TOKYO — A strong earthquake with a preliminary magnitude of 6.8 struck the northeast coast of Japan on Tuesday, triggering a tsunami warning and shaking buildings more than 300 kilometres away in Tokyo, the Meteorological Agency said.

Eighty people were injured when the roof of an indoor pool collapsed in the coastal city of Sendai, the national broadcaster NHK reported. None of the injuries was critical, according to the Kyodo News Agency, which also reported two people injured in the neighbouring state of Iwata.

Bullet train service was suspended in northern Japan, while Tokyo's Haneda airport temporarily halted flights, Kyodo said.

A 10-centimetre tsunami reached the Japanese shore about 15 minutes after the quake struck, NHK said, with no major damage immediately reported. While a tsunami can rise to great heights when it arrives at the shore, such waves are often barely noticeable in the ocean.

The quake was centred nearly 20 kilometres beneath the ocean off the coast of Miyagi state in northeastern Japan, NHK said. The quake hit around 11:46 a.m. local time.

The U.S. Geological Survey registered the quake with a 7.2 magnitude.

Japan sits at the juncture of four tectonic plates -- or moving slabs of the earth's outer crust -- and is one of the world's most quake-prone regions.

Earlier in the day, a 4.9 magnitude earthquake shook Japan's northern island of Hokkaido. There was no danger of a tsunami in that one.

A magnitude-6.0 quake shook the Tokyo area July 23, injuring more than two dozen people and suspending flights and trains for hours. A magnitude-5 quake can damage homes and other buildings if it is centred in a heavily populated area.

A Dec. 26 earthquake with a magnitude estimated at 9.1 to 9.3 and the subsequent tsunami killed more than 131,000 people in Indonesia and left half a million homeless. Three months later, another strong temblor left more than 900 dead on Nias and smaller surrounding islands.