The United States Geological Survey recorded a Magnitude 4.9 Earthquake at a depth of 5.0 km, roughly 15 km north of Chalis City in Custer County, Idaho, slightly after 6.00 pm local time on Saturday 12 April 2014 (slightly after midnight on Sunday 13 April, GMT). This is a large quake for the area, and was felt across much of Idaho as well as western areas of neighbouring Montana, but there are no reports of any injuries or damage beyond items being shaken from shelves and off walls.
The approximate location of the 12 April 2014 Custer County Earthquake. Google Maps.
The quake took part in an area known as the Central Idaho Seismic Zone, which is bisected by six active faults, the Madison, Centennial, Beaverhead, Lemhi, Lost River, and Sawtooth. These faults are associated with the Yellowstone Magma Chamber in the northeast of Wyoming, 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.
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