Wednesday 22 July 2020

Magnitude 7.8 Earthquake off the south coast of the Alaskan Peninsula.

The United States Geological Survey recorded a Magnitude 7.8 Earthquake at a depth of 28.0 km, off the south coast of the Alaskan Peninsula, slightly after 10.10 pm local time on Tuesday 21 July 2020 (slightly after 6.10 am on Wednesday 22 July GMT). There are no reports of any damage or casualties associated with this event, but people have reported feeling it over a wide area of southern Alaska, and a tsunami warning was initially issued.

The approximate location of the 21 July 2020 Alaska Earthquake. USGS.

Alaska lies on the North American Plate, with the Pacific Plate underlying the ocean to the south. The Aleutian Trench runs along much of the south coast of Alaska, with the Pacific Plate being subducted beneath this and passing under Alaska as it sinks into the Earth. Off the coast of southeast Alaska the Pacific and North American Plates pass one-another horizontally, with the Pacific Plate moving northward and the North American to the south (a transform plate margin). This is not a smooth process, and the plates frequently stick together, then break apart as the pressure builds up, causing Earthquakes.

Model of the subduction of the Pacific Plat beneath the North American Plate along the southern coast of Alaska and the Aleutians. USGS.

Earthquakes along subductive margins are particularly prone to causing tsunamis, since these often occur when the overlying plate has stuck to the underlying plate, being pulled out of shape by its movement.. Eventually the pressure builds up to far and the overlying plate snaps back, causing an Earthquake and a tsunami. 

 Simplified graphic showing tsunami generation along a convergent margin.NASA/JPL/CalTech.

Witness accounts of quakes can help geologists to understand these events and the rock structures that cause them. If you felt this quake (or if you were in the area but did not, which is also useful information) you can report it to the United States Geological Survey here. 

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