A major 7.3-magnitude offshore earthquake rattled Japan on Wednesday, swaying Tokyo buildings, triggering a small tsunami and reminding the nation of the ever-present threat of seismic disaster.
Police reported no casualties or property damage, and operators of nuclear power plants and Shinkansen bullet trains quickly gave the all-clear, while the wave hitting the Pacific coast measured just 60 centimetres (24 inches).
The tremor struck in the late morning about 160 kilometres (100 miles) offshore and 430 kilometres northeast of Tokyo, at a shallow depth of 10 kilometres beneath the Pacific seafloor, authorities said.
In greater Tokyo -- the world's most populous urban area with more than 30 million people -- the earthquake and a succession of tremors that quickly followed were uncomfortably felt as they shook buildings.
The state Meteorological Agency issued a coastal tsunami advisory just a few minutes after the quake, but lifted it three hours later.
Television channels immediately cancelled their programming to transmit information on the quake and the tsunami alert.
It soon became clear the quake had left Japan unscathed, but it was yet another uncomfortable reminder that the threat of "the Big One" is a reality of daily life.
Japan is located on the "Pacific Ring of Fire" and dotted with volcanoes, and Tokyo is in one of its most dangerous areas.
The mega-city sits on the intersection of three continental plates -- the Eurasian, Pacific and Philippine Sea plates -- which are slowly grinding against each other, building up enormous seismic pressure.
The government's Earthquake Research Committee warns of a 70 percent chance that a great, magnitude-eight quake will strike within the next 30 years in the Kanto plains, home to Tokyo's vast urban sprawl.
The last time a "Big One" hit Tokyo was in 1923, when the Great Kanto Earthquake claimed more than 140,000 lives, many of them in fires. In 1855, the Ansei Edo quake also devastated the city.
More recently, the 1995 earthquake in the city of Kobe killed more than 6,400 people.
Small quakes are felt every day somewhere in Japan, and people take part in regular drills at schools and workplaces to prepare for a calamity.
Families are urged to keep earthquake survival kits at home, quake alerts can be sent via mobile phones, and parks and schools are signposted as quake shelters.
Nuclear power plants and bullet trains are designed to automatically shut down when the earth rumbles, while many buildings have been quake-proofed with steel and ferro-concrete at great cost in recent decades.
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