Mount Ontake Eruption Case Study

Distribution and characteristics of the eruption vents

The eruption vents produced by the 2014 eruption were located on the southwestern side of the summit and oriented along a west-northwest to east-southeast axis (Figs. 1b, 2a). The three vent areas included the W1 vent and fissures on the western slope of the Ichinoike cone, the J1–J7 vents in the Jigokudani valley, and the E1 vent west of Ohtakisancho peak. Although the location and general orientation of the vents were similar to those for the 1979 eruption, they had shifted 200–300 m to the south-southwest and had extended by about 300 m to the west-northwest.
Three large, adjacent vents (J3–J5) that appeared to be connected were located on the floor of the Jigokudani valley. On September 28, three smaller vents (J1–J2 and J6) were observed on the valley wall (Figs. 2a, 3e; the J7 vent was inactive on the 28th). The large vents were associated with piles of pyroclastic materials and formed a pyroclastic cone on the southern slope of the valley. In the photograph taken at 14:13 on September 27 (Figs. 2b, 3a), the J4 vent appeared as it was on September 28 (Figs. 2a, 3e); however, the J3 vent appeared somewhat smaller in size, and the J5 vent could not be identified. In the photograph taken at 15:11 (Fig. 3b), the J3 vent was larger in size, reaching its final shape in the photographs taken at 16:02 (Fig. 3c), and parts of the J5 vent could be recognized near the eastern side of the valley wall (Fig. 3c). Thus, these two vents are considered to have formed between 14:13 and 16:02 on September 27. Because no explosive eruption occurred during that time, they are considered to have been generated by non-explosive processes such as the collapse of a part of the vent area into the conduit. Thus, it is likely that the J4 vent was the main vent involved in the eruption on September 27. In the photograph taken at 16:44 (Figs. 2c, 3d), a mudflow was observed on the slope of the pyroclastic cone. The mudflow appeared to have effused between 16:02 and 16:44. In the image taken on September 28, the interior of the J4 vent appeared dark, suggesting that it was filled with mud or water. In addition, a rill-like structure formed on the slope of the pyroclastic cone (Fig. 3e, f), probably due to erosion caused by repeated mudflows from the vent. Small-scale mudflows were also observed, which probably effused from other very small vents and fractures (Fig. 3a, b).
On the western slope of the Ichinoike cone, the fissures generated were arranged along an east–west trending axis, with the W1 vent in the center (Fig. 2a). Although the W1 vent was not surrounded by a pile of pyroclastic materials, it ejected a fine brownish ash layer that covered the surrounding areas, particularly to the north. In addition, a mudflow was observed to have effused from the vent (Fig. 4). All of the fissures emitted fumarolic gas. The E1 vent located 400 m west of Ohtakisancho peak (Fig. 2a) may have ejected ballistic materials over the surrounding areas, but pyroclastic materials did not appear to have collected around it.

Distribution and characteristics of the pyroclastic flows

Pyroclastic flow-like units occurred during this eruption. Although no examples of pyroclastic flow typical of magmatic eruptions were observed, flows comprising a mixture of ash and gas moved down the slope by gravity (Yamamoto 2014). We referred to these as “pyroclastic flows.”

Based on images captured by hikers (e.g., Asahi 2014), when the initial eruption plume reached an altitude of several hundred meters above the summit, the plume began to expand laterally before flowing down the volcano slopes as pyroclastic flow in almost all directions. Because the temperature of the plume was relatively low, the entire plume may not have been able to rise very high above the volcano. Based on the time at which the dense ashy plume that covered the cottage at the summit had disappeared (Kaito 2014), pyroclastic flows are considered to have been generated intermittently until around 12:20 on September 27. Video footage taken by a hiker who was caught in moving pyroclastic flow (https://www.youtube.com/watch?v=GGTp0bNb608) revealed that the flow consisted of aggregated ash particles blowing laterally as in a snow storm. The majority of the pyroclastic flow deposits were distributed within approximately 1 km of the vents, except in the area southwest of the vent (Fig. 5a).

