A magnitude 6.2 earthquake shook part of northern Japan early Monday, but no damage or casualties have been reported.No tsunami advisory was issued by the Japan Meteorological Agency.The quake struck 18 kilometres west of the small town of Sarabetsu on Japan’s northernmost main island of Hokkaido at a depth of 81 kilometres, according to the US Geological Survey. It measured the quake’s strength at 6.1 magnitude.A week ago, a 7.7-magnitude earthquake prompted Japan to issue an advisory of a slightly higher risk of a possible megaquake for the nation’s northeastern coastal areas.