Active Volcanoes In Japan Research Articles

Effects on local atmospheric environment of volcanic ash from Sakurajima volcano, inferred from atmospheric deposition of Potassium-40 at Kagoshima City, Japan

In this study, we statistically demonstrated that an anomalous high of 40K fallout in the atmospheric fallout in Kagoshima City is caused by heavy ashfall associated with eruptions of Sakurajima volcano. Sakurajima is one of the most active volcanoes in Japan, and its repeated explosive eruptions cause large amounts of ash to fall on Kagoshima City. The fallout of crust-derived natural radionuclides, 40K, 212Pb, and 214Bi, from the atmosphere in Kagoshima City showed a significant correlation with the number of eruptions of Sakurajima volcano and the amount of ashfall in Kagoshima City. In contrast, no significant correlation was found between 40K and 7Be fallout. The 40K fallout indicates that almost all of the atmospheric fallout in Kagoshima City is composed of volcanic ash particles. The contribution from mineral and sea salt particles other than volcanic ash is minimal. The mass balance of the observed 40K fallout, ashfall, and atmospheric fallout yield indicates that there is a significant amount of volcanic ash deposition that is not accounted for as ashfall. In most cases, the ash deposition observed as ashfall is only 30–70 wt% of the total deposition collected as atmospheric fallout samples, and the remaining portion is fine-grained and behaves as suspended volcanic ash particles, which significantly impact the atmospheric environment.

Open Science Initiatives by Sakurajima Volcano Observatory

The sudden eruption and tragedy of Mt. Ontake in 2014, a volcano located in central Japan, showed the fact that the volcanic eruption is the event with uncertainty, and it is important to let citizen to be aware of such uncertainty. To find measures to raise citizen’s disaster awareness, this study focuses on the risk communication between citizens and volcano observatories, which are attached to universities. It examines the role of observatories, focusing on the activities of the Sakurajima Volcano Research Center, which monitors Mt. Sakurajima, one of the most active volcanoes in Japan, and suggests the necessity of human resource development that is able to connect citizen and science.

Temporal changes in lahar sediment run‐off characteristics and run‐off coefficients in the Arimura River basin of Sakurajima volcano, Japan

AbstractThe rapid flowing nature of lahars can cause serious hazards in the proximal to distal areas of volcanoes. The accumulation of ash can form an impermeable layer on the ground and prevent the infiltration of precipitation. The surface water thus formed erodes the ash layer and causes the surface permeability to rise again. Over time, this accumulation and erosion sequence changes the lahar risk. Large numbers of lahars are observed each year around Sakurajima, which is known as one of the most active volcanoes in Japan. In this study, the temporal change of lahar sediment run‐off properties and run‐off coefficients have been investigated between 2015 and 2020 in the Arimura River basin of Sakurajima volcano, Japan. X Band Multi‐Parameter (XMP) radars are used to gather precipitation data in the basin. The sediment data have been obtained by Load, Vibration, Pressure (LVP) sensors, and a kinematic wave model is adopted to calculate surface run‐off. Lahars are categorized as regular and irregular considering the correspondence of measured and calculated waveforms of the hydrographs based on the kinematic wave model. Analyses reveal that run‐off coefficient changes depend on the volcanic activity and decreases after an irregular lahar compared with a previous regular lahar. LVP sensor analysis results show that sediment concentrations of the lahars change in time. It is found that irregular and regular lahars are distinct in terms of velocity and sediment concentrations. It is estimated that irregular lahars carry larger material than regular lahars based on the vibration data. This study underlines that the properties of lahars change over time under the influence of ash fall thickness, and these changes can be detected in detail with our kinematic wave model and LVP sensors.

The proximal volcaniclastic materials of Fukutoku‐Oka‐no‐Ba in the Izu‐Bonin arc show contrasting characteristics to the drift pumice of the 2021 eruption

