BULLETIN OF THE VOLCANOLOGICAL SOCIETY OF JAPAN
Online ISSN : 2189-7182
Print ISSN : 0453-4360
ISSN-L : 0453-4360
Volume 50, Issue Special
Displaying 1-23 of 23 articles from this issue
  • [in Japanese]
    2005 Volume 50 Issue Special Pages I-
    Published: December 20, 2005
    Released on J-STAGE: March 20, 2017
    JOURNAL FREE ACCESS
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  • Daisuke SHIMOZURU, Kazuhiro ISHIHARA
    Article type: Article
    2005 Volume 50 Issue Special Pages S1-S6
    Published: December 20, 2005
    Released on J-STAGE: March 20, 2017
    JOURNAL FREE ACCESS
    This chapter illustrates briefly why and how the unified volcano surveillance system in Japan has been established called as “Program for the Prediction of Volcanic Eruptions” which has included strategic planning of manifold types. The program successively continues with every five year period improving the substance of the surveillance technique. Some successful case histories relating the commencement of eruptions are described.
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  • Hitoshi YAMASATO
    Article type: Article
    2005 Volume 50 Issue Special Pages S7-S18
    Published: December 20, 2005
    Released on J-STAGE: March 20, 2017
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    Volcano observation is a basic to the fundamental research on volcanism and to the surveillance of volcanic activity for disaster mitigation. In this paper, the author reviews the history of volcano observation in Japan since 19th century, especially the history of volcano surveillance of the Japan Meteorological Agency (JMA) and the recent advancement of the monitoring technique of JMA. The volcano observation in Japan started by Prof. Sekiya on occasion of the eruption of Bandai volcano in 1888. Prof. Omori carried out pioneering observation at Usu volcano in 1910 and established the first volcanological observatory at Asama volcano in 1911. Since then, national universities established observatories at several active volcanoes. Their recent volcanological researches have been endorsed by the National Project of the Prediction of Volcanic Eruptions that started in 1974. JMA's continuous volcano observation started in 1888. JMA had started continuous observation at 10 volcanoes by 1950. In 1962-66, JMA divided active volcanoes into three classes (A, B and C) depending on the level of volcanic activity and the risk of disaster and installed seismographs at three and one stations for A class (4 volcanoes) and B class volcanoes (13 volcanoes), respectively, and organized mobile observation teams for B and C class volcanoes. Since late 1980’s, public concern for volcanic disaster mitigation has risen because of the eruptions at Izu-Oshima volcano in 1986, Unzen volcano in 1991, Usu volcano and Miyakejima volcano in 2000. To promote the disaster preparedness, JMA strengthened observation system and established Volcano Observation and Information Centers (VOIC) at Sapporo, Sendai, Tokyo and Fukuoka and centralized the volcano observation in 2001. VOIC installed TV cameras, seismometers, GPS stations, tiltmeters and infrasonic stations at each volcano. Most of the TV cameras are as sensitive as to detect visual phenomena even at night. These data obtained at each station are telemetered to VOICs and are monitored on a real-time in 24 hours. Each VOIC has Mobile Observation Team, which periodically collects basic observational data from active volcanoes. The observations consist not only of the installation of temporal observation stations but also of periodical thermal, geomagnetic and geodetic surveys. They carry out temporal observations to enhance monitoring capability whenever an unusual phenomenon is detected at volcanoes. VOIC issues Volcano Information to the disaster prevention authorities and to the public to initiate and take relevant disaster mitigation measures. There are three types of Volcano Information, Volcanic Alert, Volcanic Advisory, and Volcanic Observation Report. Since November 2003, JMA has introduced Volcanic Activity Levels as an additional index to Volcano Information for some volcanoes. To indicate the Level, JMA uses 6-level of numerical scheme to reflect an increasing order of unrest: 0 as dormant to 5 as large-scale eruption.
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  • Yasuaki SUDO
    Article type: Article
    2005 Volume 50 Issue Special Pages S19-S25
    Published: December 20, 2005
    Released on J-STAGE: March 20, 2017
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    Volcano observatories in affiliation with universities have been clarifing the volcanic eruption process, educating students, and playing a major role to the enlightenment on volcanic activity to the local goverment and the resident. However, recently the programs, such as strategy for establishing the international headquarters of universities, has been designed as a competitive fund and realized as a competitive research environment in Japanese universities. In addition, the mid-term evaluations will be implemented in the third year for 5 years. As a result, volcano observatories in affiliation with universities will take on new, serious and difficult situations. Here, the significance of volcano observatories will be reported.
