TOKYO - A powerful 6.9-magnitude earthquake struck northern Japan on Saturday, causing strong tremors across Hokkaido island but generating no tsunami or immediate reports of damage, authorities said.The quake, which was preceded by an early warning broadcast on television and radio, hit near the town of Obihiro at a depth of 103 kilometres (64 miles) at 23:17 pm (1417 GMT), according to US Geological Survey data.Japan’s meteorological agency said there was no threat of a tsunami from the quake, and public broadcaster NHK cited police in Hokkaido as saying there were no immediate reports of damage or injuries.Blackouts were reported in some areas and a number of highways were closed, Kyodo News reported, citing officials.Strong tremors were felt throughout Hokkaido — the nation’s second-largest island and a popular destination for skiing — as well as the main island of Honshu which lies to its south, the Japan Meteorological Agency said. Several of Japan’s nuclear facilities are located in Aomori prefecture in Honshu’s north, but the companies which run the facilities said there were no abnormalities reported after the quake. There was also no abnormality reported in Tomari, another nuclear plant in Hokkaido.Tremors from the powerful quake lasted for about a minute, according to NHK which warned of possible aftershocks.Shortly before it hit, an alert was broadcast on television and radio, forecasting the impending quake, through an early warning system established by the weather agency. Programmes were interrupted on NHK to broadcast information about the earthquake. “Make yourself safe, turn off the gas, beware of falling objects, and if you are outside do not approach the coast,” the broadcaster said.A press conference by the national seismological agency was scheduled for later in the night.In March 2011 a devastating earthquake and tsunami in Japan’s northeast left some 19,000 people dead or missing and crippled the Fukushima nuclear power plant in the world’s worst atomic disaster in 25 years.A powerful 7.3-magnitude undersea quake in the same area in December triggered a one-metre-high tsunami but no reports of fatalities.