A few minutes before midnight on Friday 4 January 2012 (a few minutes before 9.00 am on Saturday 5 January, GMT), the United States Geological Survey recorded a magnitude 7.5 Earthquake at a depth of 9.8 km, off the southeast coast of Alaska, roughly 335 km south of Juneau. This is a large quake, and quite shallow, and the USGS predicted there to be a 31% chance of fatalities occurring, though no reports of casualties or serious damage have emerged. The National Oceanic and Atmospheric Administration initially issued a tsunami warning, but this was withdrawn after two hours.
The location of the 4 January quake (black star), and the areas likely to have suffered the worst shaking. Areas within the blue circle will have felt shaking, and within the green circle damage to buildings may have occurred. 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. The 4 January quake occurred in the far southeast of the state; east of the extent of the Aleutian Trench. 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.
Witness reports can help geologists to understand the processes going on in Earthquakes. If you felt this quake you can report it to the USGS here.
See also Earthquake off Vancouver Island, Magnitude 7.7 Earthquake hits coast of British Columbia; tsunami threatens Hawaii, Seismic activity on Little Sitkin, Seismic activity on Mount Spurr, Alaska and Earthquakes on Sciency Thoughts YouTube.
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