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  • Makoto MURAKAMI
    Article type: Article
    2005 Volume 50 Issue Special Pages S27-S47
    Published: December 20, 2005
    Released on J-STAGE: March 20, 2017
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    During the past decade the space geodetic technologies have become a standard tool for monitoring of crustal deformations associated with volcanism. Since early 1990’s the GPS continuous measurement at 1,200 sites distributed throughout Japan have provided relevant information to understand ongoing magmatic processes beneath active volcanoes. During the crises of 1998 Iwate, 2000 Usu, 2000 Miyake and 2004 Asama, the GPS data played an important role to know magma migration sequence related to those volcanic activities. Synthetic Aperture Radar Interferometry (InSAR) also demonstrated its usefulness in volcanological applications. This technology enables us to map deformation field over a large spatial area without installing ground based observational instruments. During the 1998 Iwate crisis InSAR data acquired by JERS-1 satellite radar (functional during 1992-1998) provided key information about the deformations overlooked by other geodetic measurements. The developments of those space geodetic technologies are continual and further improvements will be achievable in terms of accuracy, rapidness of processing and completeness of temporal and spatial coverage. The geodetic data set will continue to be one of the major sources of information to understand volcanism in the forthcoming future.
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  • Shuhei OKUBO
    Article type: Article
    2005 Volume 50 Issue Special Pages S49-S58
    Published: December 20, 2005
    Released on J-STAGE: March 20, 2017
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    We describe in this paper recent development of gravity observation and theoretical works on gravity change that enable us to infer mass transport during volcanism with reasonable accuracy. Combined use of both absolute and relative gravity measurement, namely hybrid gravity measurement, gives us "absolute" gravity data on local to regional scales. The data merged with crustal movement can be inverted for physical models of volcanism. The strategy gives us invaluable information how underground masses were transported during the event of Miyake-jima eruption in 2000 and that of Mt. Asama in 2004.
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  • Izumi YOKOYAMA
    Article type: Article
    2005 Volume 50 Issue Special Pages S59-S76
    Published: December 20, 2005
    Released on J-STAGE: March 20, 2017
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    One of the outstanding targets of physical volcanology is searches for the real nature of magma reservoirs. Calderas and their formation have been long disputed in relation with magma reservoirs. Calderas must be defined by their subsurface structures not by their localities. From such a standpoint, calderas are classified into two: high gravity-anomaly type and low gravity-anomaly type because gravity anomalies reflect basement structures and density contrasts of caldera fills. The hypotheses of caldera formation in the early 20th century had not taken consideration of the following factors: Their subsurface structures, depths of magma reservoirs and strength of the crust. Some models of caldera formation depend on inappropriate analogy between small-scale geologic events and large-scale ones. First, strength of the earth crust is reviewed in laboratory and field measurements, with the distinction between shallow and deep parts of the crust. The two mechanical models of caldera formation are criticized from the standpoint of strength of the upper crust: a) “Model of cap rock”: After a simple calculation, it is proved that calderas smaller than 3km in diameter may be interpretable by this model. b) "Model of collapse into an emptied magma reservoir": Stress distribution due to collapse of a magma reservoir is estimated under some assumptions. The stress decreases in inverse proportion to square of the distance from the origin. Then, depths of magma reservoirs have much importance to the stresses at the earth surface caused by their collapses. Then, reliable geophysical data of locations of magma reservoirs are collected, and their average depth is provisionally determined as 10km. After the above estimates, it may be said that calderas can be scarcely formed by collapses of magma reservoirs measuring a few kilometers in diameter and locating at depths deeper than 5km. The hypothesis that calderas were formed by collapses of pre-caldera volcanoes in a body into magma reservoirs, is examined by their seismic effects in some historical eruptions resulting calderas or larger craters. A few eruptions were accompanied with earthquakes of M 7; However, they can be interpreted as magmatotectonic earthquakes. Many eruptions were usually accompanied with earthquakes of M 5 or smaller than M 6, and the earthquakes were not always simultaneous with the large eruptions. The hypothesis proved to be unreasonable. To exemplify the actual state of understanding for caldera structure, the three typical calderas in Japan, Hakone, Aso and Aira calderas are briefly described. Some volcanological data newly acquired on these calderas are introduced and additionally interpreted. Volcanic processes confirmed on such Japanese calderas should be substantially common to all calderas on the earth even if they may take some different features. It is finally concluded that calderas are composites (or amalgamations) of plural explosion craters which are often accompanied with violent eruptions of pyroclastic material and that calderas are not produced by collapses of volcanoes into emptied magma reservoirs.