AbstractFukutoku‐Oka‐no‐Ba is a submarine volcano located at 24°17.1′ N/141°28.9′ E in the Izu–Bonin arc, and is one of the most active volcanoes in Japan. This volcano produced an explosive eruption in August 2021 that generated a large amount of volcaniclastic material, some of which drifted westward to Japan and the coastal area of East Asia as a pumice raft. The pumice clasts that drifted for >1000 km were mostly homogeneous and identical to those produced by past historical eruptions. The clasts have trachytic compositions (SiO2 = 61–63 mass% and Na2O + K2O = 8.6–10.0 mass%) and contain augite, plagioclase, olivine (Mg# ~65), and magnetite, along with a small number of mafic enclaves containing diopside and high‐Mg olivine (Mg# ~ 92). We undertook a research cruise to investigate the proximal volcaniclastic materials by dredging. The proximal materials include pumice, weakly vesiculated lapilli, and volcanic blocks, which have trachytic composition (SiO2 contents up to 64.5 mass%). The main minerals in the proximal material are similar to those in the drift pumice, although remnants of mafic magma do not occur in the SiO2‐rich samples. The petrographic and geochemical characteristics of the proximal and drift ejecta from Fukutoku‐Oka‐no‐Ba suggest the magma reservoir was stratified into two parts. The major part experienced magma mixing with a limited volume of mafic magma, whereas the other part was more differentiated. The differentiated high‐SiO2 magma accumulated in the upper part of the magma reservoir and avoided the mixing with and feed of volatile from the mafic magma, then were pushed out from the volcanic vent without extensive bubbling to sunk in the proximal area.

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.

Influence of hydrothermal recharge on the evolution of eruption styles and hazards during the 2018\u20132019 activity at Kuchinoerabujima Volcano, Japan

The activity of the 2018–2019 eruption of Kuchinoerabujima Volcano in Japan changed from continuous ejection of ash-laden plumes between October 21 and the middle of December, to intermittent explosive activity accompanied by several pyroclastic density currents until January 2019. To understand the behaviors of magma and hydrothermal fluid that controlled the eruptive sequence, we carried out component analysis, X-ray diffractometry, and leachate analysis for ash samples. The proportion of non-altered volcanic ash particles is ~ 15% in the earlier phase, then it decreased to less than 10% in the later explosive phase. Accordingly, the mineral assemblage of the volcanic ash samples changed from plagioclase-dominant to sulfate minerals-dominant. Concentration of SO42− and Cl/SO4 values of the ash-leachates decreased toward the later activity. These results indicate that the proportion of fresh volcanic rocks decreased and sulfuric acid fluid-derived sulfate minerals increased toward the later activities. Consequently, the 2018–2019 eruption at Kuchinoerabujima Volcano changed from magmatic activity to phreatomagmatic activity. Weak glowing of the crater was observed during the magmatic activity, indicating the volcanic conduit was hot enough to dry up the sub-volcanic hydrothermal system. The following phreatomagmatic activity indicates that the hydrothermal fluid recharged after the magmatic eruption phase. Recharge of the hydrothermal fluid likely caused the variation of the eruption style, and is a process that may control the evolution of hazards during future eruption scenarios at similar active volcanoes in Japan and worldwide.Graphical

An improvement on shelter airport selection model during large-scale volcanic disasters: A case study of Hakoneyama Japan

Through the movement of passengers and cargo, air transportation contributes to economic growth and prosperity. As it expands in size and complexity, it becomes more susceptible to disruptions caused by weather conditions and natural disasters, such as volcanic eruptions. The eruptions of the Eyjafjallajökull and Merapi volcanoes caused significant problems in Europe and central Indonesia concerning air traffic and the economy in 2010. Numerous active volcanoes in Japan could severely disrupt the country's airspace and damage aircraft. The study is aimed at the challenge of determining how to select an airport as a safe place to land during an aircraft evacuation in the event of a volcanic eruption. Airside, airline, and historical data were used to design the shelter airport selection model. Later, it was used to discover an approximate solution for aircraft evacuation using a genetic algorithm (GA).

Detection of low-frequency earthquakes by the matched filter technique using the product of mutual information and correlation coefficient

The matched filter technique is often used to detect microearthquakes such as deep low-frequency (DLF) earthquakes. It compares correlation coefficients (CC) between waveforms of template earthquakes and the observed data. Conventionally, the sum of CC at multiple seismic stations is used as an index to detect the DLF earthquakes. A major disadvantage of the conventional method is drastically reduced detection accuracy when there are too few seismic stations. The new matched filter method proposed in this study can accurately detect microearthquakes using only a single station. It adopts mutual information (MI) in addition to CC to measure the similarity between the template and target waveforms. The method uses the product of MI and CC (MICC) as an index to detect DLF earthquakes. This index shows a distinct peak corresponding to an earthquake signal in a synthetic data set consisting of artificial noise and the waveform of a DLF earthquake. Application of this single-station method to field observations of Kirishima volcano, one of the most active volcanoes in Japan, detected a total of 354 events from the data in December 2010, whereas the catalog of the Japan Meteorological Agency shows only two. Of the detected events, 314 (89%) are likely DLF earthquakes and other events may be false detections. Most of the false detections correspond to surface-wave arrivals from teleseismic events. The catalog of DLF earthquakes constructed here shows similar temporal behavior to that found by the conventional matched filter method using the sum of the CC of the six stations near the volcano. These results suggest that the proposed method can greatly contribute to the accurate cataloging of DLF earthquakes using only a single seismic station.Graphical

