A strong 6.1 magnitude earthquake struck off the northern Japanese island of Hokkaido early Tuesday, the US Geological Survey said. The quake hit off the west coast of the island, about 170 kilometres (106 miles) north of Sapporo, at a relatively deep 236 kilometres. The Japan Meteorological Agency recorded it as a weaker 6.0 quake. Japan sits at the meeting place of four tectonic plates and experiences around 20 percent of the world's most powerful earthquakes every year.