imaged in our previous waveform inversion study), and the VLP signal is caused by resultant inflation of the same cavity. The centroid of inflation (VLP source) is near the roof of the cavity, where gas accumulates, whereas the HF oscillation is emitted more intensely from lower and narrower portions of the cavity. This results in source depth differences between the two signal bands. In this model, the inflation rate of the cavity is controlled by the volume flux of the gas inflow, producing similar temporal variations between the VLP and HF signals. According to this model, the eruptive VLP is associated with a greater amount of gas inflow and accumulation, which likely played a key role in the eruption.

High precision and accuracy in volcanic SO2 emission rate quantification is critical for eruption forecasting and, in combination with in-plume gas ratios, quantifying global volcanic emission inventories. Light dilution, where scattering of ultraviolet light dilutes plume SO2 absorbance signals, has been recognized for more than 50 years, but is still not routinely corrected for during gas flux quantification. Here we use modeling and empirical observations from Masaya volcano, Nicaragua, to show that light dilution produces: i) underestimates in SO2 that can reach a factor of 5 and, at low column densities, cause little impact on standard retrieval fit quality, even for heavily diluted spectra; ii) retrieved SO2 amounts that are capped by a maximum value regardless of the true amount of SO2, with this maximum amount being reduced as light dilution increases. Global volcanic volatile emission rates may therefore be significantly underestimated. An easily implementable dual-waveband analysis provides a means to detect, and in clear sky conditions, correct dilution effects directly from the spectra, opening a path to more accurate SO2 quantifications.

The Kuril Island arc extending for about 1,200 km from Kamchatka Peninsula to Hokkaido Island is a typical active subduction zone with ∼40 historically active subaerial volcanoes, some of which are persistently degassing. Seven Kurilian volcanoes (Ebeko, Sinarka, Kuntomintar, Chirinkotan, Pallas, Berg, and Kudryavy) on six islands (Paramushir, Shiashkotan, Chirinkotan, Ketoy, Urup, and Iturup) emit into the atmosphere > 90% of the total fumarolic gas of the arc. During the field campaigns in 2015–2017 direct sampling of fumaroles, MultiGas measurements of the fumarolic plumes and DOAS remote determinations of the SO2 flux were conducted on these volcanoes. Maximal temperatures of the fumaroles in 2015–2016 were 510°C (Ebeko), 440°C (Sinarka), 260°C (Kuntomintar), 720°C (Pallas), and 820°C (Kudryavy). The total SO2 flux (in metric tons per day) from fumarolic fields of the studied volcanoes was measured as ∼1,800 ± 300 t/d, and the CO2 flux is estimated as 1,250 ± 400 t/d. Geochemical characteristics of the sampled gases include δD and δ18O of fumarolic condensates, δ13C of CO2, δ34S of the total sulfur, ratios 3He/4He and 40Ar/36Ar, concentrations of the major gas species, and trace elements in the volcanic gas condensates. The mole ratios C/S are generally 7RA (where RA is the atmospheric 3He/4He). The highest 3He/4He ratios of 8.3RA were measured in fumaroles of the Pallas volcano (Ketoy Island) in the middle of the arc.

Eruption mass and mass flow rate are critical parameters for determining the aerial extent and hazard of volcanic emissions. Infrasound waveform inversion is a promising technique to quantify volcanic emissions. Although topography may substantially alter the infrasound waveform as it propagates, advances in wave propagation modeling and station coverage permit robust inversion of infrasound data from volcanic explosions. The inversion can estimate eruption mass flow rate and total eruption mass if the flow density is known. However, infrasound-based eruption flow rates and mass estimates have yet to be validated against independent measurements, and numerical modeling has only recently been applied to the inversion technique. Here we present a robust full-waveform acoustic inversion method, and use it to calculate eruption flow rates and masses from 49 explosions from Sakurajima Volcano, Japan. Six infrasound stations deployed from 12–20 February 2015 recorded the explosions. We compute numerical Green's functions using 3-D Finite Difference Time Domain modeling and a high-resolution digital elevation model. The inversion, assuming a simple acoustic monopole source, provides realistic eruption masses and excellent fit to the data for the majority of the explosions. The inversion results are compared to independent eruption masses derived from ground-based ash collection and volcanic gas measurements. Assuming realistic flow densities, our infrasound-derived eruption masses for ash-rich eruptions compare favorably to the ground-based estimates, with agreement ranging from within a factor of two to one order of magnitude. Uncertainties in the time-dependent flow density and acoustic propagation likely contribute to the mismatch between the methods. Our results suggest that realistic and accurate infrasound-based eruption mass and mass flow rate estimates can be computed using the method employed here. If accurate volcanic flow parameters are known, application of this technique could be broadly applied to enable near real-time calculation of eruption mass flow rates and total masses. These critical input parameters for volcanic eruption modeling and monitoring are not currently available.

