Japan Earthquake: Why Japan issued its first-ever 'megaquake advisory'

Japan has issued its first-ever advisory on higher-than-usual risks of a megaquake after an earthquake of magnitude 7.1 jolted on Thursday at the edge of a tremulous seabed zone along the Pacific coast known as the Nankai Trough. A possible Nankai megaquake and tsunami disaster could kill hundreds of thousands of people and cause a trillion-dollar damage to Japan.