An earthquake, also known as a quake, tremor or temblor, is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time.
At the Earth's surface, earthquakes manifest themselves by shaking and sometimes displacement of the ground. When the epicenter of a large earthquake is located offshore, the seabed may be displaced sufficiently to cause a tsunami. Earthquakes can also trigger landslides, and occasionally volcanic activity.
Worst earthquake since Tohoku sparks tsunami alert in Japan
Bloomberg in Tokyo
A 7.3-magnitude earthquake struck off northeastern Japan yesterday, triggering a tsunami alert that sent people running for higher ground.
It was the most powerful in the country since last year's devastating quake and caused buildings in Tokyo to sway, temporarily disrupting rail and air traffic.
The quake hit at 5.18pm, with its epicentre about 240 kilometres off the coast of Miyagi prefecture at a depth of 10 kilometres, according to the Japan Meteorological Agency.
Authorities issued a warning that a tsunami potentially as high as two metres could hit. A one-metre tsunami later hit Ishinomaki, a city in Miyagi, with no major damage visible in images broadcast on NHK. Other towns reported smaller tsunamis.
A presenter on state broadcaster NHK repeatedly urged viewers to get to safety after the initial tremors. "Remember last year's quake and tsunami," he said. "Call on your neighbours and flee to higher ground now!"
Miyagi prefectural police said there were no immediate reports of damage, although traffic was being stopped in some places to check on roads.
NHK reported that five people were injured, including a 75-year-old woman in Miyagi who fell while fleeing the tsunami. Police could not immediately confirm the reports. In the northern prefecture of Aomori, 220 homes were without power.
About two hours after the quake struck, the tsunami warning was cancelled. The Pacific Tsunami Warning Centre earlier said there was no risk of a widespread tsunami.
While buildings in Tokyo shook for up to a minute, the quake did little damage. Last year's record 9-magnitude earthquake was 355 times more powerful than the one yesterday, according to the US Geological Survey. The Fukushima nuclear reactors were unaffected by the quake, said Tokyo Electric Power, the operator.
Aiko Hibiya, a volunteer for the recovery in Minami-Sanriku, a coastal town devastated by last year's tsunami, said she was at a friend's temporary accommodation when the quake struck. "It shook for such a long time."
She said other volunteers who had been in coastal areas were evacuated to a square and a parking lot as they waited for the tsunami warning to be lifted.
Bullet trains operated by East Japan Railway were automatically halted. Narita, the country's main international airport, reopened its runways after checks for damage, while Haneda airport in central Tokyo was operating normally. Tokyo's subway was also without problems.
Japan has barely begun to rebuild from last year's magnitude-9.0 earthquake, known as the Tohoku quake, which triggered a tsunami that swelled to 20 metres high in some areas, ravaging dozens of coastal communities in Miyagi and elsewhere.
About 19,000 people were killed and 325,000 people remain displaced from their homes.
Additional reporting by Associated Press, Agence France-Presse