著者

宮町 宏樹 小林 励司 八木原 寛

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.67, no.4, pp.471-478, 2022-12-31 (Released:2023-01-30)

参考文献数

9

---

We carried out a temporary seismic observation using 4 ocean bottom seismographs in and around the Aira caldera to obtain the precise hypocenter distribution, especially in the Wakamiko caldera. During the observation period from July to September 2020, we observed two short-time seismic swarms that occurred inside the Wakamiko caldera and revealed the precise distribution of these hypocenters and the focal mechanism solution. In contrast to our results, the JMA unified hypocenters of the two swarms are widely distributed in the western region outside the Wakamiko caldera. This difference in the hypocenter distributions insists that the seismic observation in the sea area around the Wakamiko caldera is required to reveal the precise seismic activity in the caldera.

著者

宮縁 育夫 飯塚 義之 大倉 敬宏

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.67, no.4, pp.441-452, 2022-12-31 (Released:2023-01-30)

参考文献数

12

---

After the July 2019-June 2020 small-scale magmatic activity, surface unrest of the Nakadake first crater, which is located at the center of Aso caldera, SW Japan, had been mostly calm for fourteen months, and a lake had reformed inside the crater by late-August 2021. An eruption producing ballistic clasts and a tephra fall deposit occurred within the first crater of Nakadake at 04:44 on October 14, 2021. A large number of ballistic clasts were distributed from the west-northwestern rim of the Nakadake first crater to the southern rim of the second crater, with ballistics also reaching at least 450 m south of the center of the first crater. The largest clast (70×32×31 cm) was ejected a distance of 300 m SW of the center of the first crater. Several impact craters, which were<1 m in diameter, were observed in the surface ash layer at the crater rim. The ballistic clasts were dominated by basaltic-andesite lithic fragments of lavas and pyroclastic rocks, and were not thought to derive from a newly ascending magma. The tephra fall deposit was distributed to the southeast, extending approximately 30 km from the source crater. At the crater rim, the fall deposits were composed mainly of sand-size particles with small amounts of lapilli (<17 %), and were aggregated at sizes of a few mm to 1 cm. The aggregated muddy ash (a few millimeters in diameter) adhered to plant leaves and the surface of man-made constructions in the southeastern part of Aso caldera (4-10 km), indicating that the rising plume contained large amounts of condensed water vapor. Based on the isomass map, the total discharged mass of the October 14, 2021 eruption was calculated at approximately 2500 tons. Gray to white lithic grains (40-50 %) were dominant in the tephra deposits (0.125-0.25 mm fraction), while black to brown glass shards (8-16 %) were also observed. Although a very small proportion of glass particles appeared to be fresh, most of glass shards showed varying degrees of alteration based on microscope observation and electron micro-probe analysis. These combined lines of evidence suggest that the October 14, 2021 eruption of the first crater at Nakadake was probably a purely phreatic eruption.

著者

渡辺 秀文

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.43, no.5, pp.271-282, 1998-10-30 (Released:2017-03-20)

参考文献数

42

被引用文献数

2

---

An evaluation of the precursors to the 1986 eruption of Izu-Oshima volcano reveals that the long-tenn precursors are clearly divided into two, magma accumulation and ascent, stages. A gradual rise of seismic activity, infiation of the volcano and anomalous decrease in the geomagnetic total intensity, had continued for more than 10 years until around 1980. After 198 1, the volcano showed a small deflation and low seismicity at the caldera area until beginning of the eruption, while we observed anomalous changes in the subsurface resistivity and the magnetic field localized around the summit crater, that indicating a gradual rise of the temperature beneath the crater. By integrating the precursors we propose that the accumulation of magma had continued until 1980, and then the basalt magma started to rise up throuth the well-developed conduit. The magma plumbing system of the summit eruption of Izu-Oshima volcano is characterized by a continuous magma supply and an well-developed conduit connecting the magma reservoir and the summit crater. Since 1987 after the eruption, the EDM and GPS measurements have revealed a re-inflation of the volcano, suggesting a continuous magma supply. Tomographic studies on the subterranean structure also delineated a low velocity zone and a melt batch at the same location (at depths of 5-10 km) beneath the caldera as that of the inflation source. The proposed model for the magma plumbing system might provide a basis for not only the short-term but also mid-term prediction of the future eruption of Izu-Oshima volcano.