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  • Yuichi MORITA, Takao OHMINATO
    Article type: Article
    2005 Volume 50 Issue Special Pages S77-S100
    Published: December 20, 2005
    Released on J-STAGE: March 20, 2017
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    Recent advances in the volcano-seismology largely depend on the advances of the technology in the field of seismic observation at volcanoes. Many new insights about the volcanic phenomena and quantitative measurements of magma emplacement are brought about by the modern seismic observation. In this paper, we mention key technologies that have brought a progress in volcano-seismology, some results and interpretations on precise hypocentral distributions around volcanoes and those on long period ground motion observed at volcanoes. Modernizations of seismic observation around volcanoes are summarized into the following two points. 1) The modern observation system has higher precision and wider dynamic range even it consumes less electric power than old-fashioned system did. The timing is synchronized to GPS timing in modern systems. These advances make us easily to construct more densely and evenly spaced seismic network with high performance even around volcanoes where there are often no commercial services of electricity and telephone. 2) The practical use of mobile broadband seismometer close to the vent of many volcanoes presents us that many kinds of long period ground motion occur around volcanoes and brings us many observational evidences of ground motions caused by magma and/or gas emplacement. The seismograms with high sampling rate, high resolution and wide dynamic ranges from densely and evenly arranged seismic network improve the precision in hypocentral distributions around volcanoes drastically. With the help of the modern hypocentral determination method, hypocenters that distribute dimly like clouds are relocated on a thin penny-shaped distribution, sharply aligned pipe, several parallel aligned planes, and so on. These systematic distributions in hypocenters are strong evidence that earthquakes around volcanoes are caused by simple volcanic processes like magma intrusion, slipping on the pre-existing fault, and so on. Several examples are introduced in this paper. Long-period ground motions (LP motion) observed in the vicinity of volcanoes are well-known in many previous studies, but systematic understanding on this vibration has not been examined enough until the last 10 years. Prevailing usage of mobile broadband seismometers provides us many observational evidences of LP motion around volcanoes and they give us much progress in interpreting their sources. Recent researches show many kinds of possible sources models that generate LP motion, and they present realistic images of magma emplacement. In many cases introduce in this paper, the generation of LP motion is related to the motions of the volcanic fluid like magma and/or volcanic gases. The feature in the wave forms of LP motion gives us important information on the physical property of volcanic fluid trapped in crack and/or magma reservoir. The advance in seismic observations around volcanoes will play more important role to better understand the volcanic process quantitatively. Further development in the seismic observation is needed to progress in volcanic seismology.
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  • Tomoki TSUTSUI
    Article type: Article
    2005 Volume 50 Issue Special Pages S101-S114
    Published: December 20, 2005
    Released on J-STAGE: March 20, 2017
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    Recent achievements in seismic studies of volcanic structure are summarized in this article. Seismic studies of volcanic structure was enhanced and advanced extensively in 1990's. The advance of the study was supported by new compact and light weighted digital recorders and by popularization of 3-D travel time inversion algorithm. Seismological characters in volcanic structure were revealed and were represented as high and low velocity anomalies, concentrated seismicity and intra-crust seismic reflectors. Important problems for seismic studies of volcanic structure are presented as more penetration, more resolution and more quantification of seismic characters in active volcanoes, which should be solved in next decade.
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  • Takeshi HASHIMOTO
    Article type: Article
    2005 Volume 50 Issue Special Pages S115-S138
    Published: December 20, 2005
    Released on J-STAGE: March 20, 2017
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    The present paper summarizes the history of applied electromagnetism for field volcanology in Japan and abroad. Achievements are presented especially on volcanomagnetic effect, resistivity structure and its time change, and volcanoelectric effect, with a special attention to the social requirements for eruption prediction. Japan has been one of the leading countries in this research field in the past half century. In particular, geomagnetic filed measurement is recently attracting a growing interest as a regular and standard item for volcano monitoring in this country. Other electric or electromagnetic methods, such as resistivity sounding or selfpotential, have also contributed to reveal shallow processes involving volcanic fluids beneath active volcanoes preceding, during, and after their eruptions. The author also proposes some future directions in developing measurement techniques to improve the spatial coverage, resolution, depth penetration as well as to seek for safer data acquisition.
