What happened
On the afternoon of 11 March 2011, local time, the Pacific plate subduction zone beneath north-eastern Japan ruptured. It was the strongest earthquake ever measured in the country. Most Japanese buildings withstood the shaking, but the tsunami surged several kilometres inland, and in places the waves ran up to heights of several tens of metres. Almost 20,000 people died or went missing. Flooding of the Fukushima Daiichi nuclear power plant led to meltdowns in three reactors and the evacuation of surrounding towns.
How to read these data
Magnitude measures the energy released at the focus: each step is about 32 times more energy, so an M 9 earthquake releases about a thousand times more energy than an M 7. Its effects, however, also depend on focal depth, distance to cities, ground conditions and building quality – a shallow M 7 beneath a city can be deadlier than a deep M 8 under the ocean.
Data: USGS ComCat (official20110311054624120_30) · Wikipedia (pl) · Wikipedia (en)