The 4th Japan-Korea Joint Symposium for Landslide Disaster Mitigation 2011, August 25 2011, Sapporo, Japan 北海道の地すべり・岩盤崩壊・巨大崩壊 Landslide, rock failure and gigantic failure in Hokkaido 伊東 佳彦 1) , Yoshihiko Ito, 1) 独立行政法人土木研究所寒地土木研究所防災地質チーム, Team Leader, Geological Hazards Research Team, Civil Engineering Research Institute for Cold Region, PWRI, [email protected]SYNOPSIS: This paper reviews geological features and recent or past big disasters by landslide, rock failure and gigantic failure in Hokkaido. In Japan, though landslide disaster of the Sea of Japan side from Hokuriku to Tohoku district is very famous, there also distributes considerably many landslides in Hokkaido. The number of landslide topography is more than 12,800. The average size of them is 516m in length, 390m in width and 0.2274km 2 in area. The average area of landslide topography in serpentine or andesite zone is about four times larger than that in none-consolidated sediments or Tertiary Mudstone. Geologically, frequent occurrence area of landslide in Hokkaido is classified into four groups; volcanic rock area, “Green tuff” area, Cretaceous to Neogene Mudstone area, and Serpentine to “Greenrock” area. Rock failure is very dominant in Hokkaido. The Sea of Japan side and northeast coast of Erimo Peninsula is two major zone of rock failure in Hokkaido. The geology of the former is pyroclastic rock and volcanic rock, and that of the latter is hornfels or sedimentary rock in accretionary prism. Rock failure size of the former varies from very large to small, while that of the latter is ordinary small without some exception. Toyohama Tunnel rock failure in 1996 is very infamous because 20 people in a bus died and one in a car was injured. Daini-Shiraito Tunnel rock failure in 1997, Hokuyo (two victims) in 2001 and Erimo (one victims and one injured) in 2004 is also well known. These four rock failures including Toyohama happened along national road, and traffic was held up for the long time. Machida (1978) extracted 333 gigantic failure topographies, and pointed out that many of them distributed in Hokkaido (132) and in Tohoku district (126). As for historical record of gigantic failure, 11 cases were reported and 3 of them happened in Hokkaido. Tsunami caused by gigantic failure is one of the worst natural disasters concerning mass movement. In Japan, Unzen-Mayuyama collapse in 1792 in Kyushu district is the worst one. The debris avalanche fell into Ariake Sea, tsunami happened, and 15,000 people died. In Hokkaido, tsunami caused by debris avalanche happened in two cases. One is Komagatake in 1640 and another is Oshima-Ooshima in 1741. Both are the slope of active volcano. About 700 people died by the former and more than 2,000 people died by the latter. By the latter, it is confirmed that the tsunami reached Tsugaru district (north of Honshu Island), Sado Island, Noto Peninsula, and even the east coast of Korea. Key Words: landslide, rock failure, gigantic failure 1.はじめに 変動帯に属する日本は,地形が急峻で地質が脆弱であり地震動や火山噴火も多い.また,台風や豪 雨・豪雪により毎年のように洪水や土砂災害が発生している.このような国土に多くの人間が生活し, 道路や鉄道等の社会基盤が稠密に整備されているため,日本は世界的に見ても斜面災害の多発国とな っている.北海道はこれら日本共通の条件に加え,凍結融解作用や融雪期の多量の地下水供給,ある いは気温の日較差や年較差が大きいなど,斜面にとって一層厳しい条件下にある. マスムーブメントには地すべり,土石流,岩盤崩壊など様々な形態がある(Fig.1).今回はこの うち,北海道で特徴的,あるいは被害が顕著な地すべり,岩盤崩壊,および巨大崩壊について述べる. 42 50
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The 4th Japan-Korea Joint Symposium for Landslide Disaster Mitigation 2011, August 25 2011, Sapporo, Japan
北海道の地すべり・岩盤崩壊・巨大崩壊 Landslide, rock failure and gigantic failure in Hokkaido
伊東 佳彦1), Yoshihiko Ito,
1)独立行政法人土木研究所寒地土木研究所防災地質チーム, Team Leader, Geological Hazards Research
Team, Civil Engineering Research Institute for Cold Region, PWRI, [email protected]
SYNOPSIS: This paper reviews geological features and recent or past big disasters by landslide, rock failure and gigantic
failure in Hokkaido. In Japan, though landslide disaster of the Sea of Japan side from Hokuriku to Tohoku district is very
famous, there also distributes considerably many landslides in Hokkaido. The number of landslide topography is more than
12,800. The average size of them is 516m in length, 390m in width and 0.2274km2 in area. The average area of landslide
topography in serpentine or andesite zone is about four times larger than that in none-consolidated sediments or Tertiary
Mudstone. Geologically, frequent occurrence area of landslide in Hokkaido is classified into four groups; volcanic rock area,
“Green tuff” area, Cretaceous to Neogene Mudstone area, and Serpentine to “Greenrock” area.
