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Why Japan issued its first-ever 'megaquake advisory' — and what that means

www.nbcnews.com Why Japan issued its first-ever 'megaquake advisory' — and what that means

A 7.1-magnitude earthquake has raised the risk of an even larger temblor on the Nankai Trough, an underwater subduction zone.

Why Japan issued its first-ever 'megaquake advisory' — and what that means
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