1741 eruption of Oshima–Ōshima and the Kampo tsunami

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1741 eruption of Oshima–Ōshima and the Kampo megatsunami
VolcanoOshima Oshima
Start date18 August 1741
End date1 May 1742
TypeUnknown
LocationOff the coast of Hokkaido.
Sea of Japan, Japan
VEI4[1][2]
ImpactSector collapse and regional tsunami
Deaths1,467–2,033 dead[3]
Ōshima is located in Japan
Ōshima
Ōshima

The devastating eruption of Oshima–Ōshima began on 18 August 1741 and ended on 1 May the next year.

Korean Peninsula
.

Background

Japan is situated along a zone of convergence between at least four major and minor tectonic plates. The

upper mantle, lowering its melting point. As the mantle begins to melt, its density decreases and rises through the upper crust
, forming a volcanic vent.

Oshima Island

Oshima Oshima island seen in 2011.

Oshima is the uninhabited island located in the

andesitic stratovolcanoes
, the highest measuring 737 meters (2,418 ft) above sea level. No records of eruptions prior to the 1741–42 eruption exists due to the remoteness of the island although some fumaroles were documented. The most recent record of an eruption was in 1790. Activity resurfaced in 1996 with seismic unrest beneath the island but no eruption occurred.[1]

Kampo tsunami

The initial eruption began on 18 August and was visible from Hokkaido by 23 August. By 25 August, so much ash had been ejected that sunlight was blocked out. Ash fall measured up to over 20 centimeters at places. On August 29 at 05:00, a second and more violent eruption took place on the island and was followed–up by a large tsunami up to 90 m (300 ft).[3][4] The tsunami engulfed many coastal villages and towns along the shores of the Sea of Japan. While the eruption itself did not result in any casualties, the ensuing tsunami drowned over 2,000 people.[6]

In

Matsumae Peninsula, heavy ash fall from the eruption blocked out the sun and plunged villages into darkness. A tsunami arrived along the shores at some time between 20:00 and 22:00. More than 729 homes were washed away and 33 others were seriously damaged. The tsunami also took with it two warehouses and destroyed 25. Wave heights reportedly exceeded 9 m (30 ft). One document states that approximately 120 km (75 mi) was inundated, and that at least 1,467 inhabitants lost their lives, a figure that excludes native Ainu inhabitants that left no written records. Estimates of the total casualties including Ainu suggest that over 2,000 people lost their lives.[7] Some 1,521 fishing boats and ships near the erupting volcanic island were also destroyed by the waves. One hundred and forty people were killed while 53 vessels and 83 houses were lost to the waves in Tsugaru on the island of Honshu.[8]

Run–up heights of 60–90 m (200–300 ft) were apparently observed by eyewitnesses at Sado Island in Niigata, at least 400 km (250 mi) from Oshima Oshima, according to a 1984 catalogue.[9][10] Oral records, however, suggest the highest tsunami waves topped 34 m (112 ft) and written documentation on the tsunami presented a height of 13 m (43 ft).[11]

On the

Joseon dynasty.[14] Estimation of wave heights along the coast range from 3–4 m (9.8–13.1 ft).[11]

Origins

The source of the Kampo Tsunami is still debated among scientists, claiming an earthquake, debris avalanche or some other phenomenon caused the tsunami. There is still no consensus in the debate but much evidence points to a landslide and debris avalanche along the flank of the volcano.

Earthquake

Off the western coast of Hokkaidō and northern Honshu, at the eastern brink of the Sea of Japan lies a

convergent plate boundary between the Amurian and Okhotsk plates, microplates of the Eurasian and North American plates respectively. The convergent boundary is the source for many historically documented tsunamigenic earthquakes in 1833, 1940, 1964, 1983 and most recently, the 1993 southwest off Hokkaido earthquake.[11]

Based on analysing records of the tsunami heights, a large magnitude 7.5–8.4 earthquake along the eastern margin of the Sea of Japan would have been sufficient enough to generate the wave heights as observed in 1741.[15][16][17][18] The earthquake hypothesis however is challenged because no records of shaking from an earthquake exists.[8] A 1995 research article suggested the 1741 tsunami may have been caused by an earthquake that ruptured a present–day seismic gap on the plate boundary between the rupture zones of the 1833 and 1983 earthquakes.[19][3] Because of the absence of documented shaking caused by a possibly large earthquake (Mt  7.5–8.4), scientists interpreted the event had characteristics similar to a tsunami earthquake.[20] Initially proposed in 1972 by Hiroo Kanamori, these events release seismic energy in long periods.[21] Such events involve a slower than usual rupture propagation along the shallow segment of a subduction zone. An event of this sort would go undetected by humans because of the low–frequency ground motions. Tsunami earthquakes rupture the shallowest section of the subduction boundary which generate large tsunami run-ups.[21]

