Why Japan issued an advisory for a possible megaquake in the country's north

Japan has issued a megaquake advisory after a magnitude 7.5 earthquake struck off the eastern coast of Aomori, the northernmost prefecture on the main island. The quake Monday caused 34 mostly slight injuries and modest building damage. Officials said the advisory is not a prediction and the probability of a magnitude 8 or larger quake is only about 1%. But there’s hope the advisory will serve as a wake-up call for a quake that could rival the 2011 disaster that killed nearly 20,000 people. Officials are urging residents, especially along coastal areas, to be well prepared for a bigger quake.