An earthquake measuring 9.0 in magnitude strikes 130 km (81 mi) east of Sendai, Japan, triggering a tsunami killing thousands of people. This event also triggered the second largest nuclear accident in history, and one of only two events to be classified as a Level 7 on the International Nuclear Event Scale.

On 11 March 2011, at 14:46:24 JST, a M_w 9.0–9.1 undersea megathrust earthquake occurred in the Pacific Ocean, 72 km (45 mi) east of the Oshika Peninsula of the Tōhoku region. It lasted approximately six minutes and caused a tsunami. It is sometimes known in Japan as the "Great East Japan Earthquake" , among other names. The disaster is often referred to by its numerical date, 3.11.

Sendai is the capital city of Miyagi Prefecture and the largest city in the Tōhoku region. As of 1 August 2023, the city had a population of 1,098,335 in 539,698 households, making it the twelfth most populated city in Japan.

On March 11, 2011, a major nuclear accident started at the Fukushima Daiichi Nuclear Power Plant in Ōkuma, Fukushima, Japan. The direct cause was the Tōhoku earthquake and tsunami, which resulted in electrical grid failure and damaged nearly all of the power plant's backup energy sources. The subsequent inability to sufficiently cool reactors after shutdown compromised containment and resulted in the release of radioactive contaminants into the surrounding environment. The accident was rated seven on the International Nuclear Event Scale by Nuclear and Industrial Safety Agency, following a report by the JNES. It is regarded as the worst nuclear incident since the Chernobyl disaster in 1986, which was also rated a seven on the International Nuclear Event Scale.

The International Nuclear and Radiological Event Scale (INES) was introduced in 1990 by the International Atomic Energy Agency (IAEA) in order to enable prompt communication of safety significant information in case of nuclear accidents.