The earthquake theory also did not rule out the possibility of the volcano collapsing because of the extreme tsunami run–ups. However, there has been no attempts to conduct submarine surveys in the Sea of Japan to confirm the claim of seismic activity along the plate boundary.[20]

Sector collapse

A landslide and debris avalanche involving a subaerial and submerged portion of the volcanic island has been the more accepted source mechanism of the large tsunami. With an initial height of 850 m (2,790 ft), the event reduced the elevation of Hishi–yama peak to 722 m (2,369 ft). An estimated 2.4 km3 (0.58 cu mi) section of the volcano came loose and fell into the seafloor and settled towards the north of the island, similar to that during the

1888 eruption of Ritter Island.[23]

A more recent study in 2019, however, stated that the slide volume was 2.2 km3 (0.53 cu mi), a significant difference from the 2001 research. The same paper also implied that the maximum thickness of the landslide is 300 m (980 ft) and an area 14 km (8.7 mi) by 9 km (5.6 mi) was buried under debris.[24]

See also

References

  1. ^ a b "Oshima-Oshima". Global Volcanism Program. Smithsonian Institution. Retrieved 2022-03-20.
  2. ^ "渡島大島 有史以降の火山活動". Japan Meteorological Agency. Retrieved 30 March 2021.
  3. ^
    S2CID 233955868
    . Retrieved 30 March 2021.
  4. ^ a b "Significant Volcanic Eruption". NGDC NCEI. Retrieved 30 March 2021.
  5. PMID 32929150
    .
  6. ^ a b 北海道・渡島大島大噴火の巨大津波 絵図に残った凄まじい風景 建築研究所特別客員研究員・都司嘉宣. Sankei Shimbun (in Japanese). Sankei Shimbun. 6 June 2016. Retrieved 30 March 2021.
  7. doi:10.1186/BF03352698
    .
  8. ^ a b "Tsunami Event Information W. Hokkaido Island". NGDC NCEI. Retrieved 30 March 2021.
  9. ^ "Tsunami Event W. Hokkaido Island". NGDC NCEI. Retrieved 30 March 2021.
  10. ^ "Catalog of Tsunamis in Japan and its Neighboring Countries". Tsunami Digital Library. 1984. Retrieved 30 March 2021.
  11. ^
    doi:10.1186/BF03352698
    . Retrieved 30 March 2021.
  12. ^ "1st article celebrated on July 27, 17, Yeongjo". The Annals of King Yeongjo. 54. Retrieved 31 March 2021.
  13. ^ "1st article in Gyeongjin on July 18, 17, Yeongjo". The Annals of Yeongjo. 54. Retrieved 31 March 2021.
  14. ^ Min Kyu Kim; Hyun-Me Rhee; In-Kil Choi (2013). "The effect analysis of 1741 Oshima-Oshima tsunami in the West Coast of Japan to Korea" (PDF). Transactions of the Korean Nuclear Society Spring Meeting. Retrieved 30 March 2021.
  15. ISSN 0736-5306
    . Retrieved 30 March 2021.
  16. ^ Katsui, Yoshio; Yamamoto, Masatsugu (1981). "The 1741-1742 Activity of Oshima-Ōshima Volcano, North Japan" (PDF). Journal of the Faculty of Science, Geology and Mineralogy. 19 (4). Japan: Hokkaido University: 527–536. Retrieved 30 March 2021.
  17. ^ 日本海における大規模地震に関する調査検討会 報告書 (PDF). Ministry of Land, Infrastructure, Transport and Tourism (in Japanese). August 2014. Retrieved 30 March 2021.
  18. ISSN 0040-1951
    . Retrieved 30 March 2021.
  19. doi:10.1111/j.1440-1738.1995.tb00140.x
    .
  20. ^ a b Yukinobu Okamura (1998). 日本海東縁海域の活構造およびその地震との関係 [Active structure in the eastern margin of the Sea of Japan and its relationship with earthquakes] (PDF). Geological Survey Monthly Report (in Japanese). 49 (1). Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology. Retrieved 31 March 2021.
  21. ^
    doi:10.1016/0031-9201(72)90058-1
    .
  22. doi:10.1029/2000GL012175
    .
  23. S2CID 224925006
    .
  24. S2CID 134267212
    .
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