The pyroclastic flow moved away from the vent in a southwesterly direction along the Jigokudani valley for approximately 2.5 km, as this area was lower than the surrounding areas (Fig. 5a, b). Areas affected by the pyroclastic flow appeared whitish because of the presence of ash on vegetation (Fig. 5d). The absence of burned or fallen trees implied that the temperatures and forces associated with the pyroclastic flow events were both low.

Camera footage recorded by the Ministry of Land, Infrastructure, Transport and Tourism, taken from near Takigoshi village on the southern foot of the volcano (http://www.cbr.mlit.go.jp/tajimi/sabo/ontake/, https://www.youtube.com/watch?v=jt36uloZ3oI), revealed that the pyroclastic flow reached a distance of 2.5 km away from the vent at 11:57 about 5 min after the beginning of the eruption. The front of the pyroclastic flow passed through the point of an altitude of 2500 m of the Jigokudani valley around 11:53; thus, it traveled 2.1 km in 4 min. This means that the pyroclastic flow in this area moved at an average speed of 32 km/h (8.8 m/s), which is considered slow for pyroclastic flow (see Cas and Wright 1987). Indeed, the pyroclastic flows observed in this study could be considered a kind of pyroclastic surge that is characterized by low speed and low temperature. Similar pyroclastic flows were also observed in the phreatic eruption of Miyakejima on August 29, 2000 (Nakada et al. 2005), indicating that this kind of pyroclastic flow might be typical of low-temperature, phreatic eruptions with no magmatic material in the ejecta.

Distribution and characteristics of the air-fall deposits

The height of the eruption plume increased over time and, based on metrological radar observations, was inferred to have reached an altitude of approximately 11 km around 12:10 (Sato et al. 2015). Although the plume, precipitating ash over an extensive area, tilted toward the northeast at low altitudes, it moved east-northeast at high altitudes. A mixture of air-fall and pyroclastic flow deposits appears to have settled in the vicinity of the summit.

The depositional axis of the air-fall ash, indicated by a whitish color on the ground, extended to the east-northeast (Fig. 5a, c). The arrow labeled “e” in Fig. 5c indicates air-fall ash deposits comprising fine ash particles aggregated with accretionary lapilli (Fig. 5e); the thickness of this layer was 2–3 mm, and the size of the particles was 1–2 mm.

At the summit of Ichinoike, which was covered by a thick layer of ashy deposits, sun cracks developed on the surface (Fig. 5f), and water collected in the bottom of the impact craters on the day after eruption, even in those on part of the inner wall of the Ichinoike cone (Fig. 5g). These findings reveal that the ash (air-fall/pyroclastic flow) was enriched with water components, corroborating the observation of accretionary lapilli. According to a hiker at the summit, although the ash was initially dry, it became wetter, taking on the form of “mud rain” in the final stages (Kaito 2014). In video footage taken immediately after the eruption (https://www.youtube.com/watch?v=ODiqlpUwcVM), the top 1–2 cm of the few tens-of-centimeters of ash that was deposited near the summit appeared wet and semisolid, looking dark in color. This water is considered to have been derived from the precipitation of water vapor contained in the eruption plume.

Distribution of ballistic ejecta

The phreatic eruption of Mount Ontake generated large amounts of ballistic ejecta. According to a hiker who sought shelter in the cottage at the summit, the generation of ejecta continued intermittently for about 1 h (Kato 2014). This report is concordant with the morphological variation observed in the impact craters produced by the ballistic ejecta, which included craters with both well-defined and indistinct outlines (Fig. 5f).

