The quake struck at 11:34 a.m. on Tuesday (2334 GMT on Monday) some 125 miles (200 km) from the city of Nikol'skoye on Bering island off the Kamchatka Peninsula. The epicentre was west of Attu, the westernmost and largest island in the Near Islands group of Alaska's remote Aleutian Islands.
The earthquake was very shallow, only 6 miles (10 km) below the seabed, which would have amplified its effect, but it was far from any mainland.
The U.S. Pacific Tsunami Warning Center said "hazardous tsunami waves were possible for coasts within 300 km (186 miles) of the earthquake epicentre."
Tsunami waves, however, were unlikely to reach Kamchatka's eastern coast, some 500 km (310 miles) away.
The quake was initially reported as a magnitude 7.7 before being revised down to 7.4 and finally upgraded to 7.8, a major quake normally capable of causing widespread and heavy damage when striking on or near land.
The quake was followed by several aftershocks, including a couple above magnitude 5.0.
(Reporting by Sandra Maler; Editing by Peter Cooney and Diane Craft)
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