A moderate earthquake hit Tokyo early Saturday, waking residents and shaking buildings in the Japanese capital, but there was no immediate report of any damage.
The 5.4-magnitude quake, with its epicentre located in Tokyo Bay, struck at 5:49 am (2049 GMT), according to the US Geological Survey.
The Japan Meteorological Agency said no tsunami warning had been issued, and that the quake was 70 kilometres (43 miles) deep.
Residents said the quake was not strong enough to knock things from shelves. It prompted a handful of reports of possible injuries, including a woman who complained about being hurt after falling from her bed, according to national broadcaster NHK.
It temporarily stopped Tokyo subways and trains, but service was quickly resumed.
The tremor did not cause any damage to the region's nuclear facilities, according to the government, and did not affect the areas that host the crippled Fukushima Daiichi nuclear plant, which went through meltdowns after a 9.0-magnitude quake and tsunami disaster in 2011.
It served as a reminder that a huge earthquake could strike the Japanese capital -- which has a population of about 13 million people -- at any time.
Experts have long warned Japan to stay vigilant for the next "Big One", and a powerful 7.8-magnitude earthquake struck off the coast in May.
Residents and officials of Tokyo and the rest of the nation routinely hold emergency drills, with the government stepping up its disaster response in the wake of the 2011 devastation.