Airborne measurements of volcanic gas composition using an unmanned aerial vehicle (UAV) and Cessna aircraft were conducted at Kuchinoerabujima volcano, Japan, between 2014 and 2016. Because eruptions occurred in August 2014, May 2015, and June 2015, access to the summit crater was limited because of the risk of sudden eruption such that airborne measurements were the only viable method to measure the volcanic gas composition. Multi-GAS and alkali-filter pack measurements were made on the leeward side of the crater and around the crater, using the Cessna and UAV, respectively. Observations using the UAV could measure the dense plume and quantify the gas species of H2O, CO2, SO2, H2S, H2, HCl, and HF, while the observations using the Cessna could measure only the diluted plume and quantify CO2, SO2, H2S, and H2. The seven airborne observations enabled us to monitor variations in the volcanic gas composition. Over the observation period, the SO2/H2S ratio decreased from 10 to 1.9. The H2O/SO2 ratio, H2/SO2 ratio, and apparent equilibrium temperatures (AET) estimated using the volcanic gas composition increased after the 2014 eruption. The decrease in the SO2/H2S ratio might be attributed to changes in the pressure of degassing magma and interactions with the hydrothermal system. The airborne methods presented here highlight the utility of using light aircrafts to safely conduct volcanic gas measurements during periods of volcanic unrest when traditional ground-based methods are not possible.

SO2 cameras are able to measure rapid changes in volcanic emission rate but require accurate calibrations and corrections to convert optical depth images into slant column densities. We conducted a test at Masaya volcano of two SO2 camera calibration approaches, calibration cells and co-located spectrometer, and corrected both calibrations for light dilution, a process caused by light scattering between the plume and camera. We demonstrate an advancement on the image-based correction that allows the retrieval of the scattering efficiency across a 2D area of an SO2 camera image. When appropriately corrected for the dilution, we show that our two calibration approaches produce final calculated emission rates that agree with simultaneously measured traverse flux data and each other but highlight that the observed distribution of gas within the image is different. We demonstrate that traverses and SO2 camera techniques, when used together, generate better plume speed estimates for traverses and improved knowledge of wind direction for the camera, producing more reliable emission rates. We suggest combining traverses and the SO2 camera should be adopted where possible.

Accurate quantification of the emission rate of sulphur dioxide (SO2) from volcanoes provides both insights into magmatic processes and a powerful monitoring tool for hazard mitigation. The primary method for measuring magmatic SO2 is Differential Optical Absorption Spectroscopy (DOAS) of UV scattered sunlight spectra, in which a reference spectrum taken outside the plume is used to quantify the SO2 slant column density inside the plume. This can lead to problems if the reference spectrum is contaminated with SO2 as this will result in a systematic underestimation of the retrieved SO2 slant column density, and therefore emission rate. We present a new analysis method, named “iFit”, which retrieves the SO2 slant column density from UV spectra by directly fitting the measured intensity spectrum at high spectral resolution (0.01 nm) using a literature solar reference spectrum and measured instrument characteristics. This eliminates the requirement for a measured reference spectrum, providing a “point and shoot” method for quantifying SO2 slant column densities. We show that iFit retrieves correct SO2 slant column densities in a series of test cases, finding agreement with existing methods. We propose that iFit is suitable for both traverse measurements and permanent scanning stations, and could be integrated into volcano monitoring networks at observatories. Finally, we provide an open source software implementation of iFit with a user friendly graphical interface to allow users to easily utilise iFit.