著者

大学合同観測班地質班

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.37, no.1, pp.47-53, 1992-04-01 (Released:2017-03-20)

参考文献数

7

被引用文献数

4

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.67, no.3, pp.app1, 2022 (Released:2022-10-27)

著者

森 俊哉 野上 健治

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.67, no.3, pp.379-388, 2022-09-30 (Released:2022-10-27)

参考文献数

17

---

When volcanic ash is exposed to water after fell on the ground, various chemical substances will be eluted to water phase. Amounts of water soluble SO4 and Cl and their Cl/SO4 ratio of ash are useful for understanding eruptive activities of the volcano. For prompt evaluation of eruptive activities by water soluble components on ash, it would be useful if the analyses are made in-situ or near by the volcano instead of sending the samples to the laboratories far away. For this purpose, we utilized and established a method using a compact handheld absorptiometer for analyses of Cl and SO4 in ash leachate. The method uses a small digital scale, a handheld absorptiometer and other equipment (PP bottles, syringes filters and etc.). The scale is for ash leachates preparation and the absorptiometer is used for the turbidimetric measurements of SO4 and Cl. The calibration curves for SO4 and Cl were linear and parabola for the concentration range of the standard solutions up to 81.9 mg/L and 39.6 mg/L, respectively. Eleven ash samples from Kirishima Shinmoedake and Sakurajima volcanoes were analyzed by turbidimetry method of this study and by ion chromatography method, and were compared for validation of the method. The analyzed concentrations were basically within about 10 % compared to those of ion chromatography, except for samples whose absorbance were smaller than 0.1 unit. We also checked for the interfering components for turbidimetry analyses by checking the compiled ash leachate data of Airys and Delmelle (2012) and came to conclusion that effect of interference can be usually ignored. On the other hand, some of the ash samples with very low water soluble SO4 and Cl may be under detection limit with the proposed method. As a conclusion, ash leachate analyses for SO4 and Cl by turbidimetry using a handheld absorptiometer used in this study is an effective method and could be used for prompt evaluation of eruptive activities especially on remotes islands where the chemical laboratories are not available.

著者

栁澤 宏彰 及川 輝樹 川口 亮平 木村 一洋 伊藤 順一 越田 弘一 加藤 幸司 安藤 忍 池田 啓二 宇都宮 真吾 坂東 あいこ 奥山 哲 鎌田 林太郎 兒玉 篤郎 小森 次郎 奈良間 千之

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.67, no.3, pp.295-317, 2022-09-30 (Released:2022-10-27)

