The 2011 Tohoku earthquake and tsunami, Japan

Subduction at a destructive plate margin

Off the Tohoku coast the Pacific plate sinks beneath the plate carrying northern Honshu at the Japan Trench, moving about 83 mm a year. The two plates were locked together, so strain built up over centuries. On 11 March 2011 a section of the boundary roughly 400 km long ruptured at once, releasing that stored energy as a magnitude 9.1 earthquake.

A megathrust slip that lifted the seabed

The fault slipped tens of metres in places, and because the focus was fairly shallow the movement thrust a vast area of seabed upwards. That displaced the entire column of ocean water above it. The bulge of water spread outwards as a tsunami, growing taller as it entered the shallow water near the coast.

A low-lying and densely settled coast

Fishing ports and towns such as Rikuzentakata and parts of Sendai sit on flat coastal plains only a few metres above sea level, and narrow bays funnelled the waves higher. The first waves arrived within about 30 minutes, so even with warnings some people, especially the elderly, could not reach high ground in time.

Primary

• Severe shaking lasted several minutes across eastern Honshu, though strict building codes stopped most buildings collapsing.
• Parts of the Tohoku coastline subsided, leaving the land more exposed to flooding.
• Roads, railways and ports across the region were cracked and buckled.

Secondary

• The tsunami destroyed more than 123,000 houses and damaged almost a million more.
• Flooding at the Fukushima Daiichi nuclear plant knocked out cooling and caused meltdowns rated level 7 on the international nuclear event scale.
• Fires broke out in flooded ports and refineries, including a large blaze at an oil facility in Chiba.

Social

• More than 18,000 people died, most of them drowned by the tsunami.
• Over 100,000 people were evacuated from the area around Fukushima Daiichi, and many never returned home.
• More than 470,000 people in total were evacuated from their homes.

Economic

• Japan's Cabinet Office estimated direct damage of about 210 billion dollars, the costliest earthquake in world history.
• Factories making cars and electronics shut down, disrupting supply chains around the world.
• Japan switched off its other nuclear plants, forcing expensive imports of gas and oil for years.

Environmental

• Seawater and debris contaminated farmland along the coast.
• Radioactive material leaked into the air and the Pacific Ocean from Fukushima Daiichi.
• Millions of tonnes of wreckage were washed out to sea, some drifting across the Pacific for years.

Immediate

• The Japan Meteorological Agency broadcast a tsunami warning within about three minutes of the earthquake.
• Around 100,000 members of the Self-Defence Forces were mobilised for search, rescue and relief.
• A 20 km evacuation zone was declared around Fukushima Daiichi and residents were moved out.
• Emergency assistance squads and medical teams were sent from 24 countries and regions, and 163 countries and regions offered aid and relief.

Long term

• A national Reconstruction Agency was set up in 2012 to run a rebuilding programme lasting more than a decade.
• Higher sea walls were built along hundreds of kilometres of the Tohoku coast.
• Whole neighbourhoods were rebuilt on raised ground, with housing moved back from the shore.
• Nuclear safety rules were rewritten and most reactors stayed offline for years while they were checked.