Accurate quantification of the sulphur dioxide (SO2) flux from volcanoes provides both an insight into magmatic processes and a powerful monitoring tool for hazard mitigation, with miniature ultraviolet spectrometers becoming the go-to method for SO2 flux measurements globally. The most common analysis method for these spectrometers is Differential Optical Absorption Spectroscopy (DOAS), in which a reference spectrum taken outside the plume is used to quantify the SO2 column density inside the plume. This can lead to problems if the reference spectrum is contaminated with SO2 as this leads to systematic underestimates in the retrieved SO2 column density. We present a novel method, named “iFit”, which retrieves the SO2 column density from UV spectra by directly fitting the measured intensity spectrum using a high resolution solar reference spectrum. This has a number of advantages over the traditional DOAS method, primarily by eliminating the requirement for a measured reference spectrum. We show that iFit can accurately retrieve SO2 column densities in a series of test cases, finding excellent agreement with existing methods without the use of a reference spectrum. We propose that iFit is well suited to application to both traverse measurements and permanent scanning stations, and shows strong potential for integration into volcano monitoring networks at observatories.

The composition of the volcanic gases discharged from the summit crater of Asama volcano has been monitored since 2004 by Multi-GAS and alkaline-filter techniques. The persistent degassing activity at Asama volcano is characterized by large variation of SO 2 flux. The CO 2 /SO 2 and H 2 O/SO 2 ratios did not show clear variation irrespective of the SO 2 flux variation and a few eruptions that occurred during active degassing periods. The estimated ratios have large uncertainty due to variable contribution of the different fumaroles in the summit crater to the volcanic plume and lack of a systematic variation can be due to the large uncertainty. The SO 2 /Cl ratio showed a systematic decrease after the eruption to the inactive period, suggesting that degassing pressure did not significantly increase after the eruption. Low-pressure degassing along with the continuous and intensive gas discharge suggests that the degassing is due to conduit magma convection. The apparently stable CO 2 /SO 2 ratios imply a lack of significant volatile differentiation in the magma reservoir, such as CO 2 -rich bubble accumulation. The large variation of the SO 2 flux along with stable gas composition implies that the large changes in magma convection rate are caused by changes in the radius of the convecting magma conduit.

Volcanic gases can provide significant information on the state of magma, which is important for interpreting and predicting volcanic activity. We present a study of the seismicity and volcanic gases from Mt. Asama, Japan from 2007 to 2011 to elucidate the change in the mechanism of volcanic gas supply for very-long-period (VLP) seismic events. Two types of degassing occur at Mt. Asama: persistent degassing and a gas burst (ash-free eruption) following a VLP seismic event. We used the linear relationship between the seismic moment and sulfur dioxide (SO 2 ) emissions, and seismic records to estimate the quantity of SO 2 gas in the ash-free eruption, which causes the VLP seismic events. The amount of SO 2 and volcanic gas composition data were used to calculate the amount of volcanic gases, including H 2 O, CO 2 , and other species. We compared the volcanic gas emission rates of ash-free eruptions (eruptive gas emission rates) and total volcanic gas emission rates including the persistent degassing. The ratio of the eruptive gas emission rate to the total gas emission rate (eruptive/total gas ratios) showed a drastic change with eruption. An increase occurred immediately before the 2008 and after the 2009 eruptions. The VLP seismic events are likely caused by sudden outgassing at a shallow depth that is induced by the propagation of the gas phase expansion and rupture. We suggest that the rates of sudden outgassing are controlled by the distribution of gas phases in the conduit. The variability of the eruptive gas emission rates suggests changes in the gas phase distribution in the conduit.

Multiple cycles of the intensive volcanic gas discharge and ground deformation (inflation and deflation) were observed at Asama Volcano, Japan, from 2000 to 2011. Magma budget of the shallow magma plumbing system was estimated on the basis of the volcanic gas emission rates and ground deformation data. Recent inflations observed in 2004 and 2008 were modeled as a dike intrusion to 2–3 km west of Asama Volcano. Previous studies proposed that magma ascends from a midcrustal magma reservoir to the dike and reaches the surface via a sinuous conduit which connects the dike to the summit. The intensive volcanic sulfur dioxide discharge of up to 4600 t/d at the volcano was modeled by magma convective degassing through this magma pathway. The volcano deflates as shrinkage of the magma in a reservoir by volcanic gas discharge. We estimated the volume change of the dike modeled based on the GPS observations, the volume decrease of the magma by the volcanic gas discharge, and the amount of degassed magma produced to calculate the magma budget. The results show that the volume decrease of the magma by the volcanic gas discharge was larger than the volume change of the dike during the inflation periods. This indicates that a significant volume of magma at least more than 2 times larger than the volume change of the dike was supplied from the midcrustal magma reservoir to the dike. The volume decrease of the dike was comparable with the volume decrease of the magma by the volcanic gas discharge during the deflation periods. The long-term deflation trend of the dike and the volume of degassed magma (108–9 m3) suggest that the degassed magma produced is not stored in the dike and the magma is mainly supplied from the midcrustal magma reservoir. In both periods, the volume of degassed magma produced was 1 order of magnitude larger than the volume change of the dike. This indicates that the actual volume of the magma supplied from the midcrustal magma reservoir is up to 1 order of magnitude larger than the volume change of the dike. These results strongly suggest that an amount of magma moved through a magma reservoir is possible to be significantly larger than volume change of the magma reservoir estimated by the geodetic observations.