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  • Masato IGUCHI
    Article type: Article
    2005 Volume 50 Issue Special Pages S139-S149
    Published: December 20, 2005
    Released on J-STAGE: March 20, 2017
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    Mechanical process of explosive eruptions revealed by seismic and ground deformation observations are reviewed at mainly Sakurajima and some volcanoes. From macroscopic view point, mechanical process of explosive eruption is approximated by expansive process due to intrusion and storage of magma in the volcanic body prior to the beginning of the explosion, and implosive process by ejection of magma and downward single force as a counter force against ejecting product after instantaneous removal of lid of the pressurized magma chamber. More detailed process of explosions has been estimated at Sakurajima volcano. Prior to the explosions, a pressurized gas pocket is formed under the lava dome as a lid of it. Increase in internal pressure in the expansive process of the conduit start several minutes to several hours before the explosions, and it may turn decrease in pressure by weak leakage of volcanic gas immediately before the explosions, as inferred from displacement seismograms of explosion earthquakes and visual observation at Suwanosejima volcano. The decrease in pressure induces sudden bubbling at a depth of 2km causing compression first motions of explosion earthquake. Contraction of volcanic conduit follows the expansion. Expansion of the gas pocket at the uppermost part of the conduit occurs ~1 s after the first expansion, destroying the lid of the pressurized gas pocket in the conduit. After removing the lid, sudden pressure and volume decrease occur at the top of the conduit, and the deflation of magma reservoir occurs at deeper part by ejecting magma.
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  • Takehiro KOYAGUCHI
    Article type: Article
    2005 Volume 50 Issue Special Pages S151-S166
    Published: December 20, 2005
    Released on J-STAGE: March 20, 2017
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    About thirty years ago Walker (1973) proposed a diagram to classify explosive volcanic eruptions producing pyroclastic falls on the basis of extensive field data. This diagram has provided us a framework to quantitatively describe explosive eruptions in terms of the characteristics of fall deposits and has also motivated us to develop theoretical models of explosive eruptions. Some of the universal relationships observed in this diagram, such as the positive correlation between the degree of tephra dispersal and the degree of magma fragmentation from sub-plinian to ultra-plinian eruptions may be useful as criteria to verify existing theoretical models. Recent progress in theoretical models of eruption columns, conduit flow, bubble nucleation and growth and magma fragmentation is reviewed from the viewpoint how these models are verified by field data of pyroclastic fall deposits.
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  • Hiroshi SHINOHARA
    Article type: Article
    2005 Volume 50 Issue Special Pages S167-S176
    Published: December 20, 2005
    Released on J-STAGE: March 20, 2017
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    Monitoring of volcanic gas composition and flux can provide us the information about the condition of the magma and hydrothermal systems beneath the volcano. Situations of the degassing volcanoes can be classified into two types, one with an underlying hydrothermal system and the other without it discharging the magmatic gases directly from magmas. In order to understand the processes preceding the eruption in the former system, we need theoretical modeling of the hydrothermal system both for physical and chemical responses to the volcanic activity changes. Various techniques are recently developed for monitoring of volcanic gas composition and fluxes directly discharged from magmas, including mini-DOAS for flux measurement and Portable Multi-Sensor System for composition measurement. Comparison of these results with magma discharge rate, volatile contents in magma estimated from melt inclusions enabled to model the degassing processes of volcanoes, such as degassing of convecting magma column. Since the volcanic gas data can show us different aspects of volcanic activities from geophysical monitoring, and interdisciplinary comparison of the data and models will be one of the promising area of volcanology. In order to compare the geophysical data with good time resolution, we also need further development and application of these volcanic gas monitoring techniques.