Crisis hazard assessment for snow-related lahars from an unforeseen new vent eruption: the 2018 eruption of Kusatsu-Shirane volcano, Japan

Two-thirds of the 111 active volcanoes in Japan are covered with snow for several months during winter and demonstrate high hazard and risk potentials associated with snow-related lahars during and after eruptions. On 23 January 2018, a sudden phreatic eruption occurred at the ski field on Kusatsu-Shirane (Mt. Motoshirane) volcano, Japan. This new vent eruption from the snow-clad pyroclastic cone required forecasting of future snow-related lahars and crisis hazards zonation of downslope areas including Kusatsu town, a popular tourist site for skiing and hot springs. In order to achieve a prompt hazard assessment for snow-related lahars, a multidisciplinary approach was carried out involving characterization of proximal tephra deposits, snow surveys, and numerical lahar flow simulations using the Titan2D model. To determine the input parameters for the flow model, the consideration of snow water equivalent (SWE) immediately after the eruption (on 29 January) and in the post-eruptive period (on 12 March), was significant. In the case of Kusatsu-Shirane volcano during the winter of 2018, linear relationships between altitude and SWE, obtained at different elevations, were used to estimate the snow volume around the new vents. Several scenarios incorporating snow and snowmelt (water), with or without the occurrence of a new eruption, were simulated for the prediction of future lahars. Three lahar scenarios were simulated, including A) rain-on-snow triggered, B) ice/snow slurry, and C) full snowmelt triggered by a new eruption, and indicated the flow paths (inundation areas) and travel distances. These were useful for lahar hazard zonation and identification of potential high-risk areas. Since the input parameters required for the Titan2D flow model can be relatively easily determined, the model was suitable for the 2018 eruption at Motoshirane where historical and geological lahar records are not available for calibration. The procedure used in the study will enable rapid lahar prediction and hazard zonation at snow-clad volcanoes. Further consideration for simulating a cohesive-type flow, which was predicted by the primary deposits containing large amounts of clay minerals and could not be expressed in the Titan2D flow model, is necessary.

Conditions of magma generation at the Me-akan volcano, northern Japan

The Me-akan volcano is one of the most active volcanoes in Japan. In this study, the conditions of magma generation were estimated for basaltic products derived from Akan-fuji, the youngest mafic volcanic edifice at Me-akan, to elucidate the magmatic system. The volcanic products show significant variations in their whole-rock compositions (50.1 to 52.6 wt% SiO2), and can be divided into a low-SiO2 group (<50.4 wt% SiO2) and a main group (>50.7 wt% SiO2) based on Pb isotopic variations. The rocks exhibit the following two groups of phenocryst assemblages: one group contains high-Fo [100 × Mg/(Mg + Fe)] olivine and high-An [100 × Ca/(Ca + Na)] plagioclase phenocrysts (A group), and the other consists of low-Fo olivine, pyroxene, and low-An plagioclase phenocrysts (B group). Our study demonstrates that both the low-SiO2 group and the main group magmas were produced by assimilation and fractional crystallization of distinct basaltic magmas and via subsequent mixing with more primitive magmas in lower crustal magma chambers. It is suggested that the A-group phenocrysts were derived from the primitive magmas, whereas the B-group phenocrysts crystallized in situ before and/or after the occurrence of magma mixing. Multi-component thermodynamic analyses suggest that the primary magmas were segregated from the mantle at 1210–1230 °C and 0.9–1.0 GPa. The observations on estimated melt segregation depths of ~30 km are consistent with seismological observations that low-frequency micro-earthquakes (M 0.0–2.5) occur actively in the uppermost mantle. The generation of primary magmas involved slab-derived fluids of similar composition for both groups. The primary magma for the main group was generated by a greater contribution of the slab-derived fluid relative to the mantle peridotite than that of the low-SiO2 group.

