A 7.8-magnitude earthquake struck off the Japanese coast Saturday, geologists said, shaking buildings in Tokyo and setting off car alarms, an AFP correspondent reported.

A 7.8-magnitude earthquake struck off the Japanese coast Saturday, geologists said, shaking buildings in Tokyo and setting off car alarms, an AFP correspondent reported.

Despite the huge power of the quake, there was no risk of tsunami, The Pacific Tsunami Warning Centre reported.

Residential buildings swayed for around a minute as the quake built in intensity at around 8.30pm (1130 GMT).

The epicenter was 676 kilometers (422 miles) below the Earth's surface. It was centered on a remote spot in the Pacific Ocean around 870 kilometers south of Tokyo, the US Geological Survey said.

Both runways at Narita Airport, the main international gateway to Tokyo, were temporarily closed while inspections were carried out.

It was the second sizable shake Tokyo has had this week, after a much less powerful -- but far shallower -- quake hit close to the capital on Monday.

Japan sits at the meeting place of four tectonic plates and experiences around 20 percent of the world's most powerful earthquakes every year.