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11 March 2011Reputable source · 2 sourcesWell documented

The Tohoku Earthquake and Tsunami Trigger a Nuclear Meltdown at Fukushima

A magnitude 9.0 earthquake off the northeast coast unleashes a tsunami that overwhelms a nuclear plant's defenses

On the timeline · around 11 March 2011 · Postwar and Contemporary JapanPostwar and Contemporary JapanThe Tohoku Earthquake and Tsunami Trigger a Nuclear Meltdown at Fukushima196019701980199020002010

Quick facts

Earthquake magnitude
9.0-9.1
Date
11 March 2011
Tsunami height at Fukushima
c. 15 meters (defenses built for 3.1 m)
INES accident rating
Level 7 (highest)

What happened

On 11 March 2011, a magnitude 9.0 earthquake, described by USGS geophysicist Bill Ellsworth as "one of the largest earthquakes that we have ever recorded" and "the largest instrumentally recorded earthquake ever to hit Japan," struck off the Tohoku coast. The resulting tsunami reached the coast within roughly 50 minutes, with waves reported as high as 30 to 40 feet reaching up to 10 kilometers inland, and coastal towns including Natori and Minamisanriku were, in USGS's words, "virtually obliterated." At the Fukushima Daiichi nuclear plant, a 15-meter tsunami overwhelmed sea walls designed for only 3.1 meters, knocking out cooling systems; all three operating reactor cores largely melted within the first three days. The accident was rated level 7, the highest level, on the International Nuclear and Radiological Event Scale, with total radioactive releases estimated around 940 PBq (iodine-131 equivalent), and more than 100,000 people were evacuated from an expanding exclusion zone around the plant.

Why it matters

Fukushima became the world's second level-7 nuclear accident after Chernobyl and forced a fundamental reassessment of nuclear safety standards worldwide, prompting Japan and other countries to review and in some cases shut down reactors. The disaster also stands as a stark reminder that Japan's modern industrial and technological strength sits on some of the most seismically active ground on Earth, the same geology that shaped the islands since the Jomon period.

How we know

The earthquake's magnitude and the plant's meltdown sequence are documented through USGS seismic monitoring data and detailed nuclear-safety incident analysis, including timeline reconstructions of reactor core temperatures and containment failures at each unit.

Sources

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Part of a timelineHistory of Japan34 events · From cord-marked pottery on a Neolithic archipelago to a nuclear disaster on a shaken coastline, sixteen thousand years of islands remaking themselvesView all →
The Tohoku Earthquake and Tsunami Trigger a Nuclear Meltdown at Fukushima · History of Japan · SourcedStory