A magnitude-6.5 earthquake struck northern Japan early on Monday, but no tsunami warning was issued. The Meteorological Agency said in a statement it was considered to be one of the aftershocks of the 2011 powerful quake that triggered tsunami waves in the north-east, killing about 18,500 people. The seismic centre of the quake that jolted the region at 2.23am (5.23pm GMT on Sunday) was off Aomori prefecture at a depth of 43 kilometres, the agency said. There were no immediate reports of casualties or damage to property. The 2011 quake and tsunami caused a triple meltdown at the Fukushima Daiichi Nuclear Power Station, forcing tens of thousands of residents to flee their homes. Earthquake-prone Japan sits at the convergence of four tectonic plates. The country is also home to more than 100 active volcanoes.