Tsunami Alert After 7.8 Magnitude Quake Strikes Off US Coast

Alaska: A 7.8-magnitude earthquake struck the Alaskan peninsula triggering tsunami warnings today, the US Geological Survey (USGS) said.

The massive quake occurred at a depth of 10 km (6 miles), some 75 miles south of the city of Chignik on Wednesday morning.

Prelim M7.8 Earthquake Alaska Peninsula Jul-22 06:12 UTC, updates https://t.co/WrYe6gD8Zz

— USGS Big Quakes (@USGSBigQuakes) July 22, 2020

The US National Tsunami Warning Center has since issued tsunami warnings for southern and coastal Alaska and the Aleutian Islands.

Authorities have asked people staying near Pacific coast to seek safe and higher ground with evacuation thought to be underway in some areas.

The US National Weather Service has said that it is still evaluating the tsunami threat to the United States mainland, even though no such warning has been issued for the West coast.

"The July 22, 2020, M 7.8 earthquake southeast of Perryville, Alaska (south of the Alaska Peninsula), occurred as the result of thrust faulting on or near the subduction zone interface between the Pacific and North America plates. The preliminary focal mechanism solution indicates rupture occurred on a fault dipping either shallowly to the northwest, or steeply to the southeast. The location, mechanism and depth – and the large size of the event – are all consistent with slip occurring on the subduction zone interface between the two plates. At the location of this event, the Pacific plate converges with North America to the northwest at a rate of about 64 mm/yr, subducting at the Alaska-Aleutians trench 125 km to the southeast of the earthquake," USGS quoted.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Thrust faulting events of the size of the July 22, 2020 earthquake are typically about 120x50 km (length x width).

Large earthquakes are common in the Alaska-Aleutian subduction zone. Since 1900, 6 other earthquakes M7 and larger have occurred within 250 km of the July 22, 2020 event. The largest of these was a M8.2 earthquake on November 10, 1938, which occurred in an almost identical location as this July 22, 2020 earthquake. The Alaska-Aleutian Trench also hosted the second largest earthquake recorded by modern seismic instrumentation, the M9.2 March 27 1964 earthquake, which ruptured to within about 250 km of this event, at the eastern end of the 1938 rupture area. The 1938 event produced a small tsunami that was recorded both locally and in Hilo, Hawaii. The remote location of the earthquake resulted in little impact to people and infrastructure.

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