2011 Japan Earthquake and Tsunami

The earthquake

The earthquake began at 2:46 p.m. local time, with its Epicentre in the Pacific Ocean east of the Tōhoku region of Honshu. It occurred on the Japan Trench, a Subduction zone where the Pacific Plate sinks beneath northern Japan. A section of the plate boundary several hundred kilometers long slipped, and near the trench the slip reached tens of meters, among the largest ever measured.

Strong shaking lasted for several minutes along much of the northeastern coast and was felt across Japan, including in Tokyo, hundreds of kilometers away. Parts of the coastline moved east by several meters and sank, which made low-lying areas more vulnerable to flooding.

Early warning in action

Japan's national earthquake early warning system detected the first waves and issued alerts before strong shaking reached many areas, including Tokyo, where people received warning seconds before the strongest shaking arrived. Trains, including high-speed Shinkansen trains, were slowed or stopped automatically.

The Japan Meteorological Agency also issued tsunami warnings within minutes. However, the initial estimates of the earthquake's size and the expected wave heights were too low, because measuring a giant earthquake quickly is very difficult. Warnings were raised as more data arrived. Our article on the earthquake early warning system explains how these systems work.

The tsunami

The sudden upward movement of the sea floor pushed up a huge volume of water, creating a Tsunami. The first waves reached parts of the coast in roughly 30 minutes or less. Along the deeply indented coastline of Iwate and Miyagi prefectures, the water rose especially high, and run-up heights in some places exceeded 30 to 40 meters above sea level.

The tsunami overtopped seawalls built for smaller events, flooded coastal plains for several kilometers inland and destroyed entire neighborhoods. Most of the people who died were killed by the tsunami rather than the shaking. The waves also crossed the Pacific and caused damage at harbors in places as far away as Hawaii and California.

The Fukushima Daiichi nuclear accident

The Fukushima Daiichi nuclear power plant on the coast of Fukushima Prefecture shut down its operating reactors automatically when the earthquake hit. However, the tsunami flooded the site and knocked out the backup power needed to keep the reactors cool. Without cooling, the cores of three reactors were severely damaged, and radioactive material was released.

Tens of thousands of residents were evacuated from the surrounding area, and some communities remained off-limits for years. The accident led to major reviews of nuclear safety in Japan and around the world, especially of protection against tsunamis and loss of power.

The human toll and recovery

Nearly 20,000 people died or remain missing, most of them in Iwate, Miyagi and Fukushima prefectures. Many were older adults who had difficulty evacuating quickly. Hundreds of thousands of people lost their homes or were displaced, and rebuilding coastal towns took many years.

Large aftershocks continued for months, and aftershock activity remained elevated for years, which is typical after an earthquake this large.

Lessons from 2011

The disaster changed how Japan and other countries prepare for tsunamis. Key lessons include:

• Plan for the largest plausible tsunami, not just the size of recent ones.
• Do not rely on seawalls alone; evacuation to high ground remains essential.
• On the coast, strong or long shaking is a warning in itself: move to high ground immediately without waiting for an official alert.
• Do not return to low areas until officials say it is safe, because later waves can be larger than the first.
• Critical facilities need protected backup power and defenses against flooding.

Frequently asked questions