What happened on 11 March 2011, and how did a wave cause a nuclear accident?
Japan's strongest recorded earthquake sent a wave that was 13-14 m high at a plant with a 10 m ground level.
▶ Start the storyA huge undersea earthquake sent a wave over the coast, and the wave cut the power that kept a nuclear plant cool. On 11 March 2011 a magnitude 9.0 to 9.1 earthquake struck the Pacific 72 km east of the Oshika Peninsula in the Tōhoku region. It lasted about six minutes and caused a tsunami. It was the most powerful earthquake ever recorded in Japan, and the fourth most powerful in the world since 1900.
The wave was fast and high. Waves may have reached 40.5 m at Miyako, and near Sendai they travelled at 700 km/h and up to 10 km inland. Residents of Sendai had only eight to ten minutes of warning. Official figures released in 2021 reported 19,759 deaths, 6,242 injured and 2,553 missing, and a report in 2015 found that 228,863 people were still living away from their homes.
At the Fukushima Daiichi nuclear plant the largest wave was 13 to 14 m high and hit about 50 minutes after the earthquake. It overtopped the and the plant's 10 m ground level. The direct cause of the accident was the earthquake and tsunami, which caused electrical grid failure and damaged nearly all the plant's backup energy sources. Without power the cooling systems halted and heat built up.
14:46
Magnitude 9.0-9.1 earthquake, about six minutes long
About 50 min later
13-14 m wave overtops the plant's
After the wave
Backup power damaged; cooling halts and heat builds
The aftermath was complicated. At least 164,000 residents were displaced, and the displacement itself resulted in at least 51 deaths, among hospital patients and elderly people moved out of the zone. Radiation is a separate question: the United Nations committee on radiation said no adverse health effects among Fukushima residents had been documented that were directly attributable to radiation exposure. Investigations, meanwhile, faulted lapses in safety and oversight.
Fukushima Daiichi
- Ground level 10 m above sea level
- Wave of 13-14 m overtopped the
- Backup power damaged; cooling halted
Onagawa
- Closer to the
- Sea wall 14 m tall
- Withstood the tsunami, preventing serious damage
Quiz me
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Recap
Earthquake, then wave, then lost power, then lost cooling: the Fukushima accident was a chain with the tsunami in the middle.
💡 A trick to remember it · Shake, wave, blackout, heat: four links in one chain, and a taller wall broke it at another plant.
Surprising fact · Waves up to 13-14 m high overtopped a seawall at the plant, while Onagawa's 14 m wall held.
Sources (2)
No source, no claim. Every fact in this lesson (16 claims) cites at least one of these.