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  • Genji SAITO
    Article type: Article
    2005 Volume 50 Issue Special Pages S177-S192
    Published: December 20, 2005
    Released on J-STAGE: March 20, 2017
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    Volatile content in magma is one of the important controlling factors of magma ascent and volcanic eruptions. Melt inclusion analysis is a powerful method to estimate volatile concentration of pre-eruptive magma in a magma chamber. In the past 10-15 years, improvements in techniques for analysis of melt inclusions have advanced, and now melt inclusions are increasingly used as a volcanological tool to unravel magmatic processes. Important information on magma ascent and eruption processes obtained by melt inclusion analysis is (1) magma degassing and evolution processes and gas saturation pressure, (2) density of magma, (3) bubble volume in a magma chamber and (4) volume of degassed magma. Melt inclusion analyses applied to basaltic to rhyolitic eruptions reveal various behavior of volatile components in magma plumbing system; bubble formation in a upper part of a chamber due to fractional crystallization, volatile exsolution with pressure decrease, supply of volatile material from deeper magma to a upper magma chamber, accumulation of bubbles in a shallow magma chamber, and degassing of a magma chamber by magma convection in a conduit. Comparison of these results with geological and geophysical observation on structure of crust beneath volcanoes enable us to model magma plumbing system and magma ascent and eruption processes.
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  • Tetsu KOGISO
    Article type: Article
    2005 Volume 50 Issue Special Pages S193-S207
    Published: December 20, 2005
    Released on J-STAGE: March 20, 2017
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    Partial melting of mantle material in the Earth’s interior is one of the essential processes responsible for the thermal and chemical evolution of the Earth. Since peridotite is the predominant lithology in the upper mantle, peridotite is thought to be the principal source for various types of magmas and there have been a number of experimental studies on partial melting of peridotite. Recent technical progresses in high-pressure melting experiments have allowed precise determination of chemical composition of peridotite partial melts produced at fairly low degrees (<5 wt%) of melting up to ~1.5GPa, which will in turn allow developing more realistic models for genesis of magmas in the context of fractional melting, especially at mid-oceanic ridges. On the other hand, partial melting of pyroxenite, the subdominant lithology in the mantle, has recently been recognized to play important roles in magma genesis. Melting interval of pyroxenite is much smaller than that of peridotite, and therefore pyroxenite-derived melt contributes to magma generation in much higher proportion than the pyroxenite proportion in the source. An important aspect of pyroxenite partial melting is the existence of garnet-pyroxene thermal divide defined by the enstatite -Ca-Tschermaks pyroxene - diopside plane in CaO-MgO-Al2O3-SiO2 projections at pressures higher than ~2GPa. Pyroxenites that plot on either side of this thermal divide produce distinct types of partial melts, which are quite similar to either alkali-basaltic or tholeiitic magmas generated at hotspot volcanoes.
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  • Mitsuru OKUNO
    Article type: Article
    2005 Volume 50 Issue Special Pages S209-S217
    Published: December 20, 2005
    Released on J-STAGE: March 20, 2017
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    Volcanic stratigraphy and radiocarbon (14C) dating have contributed in establishing the chronological framework of eruptive histories of Japanese volcanoes during the past 10,000 years. This paper presents an overview of 14C dating as well as other dating methods used for eruptive products with emphasis on new technical aspects that were developed in the last decade. A number of eruptive ages of many volcanoes in Japan were determined by 14C dating using accelerator mass spectrometry (AMS), which can measure very small samples i. e. 1mg of carbon. AMS is the most convenient method to use in dating paleosols which directly underlie tephra layers. The 14C calibration curve and programs are widely available. The 14C wiggle matching for single annual-ring is useful for high-accurate dating. Other methods such as fission-track dating, thermoluminescence dating, and paleomagnetic age determination are also applicable for the Holocene eruptive products. To establish and refine the high-resolution eruptive histories, the 14C dating method and other dating methods must be examined in conjunction with detailed stratigraphy.
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  • Shun NAKANO, Hideo HOSHIZUMI
    Article type: Article
    2005 Volume 50 Issue Special Pages S219-S231
    Published: December 20, 2005
    Released on J-STAGE: March 20, 2017
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    Multicolored geological maps of active volcanoes in Japan have been published one after another by Geological Survey of Japan (GSJ). Those published by local governments are rare except by Hokkaido Prefecture during the years 1971-1993, where several highly active volcanoes are located. GSJ has been taking part in the volcanic eruption prediction plan in Japan, and has been preparing geological maps of volcanoes as basic materials of volcanic activities to benefit the prediction of future eruptions. Geological maps contain information concerning history and products of past eruptions which are indispensable to reduction for volcanic disasters. Geological maps of volcanoes published by GSJ are available as a series of “quadrangle series (scale 1:50,000)” or as a series of “geological map of volcanoes (scale un fixed, 1:10,000 to 1:50,000).” The former is accompanied by a voluminous explanatory text, and the latter includes a brief explanation on the back. As of 2005, published by GSJ after the year 1980 are 9 of 13 rank-A (most active), 13 of 36 rank-B and 11 of 36 rank-C volcanoes, whose classifications are defined by Japan Meteorological Agency. GSJ will continue publishing the maps of active volcanoes. Additionally, databases of the above-mentioned “geological maps of volcanoes” series and “Quaternary volcanoes in Japan” are already open to the public on the web site (http://www.aist.go.jp/RIODB/db099/, http://www.aist.go.jp/RIODB/strata/VOL_JP/). In the near future, GSJ will construct an integrated database of Japanese volcanoes including detailed and up-to-date geological information.
