The Alaska Earthquake Centre recorded a Magnitude 7.5 Earthquake at a depth of 30 km, about 156 km to the south of Perryville on the Alaskan Peninsula, slightly before 12.55 pm local time (slightly before 8.55 pm GMT) on Monday 19 October 2020. 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 issued by the National Tsunami Warning Center, citing the possibility of a tsunami over a metre high reaching the Alaskan coast, although witnesses on the shore report that two such waves were seen, one reaching a maximum height of 60 cm, the other about 45 cm.
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.
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.
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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