Scientists probing under the seafloor off Alaska have found a geologic fault they say signals significant risk of a mega-tsunami in future.
The feature closely resembles one that produced the 2011 Tohoku tsunami off Japan, killing some 20,000 people and melting down three nuclear reactors.
Writing in Nature Geoscience, the team warn a large tsunami in in the area 'could have devastating consequences to coastal communities locally in Alaska and around the Pacific Ocean'
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A tsunami can occur as ocean crust (brown area) dives under continental crust (orange), causing the ocean floor to suddenly move. In a region off Alaska, researchers have found a large fault indicating a tsunami-prone area where the floor can move more efficiently
The 2011 Japan tsunami was a surprise because it came partly on a 'creeping' segment of seafloor, where the plates move steadily, releasing tension in frequent small quakes that should prevent a big one from building.
But researchers are now recognising it may not always work that way.
Off Japan, part of the leading edge of the overriding continental plate had become somewhat detached from the main mass.
When a relatively modest quake dislodged this detached wedge, it jumped, unleashing a wave that topped 130 feet (40 metres) in places.
The telltale sign of danger, in retrospect, was a fault in the seafloor that demarcated the detached section's boundary landward of the 'trench,' the zone where the two plates initially meet.
The fault had been known to exist, but no one had understood the danger it posed.
The researchers in the new study have now mapped a similar system in the Shumagin Gap, a creeping subduction zone near the end of the Alaska Peninsula around 600 miles (970 kilometres) from Anchorage.
Tsunami waves triggered by the newly discovered fault could hit more southerly North American coasts, Hawaii and other parts of the Pacific, the researchers claim.
Similar structures may lurk undiscovered in other areas of the world, they warn.
The discovery 'suggests this part of Alaska is particularly prone to tsunami generation,' said study lead author Dr Anne Bécel of Columbia University, New York.
'The possibility that such features are widespread is of global significance.'
Tsunamis can occur as giant plates of ocean crust dive under adjoining continental crust, a process called subduction.
Some plates get stuck for decades or centuries and tension builds, until they