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  • Takayuki KANEKO
    Article type: Article
    2005 Volume 50 Issue Special Pages S233-S251
    Published: December 20, 2005
    Released on J-STAGE: March 20, 2017
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    Satellite remote sensing has been used for the analysis of active volcanoes since the mid 1980s when Landsat TM, a high spatial resolution sensor with wide spectral range, became available. Satellite remote sensing technique has advantages in spatial coverage, repeated observation capability, easy global access including remote areas and cost efficiency, compared with conventional ground-based methods. Infrared thermal analysis has been applied to investigation of various volcanic phenomena -degassing of the magma during lava dome growth, cooling and emplacement mechanism of lava flows, documentation of the detailed eruption processes and so on. Frequent time-series images from the metrological satellites have been utilised for investigating relationship between temporal variation of thermal anomalies and eruptive activities. These types of satellites also have been used for developing automated realtime monitoring systems by several institutes to monitor active volcanoes distributing wide areas. On the other hand, thermal infrared images from the hyperspectral satellites have been applied to estimation of SO2 or existence of ash particles in eruption plumes. Next important issue to be explored in volcano remote sensing will be combined analysis among different types of remote sensing data or those and the ground-based observations.
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  • Kazuhiko KANO
    Article type: Article
    2005 Volume 50 Issue Special Pages S253-S272
    Published: December 20, 2005
    Released on J-STAGE: March 20, 2017
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    This paper gives a brief review on the gravity flows sourced from volcanoes on land and under water. Pyroclastic flows are supported by internal gas and the air incorporated during flowage and run out a long distance as density currents. Ash-cloud umbrella is a special case of density current and the particle fallout from the umbrella is a transition to a dilute, pyroclastic density current. Subaqueous equivalents of pyroclastic flows are supported by internal gas and/or the water incorporated during flowage and are thus interpreted as either subaqueous pyroclastic flows in the strict sense or eruption-fed density currents. Debris avalanches and lahars are also important elements of volcaniclastic gravity flows both on land and under water. These pyroclastic and volcaniclastic gravity flows are thought to transform into traction-dominated flow, particle dispersion-dominated flow (grain or granular flow), fluid escape-dominated flow, or debris flows during flowage in response to the changes mainly of flow velocity, particle concentration, and shear stress. The details of these processes still remain in debate. The role of the heat in pyroclastic density current and subaqueous eruption-fed density current is a future subject to be solved.
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  • Kozo UTO, Toshitsugu FUJII, Setsuya NAKADA, Hideo HOSHIZUMI
    Article type: Article
    2005 Volume 50 Issue Special Pages S273-S288
    Published: December 20, 2005
    Released on J-STAGE: March 20, 2017
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    Drilling is a powerful scientific tool to directly investigate the interior of volcanoes, and to collect hidden volcanic materials. Two scientific drilling projects were organized in Japan to investigate the internal structure, to reconstruct the eruptive history, and to reveal the magmatic processes. One is at Fuji and the other is at Unzen. Project of scientific drillings at Fuji volcano was a part of three-year term comprehensive study (April 2001-March 2004) on Fuji, whose future eruption may have serious social impacts. Five drillings were made to the depth between 75 and 650m. Three of them were aimed to install geophysical instruments such as seismometers and tiltmeters. Cores obtained from all drillings were examined to reconstruct the eruptive history and magmatic evolution of Fuji and older volcanoes beneath it. A previously unknown andesitic volcanic body was identified beneath the Komitake volcano. Dominance of debris flow deposits may be due to the cooler climate during the growth stage of Fuji experiencing more frequent interactions between erupted hot materials with snows capping the edifice. Unzen Scientific Drilling Project (USDP) was a six-year term international project (April 1999-March 2005) to understand the growth history, subsurface structure and magma ascending mechanism of Unzen Volcano. Four drillings were made in total, but first two boreholes were vertically drilled at the northeastern flank (USDP-1: 752m) and the eastern flank (USDP-2: 1462m) to reveal the three-dimensional structure and the growth history of Unzen. Later two drillings were to drill into the magmatic conduit of 1990-95 eruption to prove the magma ascent model. USDP-3(350m) was a pilot drilling to construct the drilling strategy for the conduit drilling. The conduit drilling (USDP-4) started from the northern slope of Unzen at 840m asl and directional to below the summit. The conduit zone consisting of multiple EW-oriented vertical dikes and explosion volcanic breccias was found near the sea level, ca. 1.3km beneath the summit area. Volcanic materials in the conduit zone were altered hydrothermally. Pyroclastic veins of the similar orientations are abundant in the conduit zone. Relatively fresh dike between 1975 to 1999m was considered to be the solidified magma of 1990-95 eruption. The temperature was as low as 160℃, much cooler than expected due to the effective circulation of hydrothermal fluids. Magma prefers to ascend forming a new dike independent from older conduits in every eruption event as far as in this depth. Pyroclastic veins may be fossils of isolated tremor events generated by the penetration of vesciculated magma into wall rocks at the tip of intruding magmas.
