The Alaska Earthquake Centre recorded a Magnitude 5.9 Earthquake at a depth of 152.6 km, about 11 km to the northwest of Mount Iliamna in Kenai Peninsula Borough, Alaska, slightly after 1.50 pm Nolocal time (slightly after 10.40 pm GMT) on Tuesday 21 December 2021. 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.
The approximate location of the 21 December 2021 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.
How the subduction of the Pacific Plate beneath the North American Plate fuels the volcanoes of Alaska. Alaska Volcano Observatory.
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.