March 11, 2011

2011 Tōhoku earthquake and tsunami

The Tōhoku earthquake, tsunami and Fukushima emergency strike Japan

Japan issued its first tsunami warning three minutes after the quake — but the height forecast was too low, because the system had not yet resolved the earthquake's magnitude.

14:46:24 JST, 11 March 2011

A magnitude-9.1 megathrust earthquake occurred beneath the Pacific near northeastern Honshu at 14:46:24 Japan Standard Time on 11 March 2011; USGS locates it at 38.297°N, 142.373°E and 29 kilometres depth. The Japan Meteorological Agency's principal technical account gives Mw 9.0 — the same earthquake, a different solution.

The rupture ran along the subduction boundary where the Pacific plate descends beneath the plate carrying northeastern Japan, and it generated a destructive tsunami.

Three minutes, then the water

JMA issued its first tsunami warnings and advisories at 14:49, about three minutes after the earthquake. The initial forecast underestimated the eventual wave heights because the magnitude could not be determined accurately within those first minutes.

Later observations exceeded 9.3 metres at Soma and 8.6 metres at Ishinomaki before gauges failed or were overtopped. Post-disaster field investigation found a localised maximum run-up of 40.1 metres at Ryōri Bay in Ōfunato, Iwate Prefecture — a run-up measurement, not a 40-metre offshore wave.

Station blackout at Fukushima Daiichi

Eleven operating reactors at the Onagawa, Fukushima Daiichi, Fukushima Daini and Tokai Daini sites shut down automatically after the earthquake. At Fukushima Daiichi the tsunami flooded electrical equipment and disabled every operating emergency diesel generator except the one at Unit 6, producing station blackouts and loss of major cooling functions.

Japan declared a nuclear emergency at 19:03. At 21:23 it ordered evacuation within 3 kilometres of Fukushima Daiichi and sheltering from 3 to 10 kilometres; the reactor explosions and the wider 20-kilometre zone came on later days.

Sources

Researched 23 Aug 2026 10 sources 2 audit passes

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What the sources leave uncertain

Magnitude depends on agency and solution: the USGS gives Mw 9.1, while the Japan Meteorological Agency's principal technical account gives Mw 9.0. Both describe the same earthquake.
Casualty totals vary because police counts emphasize directly confirmed dead and missing, while broader government totals add certified disaster-related deaths. Every total should identify its category and reporting date.
Tsunami “height,” gauge height, inundation depth and run-up are different measurements. The 40.1 m figure is a localized run-up, not a 40.1 m offshore wave along the entire coast.
The initial tsunami forecast underestimated the eventual wave heights because the earthquake's magnitude could not be determined accurately within the first few minutes.