Oshima-Ōshima

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Oshima-Ōshima
Oshima-ooshima.jpg
Waters Japanese sea
Geographical location 41 ° 30 '40 "  N , 139 ° 21' 36"  E Coordinates: 41 ° 30 '40 "  N , 139 ° 21' 36"  E
Oshima-Ōshima (Hokkaidō Prefecture)
Oshima-Ōshima
length 4 km
width 3.5 km
surface 9.5 km²
Highest elevation Era-dake
732.4  m
Residents uninhabited
Oshima-Ōshima in the aerial photo
Oshima-Ōshima in the aerial photo

Oshima-Ōshima ( Japanese. 渡 島 大 島 ), officially just Ōshima ( 大 島 ), denotes a volcanic island in the Sea of ​​Japan west of Hokkaidō , the northernmost main island of Japan . The island is also called Matsumae-Ōshima ( 松 前 大 島 ) and belongs to the city of Matsumae , Matsumae-gun County in the sub-prefecture of Oshima .

geography

Oshima-Ōshima, the "big Oshima island", is located near the Tsugaru Strait and around 55 kilometers west of the Oshima Peninsula , a peninsula in the extreme south of Hokkaidō, and 38 kilometers northwest of Oshima-Kojima , the "small Oshima island ". The uninhabited island has a length of around 4 kilometers and a width of around 3.5 kilometers and an area of ​​around 9.5 km².

The island consists of the peaks of two stratovolcanoes that have grown together , with a base diameter of around 18 kilometers and a height of around 2300 meters above the sea floor. The volcanoes extracted about 70% basalt ; the rest is andesite . Both basalt and andesite have been extracted in several outbreaks.

In the east of the island which is Era-dake ( 江良岳 ; also Higashi-yama ( 東山 , "Ostberg") called), with 732.4 meters ( 41 ° 31 '  N , 139 ° 22'  O ) above sea level, the highest Point of the island. It is the older of the two volcanoes; its most recent eruptions may have occurred in the late Pleistocene . Large parts of Oshima-Ōshima are covered by lavas and pyroclastics of Kiyobe-dake ( 清 部 岳 ; also Nishi-yama ( 西山 , "West Mountain")). The northern part of the western volcano has collapsed and slid into the sea, so that the summit is occupied by a horseshoe-shaped crater open to the north. The southern rim of the crater reaches a maximum height of 722 meters ( 41 ° 31 '  N , 139 ° 21'  E ). Inside the crater, the pyroclastic cone Kampō-dake ( 寛 保 岳 ) formed, which rises around 200 meters above the crater floor and has a height of 648 meters ( 41 ° 31 ′  N , 139 ° 21 ′  E ) above sea level.

volcanic eruptions

Using the radiocarbon method , two eruptions of the Kiyobe-dake were found around 800 BC. And dated about 250 AD.

Historical reports describe eruptions between 1741 and 1790. Since the island was uninhabited, the effects of the eruptions on the islands of Hokkaidō and Honshū are described.

A volcanic eruption on Oshima-Ōshima was first noticed on August 18, 1741. From August 25, large quantities of ash fell on villages on the coast of the Oshima Peninsula. A huge explosion was heard on August 29th. An hour later, a 3 to 13 meter high tsunami hit the coast in southwest Hokkaidō. The tsunami spread south in the Sea of ​​Japan, reaching heights of 4 to 6 meters in the north of Honshū, 2 to 5 meters on the island of Sado and up to 4 meters on the east coast of the Korean Peninsula . Estimates of the number of deaths vary between around 1600 and over 2000 people; numerous houses and ships were also destroyed.

In 1742 the eruptions continued, with ash and pele hair falling on the Oshima Peninsula. In August 1759, the city of Aomori in northern Honshūs was affected by ash fall. The last known eruptions were in 1786 and 1790; at that time clouds of smoke were noticed over Oshima-Ōshima . In the second half of the 20th century, fumaroles were active on Kampō-dake .

As far as we know today, the north side of the Kiyobe-dake collapsed during the eruption on August 29, 1741 , which triggered an avalanche of debris that was deposited in the sea. The result was the horseshoe-shaped crater with a diameter of around 1.3 kilometers and a depth of up to 160 meters. The eruptions of 1742 built up the pyroclastic cone Kampō-dake ; at the same time, 'A'a lava came out in the crater and reached the sea.

The origin of the tsunami is controversial, as the mass moved by the debris avalanche on the island is not sufficient to cause a tsunami of the magnitude observed. One possible explanation was seen in an earthquake that triggered both the collapse of the volcano and the tsunami. Bathymetric surveys of the sea floor north of Oshima-Ōshima , carried out from 1993 onwards, showed that the flank collapse continued to a depth of over 1000 meters and was up to 5 kilometers wide. At greater depths, there are Hummocks - hill shapes that often arise when debris avalanches are deposited. The avalanche of debris moved a volume of around 2.5 km³ up to 16 kilometers. According to model calculations, a debris avalanche of this magnitude alone could have triggered the observed tsunami.

literature

Web links

Individual evidence

  1. ↑ a b 渡 島 大 島 . In: 日本 の 地名 が わ か る 事 典 / デ ジ タ ル 大 辞 泉 at kotobank.jp. Retrieved March 15, 2013 (Japanese).
  2. ↑ a b topographical map of the National Land Surveyor's Office
  3. ↑ a b Satake, Kato, 1741 Oshima-Oshima Eruption , p. 428.
  4. ↑ Katsui, Yamamoto, 1741-1742 Activity , pp. 527, 535.
  5. ^ Eruptive History in the Global Volcanism Program (accessed March 15, 2013).
  6. ↑ Katsui, Yamamoto, 1741-1742 Activity , p. 529.
  7. ↑ Satake, Volcanic origin , p. 385.
  8. ^ Entry in the NOAA / WDC Tsunami Event Database of the NOAA (English, accessed on March 15, 2013).
  9. ↑ Satake, Volcanic origin , p. 384.
  10. ↑ Katsui, Yamamoto, 1741-1742 Activity , p. 530.
  11. ↑ Katsui, Yamamoto, 1741-1742 Activity , pp. 533, 535.
  12. ↑ Katsui, Yamamoto, 1741-1742 Activity , p. 534.
  13. ↑ Satake, Kato, 1741 Oshima-Oshima Eruption , pp. 428f.
  14. ↑ Satake, Volcanic origin , p. 389.