The United States Geological Survey recorded a Magnitude 3.8 Earthquake at a depth of 4.6 km on the state boundary between Colorado and New Mexico, at approximately 2.15 am local time (8.15 am GMT) on Sunday 8 September 2013. There are no reports of any damage or casualties arising from this quake, though it was felt in the city of Trinidad on the Colorado side of the boundary.
The approximate location of the 8 September 2013 Trinidad Colorado Earthquake. Google Maps.
The New Mexico/Colorado state boundary os crosses by the Sangre de Cristo fault system, which is essentially a massive graben system; an area within a tectonic plate being stretched, causing thinning of the plate, and resulting in a series of linked faults that allow the center of the affected area to sink. Grabens can eventually develop into rift valleys and even full oceanic rifts, though this is rare.
Simplified section through the Sangre de Cristo fault system. Ruleman & Machette (2007).
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 USGS here.
See also Earthquake shakes Cleveland, Ohio, Earthquake in southeast Missouri, Earthquake in eastern Kentucky, Earthquake under Maine shakes New England and Oklahoma shaken by series of earthquakes.
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