An earthquake with a preliminary magnitude of 7.7 off northern Japan on Monday sparked a short-lived tsunami alert and prompted authorities to advise of a slightly higher risk of a possible mega-quake for coastal areas there.

The April 20 earthquake off the Tohoku region caused minor injuries, rekindled haunting memories of the 2011 disaster and prompted the government to warn about a heightened risk of a catastrophic mega-quake.

A large earthquake prompted a short-lived tsunami warning and a slightly higher-than-average risk of a mega-quake. The Cabinet Office, along with the Japan Meteorological Agency, said the risk of a mega-quake was increased from the typical .

'They can be very damaging as they produce strong shaking.'