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 U.S. Geological Survey said the epicenter was west of the small town of Sarabetsu on Japan’s northernmost main island of Hokkaido. The USGS said the magnitude was 6.1 and the quake was about 50 miles deep. 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.