TOKYO, April 21 -- At least six people were reported injured on Tuesday, a day after a powerful quake rattled northern Japan, but there appeared to be no major damage from the tremor that also triggered tsunami waves up to 80 centimetres (31 inches).

However, the Japan Meteorological Agency (JMA) also warned of an increased risk of a megaquake - a tremor with a magnitude of 8.0 or stronger - hours after Monday's 7.7 magnitude quake in Pacific waters off northern Iwate prefecture.

The jolt was so intense that it shook large buildings in the capital Tokyo, hundreds of kilometres from the epicentre.

Six people were reported injured by 8:00 am (2300 GMT Monday), two seriously, the Fire and Disaster Management Agency (FDMA) said in a statem...