In Situ, Rotor-Based Drone Measurement of Wind Vector and Aerosol Concentration in Volcanic Areas

Unmanned aerial vehicles (UAVs), represented by rotor-based drones, are suitable for volcanic observations owing to the advantages of mobility and safety. In this study, vertical profiles of wind and aerosol concentrations at altitudes up to 1000 m around Mt. Sakurajima, one of the most active volcanoes in Japan, were measured in situ using a drone equipped with an ultrasonic anemometer and aerosol sensor. The drone-measured wind profiles were compared with Doppler LiDAR data and analysis values derived from a meteorological model. Drone-measured vertical profiles collected at a vertical speed of 1 m·s−1 (upward and downward) showed strong agreement with the LiDAR observations, as did the averaged values of hovering drone measurements. Obvious vertical wind shear was found by the drone in the vicinity of Mt. Sakurajima. An aerosol sensor was installed on the drone with the capability to measure fine (PM2.5) and coarse particles (PM10) simultaneously; in this manner, volcanic ash and aerosol pollutants around the volcano could be distinguished. Thus, it was proven that drones could be applied to investigate wind conditions and aerosols in situ, even at dangerous locations near active volcanoes.

Message from the Winner

A volcanic eruption is a phenomenon in which ballistic bombs, lapilli, volcanic ash, lava, and gas are discharged. Volcanic ash and gas are carried by the wind, and pyroclastic flows and lava flows are carried away by the force of gravity. These cause disasters of various forms in the areas around volcanoes, sometimes far from eruptive center. Accordingly, volcanic countries, particularly Asian countries such as Japan, Indonesia, and the Philippines, have been the scenes of volcanic disasters. We conducted the research project “Integrated study on mitigation of multimodal disasters caused by the ejection of volcanic products” with the Center for Volcanology and Geological Hazard Mitigation and other institutes in Indonesia under the SATREPS project from FY2013 to 2018. The aim of the project was to advance volcanic hazard mitigation, and I served as the guest editor of “Special Issue on Integrated Study on Mitigation of Multimodal Disasters Caused by Ejection of Volcanic Products” (2016) and “Special Issue on Integrated Study on Mitigation of Multimodal Disasters Caused by Ejection of Volcanic Products: Part 2” (2019) of the Journal of Disaster Research. The articles in the Special Issues have been downloaded by many researchers. The Special Issues cover many topics related to volcanic disasters, but the main theme is how to forecast real-time volcanic hazards using data monitoring, since it is this monitoring that triggers the issuing of warnings. I have studied the volcanic activity of Sakurajima, the most active volcano in Japan, for 40 years, primarily to forecast its eruptions. Forecasting the eruptions is not as important as forecasting the hazards and risks posed by volcanic actions. Research done on the mitigation of the volcanic hazards of Sakurajima as well as Indonesian volcanoes has been enhanced by interaction. The cumulative volume of magma stored in the past 100 years indicates that Sakurajima has the potential for a large-scale eruption (VEI &gt; 4). An eruption and its dispersal of volcanic ash in particular would cause a variety of disasters over a wide area, as described in the other issues of Journal of Disaster Research. I hope that the research results will be utilized for hazard mitigation in the event of future large-scale eruptions. The research could be advanced through collaboration with studies aimed at the enhancement of resilience and recovery.

Can Eruptions Be Predicted? Short-Term Prediction of Volcanic Eruptions via Attention-Based Long Short-Term Memory

Short-term prediction of volcanic eruptions is one of the ultimate objectives of volcanology. At Sakurajima volcano, an active volcano in Japan, experts monitor the volcanic sensor data and analyze the prior signal to predict the eruptions. Even though experts derived some patterns, it is hard to make a good prediction due to handcrafted features. To address this issue, we propose to predict eruptions using machine learning. In this paper, we attempt to predict the eruptions hourly by adapting several machine learning methods including traditional and deep learning approaches. As recurrent neural network is well-known for extracting the time-sensitive features, we propose the model especially for volcanic eruption prediction named VepNet. The assumption is based on domain knowledge that some specific triggers are the main causes of future eruptions. To take this advantage, VepNet deploys an attention layer to locate and prioritize these triggers in decision making. The extensive experiments ever conducted using data from Sakurajima volcano showed the effectiveness of deep learning approach over the traditional approach. On top of that, VepNet showed its effectiveness on prediction with AUC-score up to 0.8665. Moreover, an attempt has been made to explain the mechanism of the eruptions by analyzing the attention layer of VepNet. Lastly, to support volcano expert in issuing warnings and the safety of living people around Sakurajima, a warning system named 3LWS is proposed. The system predicted the eruptions hourly with high accuracy and reliability with the eruption rate up to 68.97% in the High-Risk level.