Instead of actual ballistic ejecta, which are difficult to identify directly, the distribution of ballistic ejecta was inferred based on the distribution of impact craters in the photographs taken on September 28 (Figs. 6, 7). Impact craters were identified based on the outline and concave topography suggested by the effect of shading in the photographs. In each photograph, the scale of the scene was estimated by matching characteristic topographic features including large rocks to the locations on topographic map using Google Maps/Google Earth and GSI Maps (Geospatial Information Authority of Japan—http://maps.gsi.go.jp). Based on the estimated scales, 5 × 5 m areas were selected on each photograph to count the number of impact craters (Fig. 7). The size of impact craters and ballistics was also estimated based on the scale obtained here and characteristic objects in the same scene, such as statues, monuments, stone stairs, or rescue workers.

The diameters of impact craters ranged between several tens-of-centimeters to 1 m, while those of ballistic ejecta ranged from 10 cm to several tens-of-centimeters (maximum c. 1 m). The distribution density was classified into four zones, based on the number of craters per unit area (5 × 5 m); these zones were called Zones A, B, C, and D, with Zone A having the highest density of impact craters and Zone D having no impact craters. Because the distribution density of craters decreased with increasing distance from the vents, the distribution density was very high between Kengamine and Ichinoike (Fig. 7, Zone A ➀–➂). The furthest impact crater was located at Ninoike pond, 950 m from the vents in the Jigokudani valley (Fig. 7, Zone C ➈). We were unable to survey the distribution of impact craters in the area to the south of the vents because they were obscured by the volcanic plume at the time the observations were made, and the deposition of ash layer was too thin to leave clear crater structures by impact.

The distribution of craters was not isotropic but slightly extended in a north-northeasterly direction (dotted line in Fig. 6). Because the Jigokudani valley extends along a north-northeast to south-southwest axis, and because the vents are located on the valley floor, it is possible that the valley walls acted as barriers to ejecta, preventing ballistics from being ejected far beyond the valley walls along both sides of the valley.

A volcanic eruption of Mount Ontake (御嶽山,Ontake-san) took place on September 27, 2014, killing 63 people. Mount Ontake is a volcano located on the Japanese island of Honshu around 100 kilometres (62 mi) northeast of Nagoya and around 200 km (120 mi) West of Tokyo. It was the first fatal volcanic eruption in Japan since the 1991 collapse of a lava dome at Mount Unzen,[2][3][4] and the deadliest volcanic eruption in Japan since Torishima killed an estimated 150 people in 1902.

Timeline[edit]

The volcanic eruption happened at 11:52 Japan Standard Time (UTC+9).[5][6] There were no significant earthquakes that might have warned authorities in the lead up to the phreatic eruption—caused by ground water flashing to steam in a hydrothermal explosion.[7] The mountain is a popular tourist attraction for hikers, being considered good for beginner climbers and relatively safe, and the weather was also good, so there were several hundred people on its slopes at the time.[8][9] The police said that they were searching for people remaining on the mountain. By 17:00 the police reported that three people were missing and were believed to be under ash. Another person was rescued from under the volcanic ash, but remained unconscious. Six people were injured, one by flying rocks.[10]

By 19:30, the number of people believed to remain buried in ash rose to six. Nine people had been reported to be injured, five of whom had fractured bones.[11] Later, at least 40 people were reported to be injured, and another 32 were believed to be missing. The JSDF began carrying out helicopter searches for missing people.[12] One woman was reported to have died from the eruption.[13][14]

On September 28, the police reported that over 30 people had been found in "cardiac arrest" near the summit.[15] Japanese emergency services often refer to people who show no vital signs, and are apparently dead, as being in cardiac arrest, as legally, only an authorised physician can pronounce a person dead.[9][15] By September 29, a total of 36 bodies had been found, and 12 people had been pronounced dead; the search was suspended due to dangerous conditions, including hydrogen sulfide gas spewing from the mountain.[16] On September 30, fears of escalating volcanic activity on Mount Ontake continued to hinder rescue efforts.[17]

On October 1, 2014, eleven new bodies were discovered by rescuers on the slopes of Mount Ontake after searching in previously unexplored areas of the ash-covered peak, bringing the total body count from 36 to 47; a revision after an erroneous initial count of 48.[18][19]