The composition of volcanic gases discharged from Miyakejima volcano has been monitored during the intensive degassing activity that began after the eruption in 2000. During the 15 years from 2000 to 2015, Miyakejima volcano discharged 25.5 Mt of SO2, which required degassing of 3 km3 of basaltic magma. The SO2 emission rate peaked at 50 kt/day at the end of 2000 and quickly decreased to 5 kt/day by 2003. During the early degassing period, the volcanic gas composition was constant with the CO2/SO2 = 0.8 (mol ratio), H2O/SO2 = 35, HCl/SO2 = 0.08, and SO2/H2S = 15. The SO2 emission rate decreased gradually to 0.5 kt/day by 2012, and the gas composition also changed gradually to CO2/SO2 = 1.5, H2O/SO2 = 150, HCl/SO2 = 0.15, and SO2/H2S = 6. The compositional changes are not likely caused by changes in degassing pressure or volatile heterogeneity of a magma chamber but are likely attributed to an increase of hydrothermal scrubbing caused by large decrease of the volcanic gas emission rate, suggesting a supply of gases with constant composition during the 15 years. The intensive degassing was modeled based on degassing of a convecting magma conduit. The gradual SO2 emission rate that decrease without changes in volcanic gas composition is attributed to a reduction of diameter of the convecting magma conduit.

Sulfur dioxide (SO2) emission rate observations were made at Sakurajima volcano, Japan, to quantify the relationship between the SO2 emission rate and inflation prior to Vulcanian explosions. The explosions associated with precursory inflation events were preceded by decreases in SO2 emission rates by 10–60 min. The amounts of accumulated gas were calculated using time series of SO2 emission rate. The amounts of accumulated SO2 and increases in strain records before the explosions showed a positive correlation. The volume increase of a deformation source calculated using the strain records was of the comparable order of magnitude as the volume of the accumulated volcanic gas. The results suggest that the inflations before the explosions were caused by the gas accumulation.

We measured diffuse carbon dioxide (CO2) flux and soil temperature around the summit of Asama volcano, Japan to assess the diffuse degassing structure around the summit area. Soil CO2 flux was measured using an accumulation chamber method, and the spatial distributions of CO2 flux and soil temperature were derived from the mean of 100 sequential Gaussian simulations. Results show that soil CO2 flux was high on the eastern flank of Kamayama cone and the eastern rim of Maekake crater, the outer cone. These areas mostly correspond to high-temperature anomalies. The average emission rate of diffuse CO2 was calculated to be 12.6 t day−1 (12.2–14.6 t day−1). Such diffuse emissions account for 12 % of the total (diffuse and plume) CO2 emissions from the summit area. The diffuse CO2 anomalies probably reflect permeable zones controlled by local topography and hidden fractures bordering Maekake crater. The permeable zones are connected to the low-electrical-resistivity zone inferred to indicate both a hydrothermal fluid layer and an upper sealed layer made of clay minerals. Magmatic gas from the main conduit ascends to the volcano surface through this fluid layer and the permeable zones. These insights emphasize that the pathways and the connection between the pathways and the source of diffuse CO2 combine to create the pattern of heterogeneous diffuse CO2 emission seen at the surface. Only by using a combination of gas measurements and geophysical tools can we begin to understand the dynamics of this system as a whole.