Estimation of water seepage rate in the active crater lake system of Kusatsu-Shirane volcano, Japan, using FDNPP-derived radioactive cesium as a hydrological tracer

We describe here a first attempt to estimate the water seepage rate of an active crater lake using radioactive cesium dispersed into the environment by the Fukushima Dai-ichi Nuclear Power Plant accident in March 2011 as a hydrological tracer. Kusatsu-Shirane volcano is one of the most active volcanoes in Japan and has an active crater lake named Yugama. There is no outflow such as a river from Yugama crater lake. The content level of stable cesium (133Cs) in Yugama water was almost constant at 15–20 nM during the sampling period of Nov. 2011 to Nov. 2014. In contrast to 133Cs, however, the radioactive cesium (134Cs and 137Cs) concentrations in Yugama water decreased at a more rapid rate than expected by radioactive decay. Based on the decreasing rates of activity concentrations of 134Cs and 137Cs in Yugama water during the three years between 2012 and 2014, it is estimated that 700–800 m3 of the Yugama water, which corresponds approximately to 0.1% of the total volume, leaks through the lakebed per day. In the estimation, balance between leakage of radioactive cesium contaminated water though lake bed and geothermal inflow of radioactive cesium-free water in the volcano was taken into account. Consequently, the water seepage rate of the Yugama crater lake was calculated to be 8.1–9.3 L s−1.

Surface deformation of Asama volcano, Japan, detected by time series InSAR combining persistent and distributed scatterers, 2014\u20122018

Asama volcano is one of the most active volcanoes in Japan. Spatially dense surface deformation at Asama volcano has rarely been documented because of its high topography and snow cover around the summit. This study presents the first interferometric synthetic aperture radar (InSAR) observation of ground deformation at Asama volcano with 120 Sentinel-1 SAR images from both ascending and descending tracks and 20 descending ALOS-2 images acquired between 2014 and 2018. We exploited both persistent and distributed scatterers to overcome decorrelation of SAR signals and applied a three-dimensional unwrapping method to retrieve the displacement time series efficiently. Our observations reveal an asymmetric deformation around the volcano with two main deformation regions on the northeast and southeast flanks, respectively. The northeast flank (NEF) exhibits line-of-sight (LOS) extensions in all the three SAR datasets with maximum velocities of − 14, − 10, and − 12 mm/year for the descending ALOS-2, ascending, and descending Sentinel-1 measurements, respectively. The southeast flank (SEF) shows LOS extensions in the ascending observations and LOS shortening in the descending observations with velocities between − 12 and 9 mm/year. Decomposition of the LOS displacements reveals nearly pure subsidence at the NEF, while the SEF exhibits a substantial eastward component as well as subsidence. Comparisons of the vertical subsidence at two continuous GNSS stations near the summit crater with our InSAR observations indicate small discrepancies smaller than 4 mm/year. We interpreted that the subsidence at the NEF of Asama is primarily due to the hydrothermal activity, while the deformation at SEF is plausibly due to flank instability. We highlight that efforts should be taken to monitor the slope instability at Asama volcano in the future.

Hydrothermal Aluminum-Phosphate-Sulfates in Ash from the 2014 Hydrothermal Eruption at Ontake Volcano, Central Honshu, Japan

