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

Japan issued a megaquake advisory Tuesday after a magnitude 7.5 earthquake struck off the eastern coast of Aomori, the northernmost prefecture of Japan's main island of Honshu, and just south of the northern island of Hokkaido. Officials said the advisory is not a prediction and the probability of a magnitude 8 or larger quake is only about 1%. Officials are urging residents, especially along coastal areas, to be well prepared so they can grab an emergency bag and run as soon as possible if a bigger quake hits.