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

TOKYO — 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. Damage from this quake was modest — 34 mostly mild injuries and some damage to roads and buildings.

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 have the devastation of the 2011 disaster that killed nearly 20,000 people and destroyed a nuclear plant.

There's said to be an increased risk of a subsequent, magnitude-8 or larger quake within the next week. Officials are urging residents, especially alon