The quake – among the strongest in US history – hit at a depth of 10km (6.2 miles) some 256km (157 miles) southeast of Chiniak at 00.31am on Tuesday local time.
Parts of Alaska and the west coast Canada have been issued with a tsunami warning and a tsunami watch is in place for the entirety of the the US west coast, including California and Oregon, part of Washington state and Hawaii, with “hazardous tsunami waves” possible within the next three hours.
“Based on all available data a tsunami may have been generated by this earthquake that could be destructive on
coastal areas even far from the epicenter,” the Pacific Tsunami Warning Center said.
No areas appear to be in immediate threat but the situation is “still under investigation” and people in coastal areas are being advised to move inland.
In a warning for Alaska and British Columbia, the Anchorage Office of Emergency Management said: “If you are located in this coastal area, move inland to higher ground. Tsunami warnings mean that a tsunami with significant inundation is possible or is already occurring.”
And in a message sent to smartphones in Alaska, the National Weather Service advised: “Emergency Alert. Tsunami danger on the coast. Go to high ground or move inland.”
Japan’s meteorological agency said it was also monitoring the situation but has not issued a tsunami alert.
Only four earthquakes with a greater magnitude have ever been recorded in the US, all of which were in Alaska and caused a tsunami.
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The most powerful came on 27 March 1964, when a 9.2 magnitude quake caused ground fissures, collapsed buildings and killed 139 people.