Failure Knowledge Database / 100 Selected Cases

Okushiri Tsunami Generated by Southwest-off Hokkaido Earthquake
July 12, 1993 at Okushiri Island off the Hokkaido southwest coast
Masayuki Nakao (Institute of Engineering Innovation, School of Engineering, The University of Tokyo)
A powerful earthquake of magnitude 7.8 occurred southwest of Hokkaido in the Sea of Japan and unleashed atsunami that devastated nearby Okushiri Island. Strong shocks, tsunami waves, fires and mudslides claimed 230
lives including the ones missing and feared dead on the small Okushiri Island with a population of 4,700. In allaffected areas, mostly in Hokkaido, the disaster killed 200 (one in Aomori), injured 236, and 34 people weremissing.
1. EventA p owerful ea rthquake of magnitude 7 .8 occurred s outhwest of Hokkaido in t he Sea o f Japan an dunleashed a tsunami that devastated near by Okushir i Island. I mmediately after the earthquake, a tsu nami
struck the island, causing fires and mudslides. The nu mber of missing and dead from the disaster stood at230.

Figure 1. Seismic Center of Southwest-off Hokkaido Earthquake1

Failure Knowledge Database / 100 Selected Cases

2. Course
At 22:17 on July 12, 1993, an earthquake occurred southwest (at latitude 42.47 north and longitude 139.12east) of Hokkaido in the Sea of Japan. The powerful 7.8 magnitude quake had its epicenter at a depth ofabout 34 km on the sea floor very close to the Okushiri Island (Figure 1). The island was hit by a tremor
measuring 6 on the Richter scale (estimated because the island did not have a seismograph).
At around 22:20, the first wav e of the tsunami struck the island, sweeping away many people and h omes
(385 homes o ut of 504) in the Aonae d istrict of the is land. Structures of 19 0 h omes remaining af ter thewave attack were destroyed by the fires that engulfed the area (total 51,000 meters squared) due to broken
gas lines and toppled fuel containers. The quake triggered a mudslide in the Okushiri district of the island,sending a hug e chunk of mu d and rocks onto a hotel. The mudslide killed 29 people including th e guests
staying at the hotel.
At the northern tip of the island, the tsunami waves swept the Inaho district, taking 70 homes.
About 5 minutes after the quake at 22:22, the Sapporo District Meteorological Observatory issued tsunami
warning.
Among the tsunami waves that struck the coastal towns of the Sea of Japan, th e ones that hit the Okushiri
Island were the strongest. Waves reported variously as being between 6.7 and 30.6 meters in height swept
through the co astal areas : 8.5 m in Inaho, 3 .5 m in Okus hiri, 16 .8 m in Hatsu- Matsumae, and 6.7 m inAonae. In the Monai district located west of the island, the wave height reached 23.2 m at the high-pitchedriver mouth, and 30.6 m at the inland riverfront. The highest inundation height of 11 m was observed in thewest part of the island, and the lower one of 5 m in the east part of the island.
The disas ter caused considerable da mages in cluding ear thquake cracks, subsidence dam age, d estroyed
structures, liquefaction of ground, and leakage of kerosene from toppled fuel containers.
The tsunami waves also struck the west part of the Oshima peninsula (located in Hi yama, Hokkaido) andTohoku prefectures.
Photo 1 shows the devastated fishing port of Aonae, and Photo 2 the landslide in the Okushiri district.
People in the Okushiri Island had experienced a tsunami generated by the Japan Sea earthquake 10 years
before, in which the tsunami struck the island 20 minutes after they felt shocks beneath their feet. 7% of theisland’s population, who assumed that ts unami would not reach the shore so s oon, took time to evacu ate
and failed to escape from the waves.
The waves propagated to the island in approximately 3 minutes, which allowed enough time for the healthy
young to run or cycle and evacuate safely to an elevated inland location. In response to the 1993 tsunami,
6.6 m-tall artificial ground facilities w as created at th e port of Aonae as ev acuation and comm unicationroutes leading to an elevated inland location.