A 40-year-long record of the sulfur dioxide (SO2) emission rate of Asama volcano, Japan, is presented including high-temporal-resolution data since the 2004 eruption. The 2004 and 2008–2009 eruptive activities were associated with high SO2 emission, and SO2 emission rates markedly fluctuated. In contrast, stable and weak SO2 emissions have been observed for the rest of the investigated interval. The fluctuation of the SO2 emission rates is correlated with the number of shallow low-frequency B-type earthquakes, implying that increased flows of gas and/or magma induced the B-type earthquakes along the shallow conduit. The total volumes of outgassed magma during the 2004 and 2008–2009 eruptive activities are estimated to be 1.9 × 108 and 1.5 × 108 m3, respectively. These volumes are about 100–200 times larger than those of the erupted magma, indicating that the large volumes of the magma were outgassed without being erupted (i.e., excess degassing/outgassing). Degassing and outgassing driven by magma convection rather than by permeable gas flow in the conduit is concluded as the probable degassing/outgassing process of Asama volcano based on model examinations, and is thought to occur regardless of the outgassing intensity. Production rates of outgassed magma related to the 2004 and 2008–2009 eruptive periods are estimated to have been 7.4 × 103 and 6.7 × 103 kg/s, respectively. These values are one order of magnitude higher than the average production rate of 0.92 × 103 kg/s for the inactive periods. Increased supply of fresh magma is thought to activate magma convection in the conduit and to thereby increase magma degassing/outgassing.

A phreatic eruption of Mt. Ontake, Japan, started abruptly on September 27, 2014, and caused the worst volcanic calamity in recent 70 years in Japan. We conducted volcanic plume surveys using an electric multirotor unmanned aerial vehicle to elucidate the conditions of Mt. Ontake’s plume, which is flowing over 3000 m altitude. A plume gas composition, sulfur dioxide flux and thermal image measurements and a particle sampling were carried out using the unmanned aerial vehicle for three field campaigns on November 20 and 21, 2014, and June 2, 2015. Together with the results of manned helicopter and aircraft observations, we revealed that the plume of Mt. Ontake was not directly emitted from the magma but was influenced by hydrothermal system, and observed SO2/H2S molar ratios were decreasing after the eruption. High SO2 flux of >2000 t/d observed at least until 20 h after the onset of the eruption implies significant input of magmatic gas and the flux quickly decreased to about 130 t/d in 2 months. In contrast, H2S fluxes retrieved using SO2/H2S ratio and SO2 flux showed significantly high level of 700–800 t/d, which continued at least between 2 weeks and 2 months after the eruption. This is a peculiar feature of the 2014 Mt. Ontake eruption. Considering the trends of the flux changes of SO2 and H2S, we presume that majority of SO2 and H2S are supplied, respectively, from high-temperature magmatic fluid of a deep origin and from hydrothermal system. From the point of view of SO2/H2S ratios and fumarolic temperatures, the plume degassing trend after the 2014 eruption is following the similar course as that after the 1979 eruptions, and we speculate the 2014 eruptive activity will cease slowly similar to the 1979 eruption.

[1] Multiple volcanic observations conducted at Mt. Asama, Japan, provide evidence of a link between single very-long-period (VLP) seismic pulses and volcanic gas emissions. SO2 flux measurements were conducted on 2 June 2009, when Mt. Asama was producing ash-free eruptions with VLP pulses. Gas bursts from a vent at the crater bottom following the VLP pulses provided an excellent opportunity to examine the relation directly. The SO2 emission for each eruption was calculated by integrating high temporal SO2 flux data obtained by the SO2 imaging system and subtracting the contribution from quiescent degassing from fumaroles around the crater bottom. A seismic moment of VLP pulse was estimated by the waveform inversion. We observed seven eruptions and obtained the proportional relation between VLP pulse moment and SO2 emission. The relation determined is consistent with the VLP source model; these observational results are the first report of a quantitative comparison between single VLP pulse moment and volcanic gas emission.

[1] We present here a new method for measuring the SO2 concentration distribution in a volcanic plume cross-section, by using the airborne traverse technique with three UV spectrometers and calculating a computed tomography (CT) reconstruction. Our observation requires only one traverse, and is as easy as conventional measurements of SO2 flux. The plume is traverse scanned using three UV spectrometers aligned at three different zenith angles, and a CT technique initially designed for air pollutant monitoring was improved by using ‘Akaike's Bayesian Information Criterion' (ABIC). We applied this method to a volcanic plume at 10 km downwind from the caldera of Miyakejima volcano in Aug. 2004. The results of the application show that the profile of the plumes significantly changed in only a matter of a few minutes. Due to the flexibility and ease of the observation procedure, this method is widely applicable to various fields and shows promise in volcanic gas monitoring.