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  • Masato KOYAMA
    Article type: Article
    2005 Volume 50 Issue Special Pages S289-S317
    Published: December 20, 2005
    Released on J-STAGE: March 20, 2017
    JOURNAL FREE ACCESS
    A critical review was made on public communication/education of knowledge and information about volcanoes and their risk in Japan. Volcanic process can socially be divided into four periods: dormant, pre-emergency, emergency, and restoration/rehabilitation periods. For better mitigation of disasters during all these periods, knowledge and information about volcanoes should enough be shared among volcanologists, officials, and residents around volcanoes. Psychologists well studied the methodology of decision-making and public communication under various risks and many of the results can be applied to volcanic risk. Many volcanologists, however, do not well know the achievements by psychologists. Several Japanese volcanological terms, which have been traditionally used in the public information/education, are ambiguous and have potential for misunderstanding. Journalists often distort the information from volcanologists. The internet may provide a better place for direct risk-communication between volcanologists and residents around volcanoes. Volcanologists should systematically survey the residents and know what method of public communication is the best for sharing risk infomation. The author summarizes the present status of the Japanese system for risk evaluation and announcement during volcanic crises and reviews the problems, which were exposed during the recent volcanic crises in Japan. The author also reviews the status of risk education using hazard maps and/or other methods, which include outreach programs for citizens and schoolchildren.
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  • Shigeo ARAMAKI
    Article type: Article
    2005 Volume 50 Issue Special Pages S319-S329
    Published: December 20, 2005
    Released on J-STAGE: March 20, 2017
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    A hazard map can be utilized as a means of mitigating volcanic disasters. Hazard maps are useful for assessing future volcanic disasters which are difficult to evaluate or forecast using the “deterministic” approach dealing with precise physical models of volcanoes. Instead, a “probabilistic” approach based on the past activity of a volcano can produce useful information such as those expressed in hazard maps. In recent years, hazard maps have been published on 33 active volcanoes in Japan covering most of the volcanic areas which are reckoned to be most susceptible to future eruptions. Hazard maps of Fuji volcano are outstanding in that they are the only maps authored by the national government with the scope of very wide area affected by the volcanic eruption. The maps utilized large amount of data recently collected including pyroclastic flow eruptions and numerical simulations. It is stressed that promotion of the knowledge base related to volcanic activity in general as well as specific details of the past volcanic disasters is the most essential in mitigation of disasters. Longevity of recurrence time of volcanic activity, in comparison with human life cycles makes it very difficult for general public to acquire correct knowledge of how volcanoes erupt. Coupled with bureaucratic systems in which individual officers may not stay in a fixed position of civil defense duty more than 2-3 years, the basic knowledge and preparedness of even the specialists are very meager. As Japanese islands are endowed with rich volcanic landscape and hot springs, it is suggested that we should make most of those natural environment to promote eco-tourism and recreation as well as to promote volcanic disaster mitigation by more closely understanding the nature of volcanic activity.
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  • Article type: Appendix
    2005 Volume 50 Issue Special Pages App2-
    Published: December 20, 2005
    Released on J-STAGE: March 20, 2017
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