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Failure Knowledge Database / 100 Selected Cases

Photo 1. Devastated Aonae Port [1]Photo 2. Landslide in Okushiri [1]3. CauseA p owerful 7.8 magnitude earthquake occurr ed s outhwest (at latitude 42.47 n orth an d long itude 1 39.12east) of Hokkaido in the Sea of Japan. Because the quake had its epicenter at a depth of 34 km on the sea
floor very close to the Okushiri Island, the tsunami struck the island in less than 4 minutes after the tremor,leaving no time for th e people of the island to ev acuate. The ma ssive tsunami s wept acr oss t he is land,
destroying homes and towns in an instant. It caused the most of the casualties.
The location of many homes by the seaside contributed to high toll of victims.
4. Immediate Action
The Sap poro Di strict Meteorol ogical Ob servatory p rocessed data quic kly and i ssued tsun ami warning at
22:22, within 5 minutes after the quake. However, it was just the propagation time of the first wave to thesouthern tip of the Okushiri Island.

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Failure Knowledge Database / 100 Selected Cases

5. Countermeasure
As the disas ter p revention measures, the local govern ment con structed tide em bankments, ins talled twotsunami sluices on a riv er ( the s luice gates sh own in Photo 3 automatically close after one-minute
emergency broadcasting w hen a q uake of magnitude 5 is d etected), and r einforced sl opes. F or me asuresthat help emergency evacuation, the local government created artificial ground facilities as evacuation andcommunication routes, distr ibuted a d isaster preven tion handbook to r esidents, and sup ported indiv idual
household for purchasing emergency broadcast receiver.

Photo 3. Tsunami Sluice [1]6. Summary
The tsunami-earthquake occurred in a quiet zone within a seismic gap, where fewer earthquakes had been
experienced h istorically. The tot al da mage ca used by this tr emendous dis aster was the b iggest in pas t 45years in Japan. The dis aster is called “Okushiri earthquake disaster” because the Okushiri Island received
the heaviest damage in the disaster from strong shocks, tsunami waves, fires and mudslides.
7. Knowledge
This disaster provided a stark lesson on safety action. When we felt an earthquake, we must:(1) Put out the fire first,(2) Never go to low ground, and(3) Immediately escape to high ground.
Evacuation war ning may arr ive to o late , i f w e w ait for a n o fficial wa rning o f a natural di saster. I n t his
Okushiri disaster, although issued quickly after the earthquake, the ts unami warning did n ot arrive in ti me
to save lives from the first tsunami wave.

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Failure Knowledge Database / 100 Selected Cases

8. Background
The Okushiri Island, so-called “Okinawa in t he north”, is surrounded by deep blue water and blessed with
an abundance of seafood such as sea urchin, abalone, flounder, squid and atka mackerel. Unex pectedly toall, a tsunami attacked this beautiful island rich in water resources and woods.
The 1 983 so uthwest-off Ho kkaido e arthquake o ccurred in a qu iet z one w ithin a s eismic g ap, w hich islocated between the epicentral area of the 1983 Japan Sea earthquake (magnitude 7.7) and that of the 1940
Shakotan earthquake ( magnitude 7.5). I t is j ust of f th e we st coast of t he main island wh ere t he N orth
American and Eurasian plates collide. These three earthquakes were all caused by the Eurasian plate being
subducted u nderneath Japan. The 1983 sou thwest-off Hokk aido earthquake h ad a s cale and a focal
mechanism similar to the 1983 Japan Sea earthquake. Crustal deformation caused by fault motion resultedin depression (20 cm – 80 cm) and movement (1 m – 2 m to the west) of the Okushiri Island.
References[1] Administrative Association for the Hokkaido Hiyama Area, “Yomigae-ru Yume-no Shima (Reviving
Beautiful Island) – Report on the Southwest-off Hokkaido Earthquake”,
http://www.hiyama.or.jp/earthqu/default.htm#saigai[2] The General Insurance Association of Japan, Jishin! Guratto Kuru Mae-ni – Jishin Saigai-no Kowasa-oMise Tsuketa Hokkaido Naisei-Oki Jishin (Earthquake! Before Shocks Hit – Southwest-off Hokkaido
Earthquake That Demonstrate the Threat), The General Insurance Association of Japan.

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