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

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The Nankai Trough is an underwater subduction zone with the potential to produce magnitude 8 or 9 temblors.

A 7.1 -magnitude earthquake shook southern islands in Japan on Thursday .

Japan ’s Earthquake Research Committee said in January 2022 there was a 70% to 80% chance of a megathrust earthquake in the next 30 years .

The Nankai Trough has the capability of producing a magnitude-9.1 earthquake and tsunami waves 80 feet in height.

The Cascadia Subduction Zone poses a similar risk for the U.S. West Coast , though megathrust earthquakes are expected there less often — every 300 to 500 years.