Why Japan issued its first 'megaquake advisory,' and what that means

Japan issued a “megaquake advisory” following a 7.1-magnitude earthquake off its coast. That raised the risk of a larger quake on the Nankai Trough, an underwater subduction zone.

After a 7.1-magnitude earthquake shook southern islands in Japan on Thursday, the country’s meteorological agency sent out an ominous warning: Another, larger earthquake could be coming, and the risk will be especially high over the next week. In the first “megaquake advisory” it has ever issued, the agency said that the risk of strong shaking and a tsunami are greater than usual on the Nankai Trough, a subduction zone with the potential to produce magnitude 8 or 9 temblors. Area residents, it said, should prepare.

The message was not a prediction, but a forecast of enhanced risk — and it shows how far seismologists have come in understanding the dynamics of subduction zone earthquakes. 

Here’s what to know about the situation. 

The Nankai Trough is an underwater subduction zone where the Eurasian Plate collides with the Philippine Sea Plate, forcing the latter under the former and into the Earth’s mantle. 

Subduction zone faults build stress, and a so-called megathrust earthquake takes place when a locked fault slips and releases that stress. “Megaquake” is a shortened version of the name. These zones have produced the most powerful earthquakes in Earth’s history. 

https://www.nbcnews.com/science/environment/japan-megaquake-advisory-meaning-reason-rcna165976

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