The United States Geological Survey recorded a Magnitude 4.9 Earthquake at a depth of 73.5 km in the Wind River Mountains in western Freemont County, Wyoming, slightly after 7.15 am local time (slightly after 1.15 pm GMT) on Saturday 21 September 2013. A quake of this size at this depth does not present any significant threat, but is likely to be felt over a wide area, this one being recorded by witnesses as far away as Rock Springs, 156 km to the south of the epicenter.
The approximate location of the 21 September 2013 Wind River Mountains Earthquake. Google Maps.
Wyoming typically suffers two or three moderate sized Earthquakes per year, though quakes large enough to cause significant damage are rare. The mountainous areas of the east of the state have a number of deeply buried faults where occasional movement occurs. This can be caused by movement around the Yellowstone Magma Chamber in the northeast of the state, an area that suffers more-or-less constant small tremors due to the movement of magma beneath the surface, but which seldom experiences larger quakes. The constant small movements around Yellowstone can lead to stress building up in rocks further away, leading to Earthquakes as the rocks release the strain by readjusting their positions, which can affect Wyoming as well as neighbouring Idaho and Montana. The same faults also suffer stress due to more distant rock movements, notably on the subduction zone on the American west coast.
Witness accounts of Earthquakes can help geologists to understand these events and the underlying structures that cause them. If you felt this quake (or if you were in the area but did not, which is also useful information) then you can report it to the United States Geological Survey here.
See also Magnitude 3.1 Earthquake in west Okfuskee County, Oklahoma, Magnitude 3.8 Earthquake on the New Mexico/Colorado state boundary, Magnitude 3.0 Earthquake in Washington State, Magnitude 6.2 Earthquake of the coast of British Columbia and Magnitude 3.6 Earthquake in Washington State.
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