Japan lifted on Thursday a week-old warning that a "megaquake", potentially causing colossal damage and loss of life could strike, with the government telling people their lives could now "go back to normal".

The alert that such a catastrophe might hit the archipelago of 125 million people prompted thousands of Japanese to cancel holidays and stock up on essentials, emptying shelves in some stores.

"Since there were no abnormalities detected in seismic activity and crustal deformation, the special call for attention ended at 5pm," disaster management minister Yoshifumi Matsumura said.

"But it doesn't mean the risk [of a major earthquake] has been eliminated," Matsumura told reporters.

"We have been asking for special precautions, such as sleeping while being prepared to evacuate immediately. But we will no longer ask for these steps, and the people of Japan are free to go back to normal lifestyles."

Last Thursday, Japan's weather agency said the likelihood of a megaquake was "higher than normal" following a magnitude 7.1 jolt earlier in the day that injured 15 people.

That was a particular kind of tremor known as a subduction megathrust quake, which in the past has occurred in pairs and can unleash massive tsunamis.

The advisory concerned the Nankai Trough between two tectonic plates in the Pacific Ocean.

The 800-kilometre undersea gully runs parallel to Japan's Pacific coast, including off the Tokyo region, the world's biggest urban area and home to around 40 million people. (AFP)