参考文献数

73

---

The 2016 eruptions of Niigata-Yakeyama volcano in central Japan consisted of several small eruptions that were accompanied by syneruptive-spouted type lahars. We have reviewed the sequence of the 2016 activity and modeled the eruptive processes based on observations of various volcanic phenomena, including ash fall and lahars, plumes, earthquakes and crustal deformation, and analysis of eruptive products. Eruptions of Niigata-Yakeyama volcano after the 20th century can be categorized into two types; 1) VEI=0-1 eruptions during which ash fall covered only the summit area and no ballistic blocks were ejected (e.g., 1997-1998 event) and 2) VEI=1-2 eruptions during which ash fall reached the foot of the mountain with ejected blocks (e.g., 1974 event). We also discuss the characteristics of the 2016 activity by comparing the sequence with those of other events of Niigata-Yakeyama volcano: the 1974 and 1997-1998 eruption events and the 2000-2001 intensified fumarolic event. The 2016 eruptions of Niigata-Yakeyama volcano are divided into the following six stages. Stage I was characterized by the onset of intensified steam plume emission activity (≥200 m). Stage II was characterized by the onset of crustal deformation, slight increase of high frequency earthquakes (approx.>3.3 Hz) and further activation of steam plume emission activity (≥500 m). The crustal deformation observed commenced at the beginning of Stage II and lasted until the end of Stage V. The total inflated volume was estimated to be approximately 7.2×106 m3. Several very small eruptions that provided only a small amount of ash to the summit area also occurred. Stage III was characterized by a rapid increase of high frequency earthquakes accompanied by tilt change, and the onset of low frequency earthquakes (approx.<3.3 Hz). A small eruption was accompanied by a syneruptive-spouted type lahar at this time. Stage IV was characterized by the occurrence of several small syneruptive-spouted type lahars. The occurrence of high and low frequency earthquakes continued, but with decreasing abundance. Stage V was characterized by the highest altitude of steam plume emission (≥1,200 m), while no ash emission nor syneruptive-spouted type lahars were observed. Stage VI was characterized by a gradual decrease in steam plume emission and earthquake activity. The aerial photographs indicate the ash fall distribution, and the maximum scale of the 2016 eruption, which is estimated to be VEI=1. The assemblage of altered minerals indicates that the volcanic ash originated from volcanic conduits affected by a high-sulfidation epithermal system and no magmatic components were detected. Judging from the depth of the crustal deformation source of magmatic eruptions at other volcanoes, the estimated source of crustal deformation during the 2016 eruption is considered to have been caused by a volume change of the magma chamber. The sequence of the 2016 event can be interpreted as follows: 1) magma supply to the magma chamber, 2) increase in seismicity and fumarolic activity triggered by volcanic fluid released from the new magma, 3) destruction of volcanic conduit by increased fumarolic activity and emission of volcanic ash, and 4) occurrence of syneruptive-spouted type lahars by the “airlift pump” effect. At Niigata-Yakeyama volcano, such small eruptions and fumarolic events have been frequently observed for the last 40 years. We thus consider that the accumulation of magma has progressed beneath the volcano, which is a potential preparatory process for a future magmatic eruption.

著者

佐藤 鋭一 和田 恵治 野口 昌宏

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.67, no.3, pp.255-271, 2022-09-30 (Released:2022-10-27)

参考文献数

37

---

Kurodake volcano in the Taisetsu volcano group was formed approximately 0.2 Ma, producing andesitic lava flows and a dome. The lavas contain numerous mafic inclusions (<20 vol.%) ranging from approximately 1 cm to about 30 cm in diameter. The mafic inclusions exhibit typically rounded to ellipsoidal shapes and have smooth contacts with the host lavas. The mafic inclusions are classified into two types, fine and coarse, based on the size of the groundmass crystals. The groundmass crystals of the fine-type inclusions are composed of acicular minerals (0.1-0.3 mm in length). On the other hand, the groundmass of the coarse-type inclusions is primarily composed of tabular minerals (>85 vol.% and 0.2-0.5 mm in length). The plagioclase core compositions of the host lavas and two types of mafic inclusions vary substantially from An38 to An90. The plagioclase phenocrysts are classified into three groups based on their core compositions: An-rich (type A: An>80), An-poor (type B: An<60), and intermediate (type C: 60<An<80). Type A and type B plagioclases were derived from mafic and silicic magmas, respectively, and type C was derived from a hybrid magma formed by the mixing of the mafic and silicic magmas. The host lavas predominantly contain type B plagioclase phenocrysts, with infrequent types A and C, and most of the plagioclase microphenocrysts and groundmass crystals are type C. In the fine-type inclusions, type A and type B plagioclase phenocrysts coexist, and most of plagioclase microphenocrysts and groundmass crystals are classified into the type C, similar to the host lavas. In the coarse-type inclusions, most of the plagioclase phenocrysts, microphenocrysts, and groundmass crystals are classified as type B. These assemblages in the host lavas and fine-type inclusions can be explained by the mixing of the magmas, whereas the coarse-type inclusions were formed in the silicic end-member magma. Initially, mafic magma containing type A plagioclase was injected into bottom of the silicic magma chamber containing type B. A small amount of mafic magma was mixed with silicic magma to form the host magma. Subsequently, mixing occurred near the boundary between the mafic and silicic magmas, producing the fine-type inclusion forming magma. We presume that the margin of the silicic magma chamber was highly crystalline and the coarse-type inclusions were derived from the margin of the silicic magma chamber.

