A 6.1 magnitude earthquake struck northern Japan early this morning, the US Geological Survey reported, but there were no immediate reports of fatalities or damage. The quake struck 18 kilometres west of the small town of Sarabetsu in Hokkaido at a depth of 81 kilometres. Last Monday, a 7.7 magnitude earthquake off northern Japan sparked a short-lived tsunami alert and an advisory of a slightly higher risk of a possible megaquake for its coastal areas.