Rock failure is very dominant in Hokkaido. The Sea of Japan side and northeast coast of Erimo Peninsula is two major zone
of rock failure in Hokkaido. The geology of the former is pyroclastic rock and volcanic rock, and that of the latter is hornfels
or sedimentary rock in accretionary prism. Rock failure size of the former varies from very large to small, while that of the
latter is ordinary small without some exception. Toyohama Tunnel rock failure in 1996 is very infamous because 20 people in
a bus died and one in a car was injured. Daini-Shiraito Tunnel rock failure in 1997, Hokuyo (two victims) in 2001 and Erimo
(one victims and one injured) in 2004 is also well known. These four rock failures including Toyohama happened along
national road, and traffic was held up for the long time.
Machida (1978) extracted 333 gigantic failure topographies, and pointed out that many of them distributed in Hokkaido (132)
and in Tohoku district (126). As for historical record of gigantic failure, 11 cases were reported and 3 of them happened in
Hokkaido. Tsunami caused by gigantic failure is one of the worst natural disasters concerning mass movement. In Japan,
Unzen-Mayuyama collapse in 1792 in Kyushu district is the worst one. The debris avalanche fell into Ariake Sea, tsunami
happened, and 15,000 people died. In Hokkaido, tsunami caused by debris avalanche happened in two cases. One is
Komagatake in 1640 and another is Oshima-Ooshima in 1741. Both are the slope of active volcano. About 700 people died
by the former and more than 2,000 people died by the latter. By the latter, it is confirmed that the tsunami reached Tsugaru
district (north of Honshu Island), Sado Island, Noto Peninsula, and even the east coast of Korea.
Key Words: landslide, rock failure, gigantic failure
Fig. 5 Rock failures for recent 50 years in Hokkaido (more than 1,000m3)
More than 10,000m3
Between 1,000m3 and 10,000m3
Toyohama
Hokuyo
Daini-Shiraito
Northeast Erimo
Coastal area of the Sea of Japan
Fig. 4 Rock failures for recent 50 years in Japan(more than 10,000m3)
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Fig. 6 Photo of recent 4 major disiasters (upper left:Toyohama, upper right:Daini-Shiraito, lower left:Hokuyo, lower right:Erimo): Photo by Hokkaido Development Bureau
北海道では1996年の豊浜トンネルや1997年の第2白糸トンネルでの大規模岩盤崩壊を契機に,
国道沿いの岩盤崩壊の調査・対策が鋭意進められた.しかし,2001年の北陽,2004年のえりも等,
大規模岩盤崩壊による被害はその後も発生しており,その備えは依然十分とは言えない(北海道
開発局の道路部門では以上の4災害を4大災害と呼んでいる).4大災害の写真と概要をそれぞ
れ,Fig.6 およびTable 2 に示す.
Table 2 Outline of recent 4 major disasters
Toyohama Daini-Shiraito Hokuyo Erimo
Date Feb.10, 1996 1st. Aug.35, 19972nd. Aug.28, 1997
Sep. 4, 2001 Jan. 13, 2004
Volume 11,000m3 1st.failure: 42,000m3
2nd.failure: 14,000m3 24,000m3 42,000m3
Casualty 20 people died No people died 2 died 1 died, 1 injuredGeology Miocene pyroclastic r
ock Pliocene pyroclastic rock
Accretionary prism(pyroclastic rock, alternation of limestone and chert, etc.)
Hornfels
Remarks Bedding plane was frozen before the failure, and outflow of groundwater from the plane was observed after the failure
Heavy rain was recorded from three weeks before to 10 days before the failure.
Heavy rain was recorded about three weeks before and just before the failure.
Fairly big earthquakes happened about four months ago, and small earthquakes happened in the morning
Sign Crash and sand fall were reported in the tunnel one hour and 30 minutes before the failure.
Small failure happened several days and about half a day before the failure