著者

小林 幹 津久井 雅志

出版者

特定非営利活動法人 日本火山学会

雑誌

日本火山学会講演予稿集 2016 (ISSN:24335320)

巻号頁・発行日

pp.151, 2016 (Released:2017-02-07)

著者

宇井 忠英 柴橋 敬一

出版者

特定非営利活動法人 日本火山学会

雑誌

火山.第2集 (ISSN:24330590)

巻号頁・発行日

vol.20, no.2, pp.51-64, 1975-08-01 (Released:2018-01-15)

---

Volcanic activity at the snow-capped summit of Mt. Chokai was first noticed by the captain of scheduled airline on March 1, 1974. The activity began with swarm of volcanic earthquakes, succeeded by fumarolic activity and finally explosion took place and a few craters were formed at eastern (late February-early March) and western (late April) foot of 1801 lava dome (Shinzan). The ejecta were exclusively fine-grained air-fall ash and accidental blocks. The blocks, formed the mud flow mixed with melted snow, rushed down the slope of volcanic edifice. Essential materials, such as bombs, air-fall scoria, or lava flows were not erupted. Rapid melting of snow was supposed to have been triggered by the formation of fumaroles caused by ascent of hot magma and associated juvenile gas. Thermal energy consumed for melting and evaporating snow is calculated as 3×1021 ergs. Total volume of mud-flow deposit is around 3×104 cubic meters, and that of air-fall ash is an order of 105 cubic meters. The entire area which showed thermal activity is approximately 700×200 meters, elongated in east-west direction. Distribution of earthquake foci was also trending east-west just passing the prehistoric summit and parasitic vents. Direction of vent alignments is the same for the most volcanoes in northeastern Japan, and is supposed to reflect regional stress field.

著者

長岡 優 西田 究 青木 陽介 武尾 実 大倉 敬宏 吉川 慎

出版者

特定非営利活動法人 日本火山学会

雑誌

日本火山学会講演予稿集 2017 (ISSN:24335320)

巻号頁・発行日

pp.88, 2017 (Released:2018-02-01)

著者

平林 順一 野上 健治

出版者

特定非営利活動法人 日本火山学会

雑誌

日本火山学会講演予稿集 2006 (ISSN:24335320)

巻号頁・発行日

pp.107, 2006-10-23 (Released:2017-02-10)

著者

山本 圭吾

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.67, no.2, pp.221-231, 2022-06-30 (Released:2022-07-28)

参考文献数

13

著者

青山 裕

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.67, no.2, pp.171-193, 2022-06-30 (Released:2022-07-28)