Aluminum-phosphate-sulfates (APS) of the alunite supergroup occur in igneous rocks within zones of advanced argillic and silicic alteration in porphyry and epithermal ore environments. In this study we report on the presence of woodhouseite-rich APS in ash from the 27 September 2014 hydrothermal eruption of Ontake volcano. Scanning electron microscope coupled with energy dispersive X-ray spectrometer (SEM-EDS) and field emission (FE)-SEM-EDS observations show two types of occurrence of woodhouseite: (a) as cores within chemically zoned alunite-APS crystals (Zoned-alunite-woodhouseite-APS), and (b) as a coherent single-phase mineral in micro-veinlets intergrown with similar micro-veinlets of silica minerals (Micro-wormy-vein woodhouseite-APS). The genetic environment of APS minerals at Ontake volcano is that of a highly acidic hydrothermal system existing beneath the volcano summit, formed by condensation in magmatic steam and/or ground waters of sulfur-rich magmatic volatiles exsolved from the magma chamber beneath Mt. Ontake. Under these conditions, an advanced argillic alteration assemblage forms, which is composed of silica, pyrophyllite, alunite and kaolinite/dickite, plus APS, among other minerals. The discovery of woodhouseite in the volcanic ash of the Ontake 2014 hydrothermal eruption represents the first reported presence of APS within an active volcano. Other volcanoes in Japan and elsewhere with similar phreatic eruptions ejecting altered ash fragments will likely contain APS minerals derived from magmatic-hydrothermal systems within the subvolcanic environment. The presence of APS minerals within the advanced argillic zone below the summit vent of Ontake volcano, together with the prior documentation of phyllic and potassically altered ash fragments, provides evidence for the existence within an active volcano in Japan of an alteration column comparable to that of porphyry copper systems globally.

Magma Plumbing System at Izu-Oshima Volcano, Japan: Constraints From Petrological and Geochemical Analyses

The Izu-Oshima volcano is one of the most active volcanoes in Japan, and has generated relatively large-scale eruptions every 30–40 years for the past 200 years. As more than 30 years have passed since the last eruptions in 1986–87, volcanic activity is expected to resume in the near future. To help elucidate the current and future state of the volcano’s magma system, the temporal evolution of the recent magma plumbing system was investigated through a petrological and geochemical analysis of its basaltic lavas and pyroclastics (<~53 wt.% of SiO2) that were erupted during the last ~1.5 kyrs. The basaltic products have variable phenocryst contents, ranging from ~0 to ~20 vol.%, and phenocryst-bearing samples commonly contain plagioclase, orthopyroxene, and clinopyroxene phenocrysts. The whole-rock compositions are significantly scattered in the Harker variation diagrams, suggesting that the compositional diversity was established by at least two independent magmatic processes. The application of principal component analysis on the whole-rock major element data suggests that one magmatic process was crystal fractionation of crystal-poor magmas, and the other process was either plagioclase accumulation or mixing of plagioclase-rich magmas. Based on this observation, and combined with the petrological analysis and previous geophysical studies, we propose that aphyric magmas, stored in an 8–10 km-deep magma chamber, progressively differentiated over time from the 7th to 20th century. Furthermore, the compositional variations in basalts resulted from the mixing of the differentiating aphyric magmas with variable proportions of porphyritic magmas derived from a 13–18 km-deep magma chamber. Because recent eruptions have been triggered by the ascent of porphyritic magma from the 13–18 km-deep magma chamber, and its injection into the 8–10 km-deep magma chamber, it is important to monitor the deeper magma chamber to predict future volcanic activity.

活火山の地質図整備(＜特集＞火山学50年間の発展と将来)

活火山の地質図整備(＜特集＞火山学50年間の発展と将来)

Structural evolution beneath Sakurajima Volcano, Japan, revealed through rounds of controlled seismic experiments

Structural evolution beneath an active volcano is detected as the variation of seismic reflectivity through controlled seismic experiments, which is interpreted as being associated with discharging magma. The target of the present study is Sakurajima Volcano, which is one of the most active volcanoes in Japan. Six rounds of seismic experiments with controlled sources have been conducted annually at the volcano. Two seismic reflection profiles are obtained from the datasets for each successful round of experiments. The experiments reveal clear annual variation in seismic reflectivity at a depth of 6.2km in the northeastern part of Sakurajima. The reflectivity is maximum in December 2009 upon the first intrusion of magma and decreases gradually until December 2013, which coincides with the inflation and deflation cycle of Sakurajima Volcano. Reflectivity variation occurred in the embedded clear reflector at depth. An evolving sandwiched structure in the intermediate layer is used as the reflector model. Lower-velocity magma embedded in the intermediate layer and its succeeding velocity increment explain the variation range of reflectivity. This is interpreted as a temperature decrease associated with discharging magma at depth. The present study describes a new approach for instantaneously sensing magma properties and for monitoring active volcanoes.

P25 日本活火山総覧(第4版)の刊行(ポスターセッション)

P25 日本活火山総覧(第4版)の刊行(ポスターセッション)