On October 4, 2014, four new bodies were discovered by rescuers on the slopes of Mount Ontake after searching in previously unexplored areas away from trekking roads.[20] Those four were confirmed to have died.[21]

Typhoon Phanfone prevented searching activities from October 5 till 6.[22] On October 7, three more bodies were discovered, bringing the total of confirmed deaths to 54.[23] As of October 11, the death toll is at 56.[1] The victims of the Mount Ontake eruption were mourned on October 27, as authorities and residents marked a month since the volcano killed 57 people and left six others missing.[24]

See also[edit]

References[edit]

  1. ^ ab"Death Toll from Mount Ontake Eruption Rises to 56". Jiji Press. Retrieved October 11, 2014. 
  2. ^"Japan volcano: 31 hikers feared dead after sudden eruption of Mount Ontake". The Independent. 
  3. ^"31 Feared Dead in Japan Volcano Eruption". The Daily Beast. 
  4. ^"Rescuers find 30 dead of 'cardiac arrest' near summit of volcano". IB Times. 
  5. ^ (in Japanese). sankei news. September 28, 2014. Retrieved September 30, 2014. 
  6. ^"御嶽山が噴火 火口から4キロ程度は警戒を". NHK. September 27, 2014. Archived from the original on September 27, 2014. Retrieved September 27, 2014. 
  7. ^"Experts warn of further eruptions". September 28, 2014. Archived from the original on October 2, 2014. 
  8. ^"日 온타케산, 초보자도 등반하기 좋은 일본 명산". 뉴스1 코리아. September 28, 2014. 
  9. ^ ab (in Russian). ITAR TASS. 
  10. ^"3 buried under volcanic ash". NHK. Retrieved September 27, 2014. 
  11. ^"御嶽山噴火、7人が灰に埋まる 山に残留44人 警察庁". The Asahi Shimbun. Retrieved September 27, 2014. 
  12. ^"Japanese troops head for volcano after eruption to search for missing climbers". Chatham Daily News. Ontario. September 27, 2014. Retrieved September 29, 2014. 
  13. ^Associated Press (September 27, 2014). "Japan volcano Mount Ontake erupts, injuring at least 40". CBC News. Retrieved September 27, 2014. 
  14. ^"Japanese volcano kills one, over 30 seriously injured". Reuters. September 27, 2014. Retrieved September 27, 2014. 
  15. ^ ab"Rescuers find more than 30 hikers in 'cardiac arrest' on Japan's Mt Ontake after sudden volcanic eruption". ABC News. September 28, 2014. Retrieved September 28, 2014. 
  16. ^"Five more bodies found on Japan's Mount Ontake after eruption". Retrieved September 29, 2014. 
  17. ^"Fears of escalating volcanic activity on Mt. Ontake hinder rescue efforts". Mainichi Japan. September 30, 2014. Archived from the original on October 6, 2014. Retrieved September 30, 2014. 
  18. ^"UPDATE: Death toll from Mt. Ontakesan eruption hits 47, worst in postwar period". Asahi Shimbun. Archived from the original on October 1, 2014. Retrieved October 1, 2014. 
  19. ^ [Mount Ontake, 11 newly discovered bodies, totaling 47 deaths] (in Japanese). Asahi Shimbun. Retrieved October 1, 2014. 
  20. ^ (in Japanese). Asahi Shimbun. October 4, 2014. Retrieved October 4, 2014. 
  21. ^ (in Japanese). NHK NEWS WEB. Retrieved October 5, 2014. 
  22. ^"御嶽山捜索、2日連続で中止…台風18号影響". Yomiuri Shimbun. 6 October 2014. Retrieved 8 October 2014. 
  23. ^"3 more bodies found on Mt. Ontake as search resumes, toll at 54". Mainichi Shimbun. Archived from the original on October 11, 2014. Retrieved October 5, 2014. 
  24. ^http://www.japantimes.co.jp/news/2014/10/27/national/ontake-victims-mourned-month-eruption-tourism-industry-scrambles-recover/#.VE5MneeLxiU

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