参考文献数

86

---

Volcanoes in Hokkaido had vigorous eruption histories in the last 400 years. Especially in the southern Hokkaido, Hokkaido-Komagatake, Usuzan, and Tarumaesan, reawakened in the 17th century after the long-dormant period and vigorous magmatic eruptions of VEI5 class have been recorded in the historical literature and also in geological layers. Contrary to these volcanoes, we have documented histories only after the 20th century for Tokachidake and Meakandake. Continuous volcano monitoring has been performed in major active volcanoes in Hokkaido since 1960s. In the 1970s, with the increase in seismic activity of Tokachidake, sponsored research from Hokkaido Government to Hokkaido University began to establish disaster management plans for future eruptions of active volcanoes in Hokkaido, and research reports was edited by Hokkaido University. Although 50 years have passed since the publication of the first report on Tokachidake, the reports of the research on active volcanoes are still one of the first documents to be referred when investigating the past activities of volcanoes in Hokkaido and the results of old scientific surveys. In addition, the reports include interdisciplinary contents for that time such as prediction of future eruptive activities and disaster prevention measures. The sponsored research by Hokkaido Government has continued to the present, and a new report on Tokachidake was published in 2014. The compilation of such research reports is very effective for volcano researchers and for relating field to share their awareness of the problems of volcanoes beyond their individual fields of expertise. Monitoring network around the active volcanoes in Hokkaido has been remarkably improved for these 20 years by Japan Meteorological Agency (JMA), Hokkaido University and other relating institutions. Recent data exchange in real-time among different organizations reduces duplication of monitoring resources and increases multi-parameter monitoring ability. The improved volcano monitoring network is expected to detect precursory activities of future magmatic eruptions concerned at the major volcanoes. Looking back on the eruption in the 20th century in Hokkaido, small phreatic eruptions preceded magmatic vigorous eruptions in many cases. Not only mountaineers but also tourists and citizens can easily approach the crater area without any special equipment at several active volcanoes, so even a small eruption can lead to severe volcanic disaster. (View PDF for the rest of the abstract.)

著者

下司 信夫

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.61, no.1, pp.101-118, 2016-03-31 (Released:2017-03-20)

被引用文献数

3

---

Large-scale pyroclastic eruption is one of the most awful natural disasters on the earth. Though their frequency is relatively low compare to the lifetime of human society, large-scale pyroclastic eruption can make serious impact on the global environment. Frequency of the volcanic eruptions shows a negative correlation against their scale: global frequency of the eruptions larger than VEI7 is approximately ten per 10,000 years, whereas more than 10 eruption of VEI4 occur every 10 years. The storage of voluminous magma within a shallow crust is a key process for the preparation for large-scale eruption. Inactive thermal convection in highly-crystallized magma bodies and visco-elastic behavior of the surrounding host rock can allow the stable storage of voluminous felsic magma at the neutral buoyancy level in the upper crust. Segregation of interstitial melt to form a melt pocket in highly-crystallized magma body can cause smaller scale of eruptions, whereas the remobilization of entire part of magma chamber will result a large-scale eruption with caldera collapse. Rupture and collapse of the roof rock of magma chamber induced by rapid decompression of magma chamber is the fundamental process of the eruption of voluminous magmas within short period. The decompression of magma chamber activates the slip of ring fault at the marginal portion of the roof and consequently the caldera starts subsidence. The collapse is controlled by the decompression inside the chamber and the strength of the roof rock. Ring fault turns to an open ring facture through which the voluminous magma can erupt to produce large ignimbrite. The volume of magma erupts during a caldera-forming eruption against the total magma chamber volume show negative correlation against the chamber size. This means that the large fraction of magma can remain even after caldera collapse particularly in large magma chamber. Evaluation of "precursory process" for catastrophic eruption is important to understand the driving mechanism of catastrophic eruption and also the hazard assessment. Accumulation of magma and building of a large-volume magma chamber within the earth’s crust is a long-term preparation process for catastrophic eruption. Short-term process for catastrophic eruption is the destabilization and rupturing process of the magma chamber.

著者

鈴木 毅彦 中山 俊雄

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.52, no.1, pp.23-38, 2007-02-28 (Released:2017-03-20)

