Japan's first-ever megaquake advisory brings worry and confusion. What does it mean?

Japan issued its first “megaquake advisory” last week after a powerful earthquake struck off the southeastern coast of the country’s southern main island of Kyushu. Though the magnitude 7.1 quake caused no deaths or severe damage, the subsequent advisory caused public unease about when the next big one will hit. The Japan Meteorological Agency concluded the Aug. 8 quake on the western edge of the Nankai Trough increased the likelihood of a stronger earthquake. But while there is a 70-80% chance of a magnitude 8 or 9 quake associated with the trough within the next 30 years, a seismologist notes that is not a prediction for a megaquake at any specific time or location.