The United States Geological Survey recorded a Magnitude 3.1 Earthquake at a depth of 14.1 km in northwest Madison County in southwest Montana slightly before 11.10 am local time (slightly before 5.10 pm GMT) on Saturday 21 September 2013. This is not a large quake, and is unlikely to have caused any damage or injuries, though people did report feeling it in the town of Twin Bridges, roughly 6 km to the northeast of the quake's epicenter.
The approximate location of the 21 September 2013 Madison County Earthquake. Google Maps.
The area affected by this quake lies on the Madison Fault, one of the most active in the Rocky Mountains and the cause of the infamous 1959 Hebgen Lake Earthquake, which triggered a string of lethal landslides killing 29 people and forming a lake by damming the Madison River. The Madison Fault is part of the Centennial Tectonic Belt, a string of faults connected to both uplift and folding in the Rocky Mountains, caused by subduction along the American west coast, and movement beneath the Yellowstone Caldera.
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 4.9 Earthquake in the Wind River Mountains of Wyoming, 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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