参考文献数

47

被引用文献数

1

---

A widespread tephra referred to here as Tamagawa-R4 Tephra (Tmg-R4) is newly recognized. Tmg-R4, derived from the Pre-Yakeyama caldera located in the Sengan geothermal area, the Northeast Japan arc, covers the area from Tohoku to Kanto, northeast of Honshu Island. At the type locality in the proximal area, Tmg-R4 comprises a non-welded pyroclastic flow deposit (ignimbrite) and an immediately overlying welded pyroclastic flow deposit (Kurasawayama Welded Tuff). Absence of plinian fall deposits in the area of ca. 25 km south of the source and the fine vitric ash nature of the distal ash-fall deposits of Tmg-R4 suggest that they are co-ignimbrite ash-fall deposits. Tmg-R4 was identified using a combination of refractive indices and chemical compositions of major and rare earth elements of glass shards (n=1.498-1.501, SiO2: 78.3-78.6 wt%, K2O: 4.2-4.5 wt%, Ba: 830-911 ppm), mineral content, refractive indices of hornblende (n2=1.665-1.686). On the basis of these properties, Tmg-R4 was identified in Boso and Oga peninsulas, Choshi area, and in the core drilled on Musashino upland around 500 km south of the source. Calcareous nannofossil biostratigraphic (Calcareous nannofossil datum 13) and magneto-stratigraphic positions in Boso peninsula and Choshi, and paleomagnetic direction and many radiometric ages determined in the proximal area by previous studies indicate that the age of Tmg-R4 is ca. 2.0 Ma, positioned just below the base of the Olduvai Subchron. The distribution of Tmg-R4 showing emplacement of co-ignimbrite ash-fall deposit in the area 530 km south of the source, emphasizes the upwind transport direction relative to the prevailing westerly winds. This distribution shows similarity to those of a few co-ignimbrite ash-fall deposits derived from calderas in the Northeast Japan arc. As a key marker horizon in this age, the widespread occurrence of Tmg-R4 provides a tie line between many different sections over a distance of 530 km. Additionally, Kd44-Nk Tephra above Tmg-R4 is recognized in Boso peninsula, Choshi, Niigata and east Lake Biwa areas. Characteristic properties and stratigraphic positions indicate that Kd44-Nk possibly derived from the Sengan geothermal area occurred at 1.968-1.781 Ma.

著者

土出 昌一

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.42, no.1, pp.89-91, 1997-03-07 (Released:2017-03-20)

参考文献数

2

著者

乙津 孝之 岩國 真紀子 本橋 昌志 新井 伸夫

出版者

特定非営利活動法人 日本火山学会

雑誌

日本火山学会講演予稿集 2021 (ISSN:24335320)

巻号頁・発行日

pp.13, 2021 (Released:2022-02-21)

著者

北川 隆洋 風早 竜之介 谷口 無我 篠原 宏志 福岡管区気象台 大分地方気象台

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.67, no.1, pp.113-123, 2022-03-31 (Released:2022-04-26)

参考文献数

21

---

Volcanic gas composition provides us a crucial clue to investigate magma plumbing and geothermal systems. Sensor-based instruments named Multi-GAS have been used for monitoring the volcanic gas compositions at volcanoes. A sensitivity of sensors changes with time caused by deterioration, masking volcanic signals especially during long-term monitoring. Frequent calibration of the sensors is desirable for precise monitoring; however, that is pragmatically not easy because a location of a targeted volcano is remote and rural in general. Sophisticated evaluation of the long-term changes in the sensor sensitivity has not been made yet. In this study, we examined the sensitivity change of the chemical sensors within the Multi-GAS during long-term observations by comparing with other methods such as gas detector tubes and gas sampling. The volcanic gas compositions were monitored using Multi-GAS at Kusatsu-Shirane volcano and Kuju volcano, Japan. Intermittent gas composition measurements using gas detector tubes and gas sampling were conducted at fumaroles around where the Multi-GAS stations are installed. Some disagreements of the CO2/H2S ratios are observed between those measured using the Multi-GAS from those measured using other methods. In such cases, large decreases of the H2S sensor sensitivity were found by the sensor calibration after the monitoring. We found a roughly linear behavior of the H2S sensor sensitivity changes with time based on a long-term sensor sensitivity monitoring in a laboratory and propose a simple linear sensitivity correction of the H2S sensors using the calibration results obtained before and after the monitoring. The corrected Multi-GAS results agree well with the results of other methods. Our results open up a possibility for extraction of volcanic signals from the long-term volcanic gas data streams monitored using the Multi-GAS that are masked by the changes in the sensitivity of the sensors.

著者

山里 平

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.50, no.Special, pp.S7-S18, 2005-12-20 (Released:2017-03-20)

---

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.