At about a quarter to midnight on Monday 22 August 2011 an earthquake occurred in Colorado, 15 km west-southwest of Trinidad, on the border with New Mexico, or 290 km south of Colorado, and was felt in Colorado, New Mexico and Kansas. The quake was shallow, at a depth of about 4 km and measured 5.3 on the Richter Scale; shallow and large enough to be dangerous. There are no reports of any casualties, but some minor damage to houses and roads has been reported.
The area is close to the Sangre de Cristo Fault, part of the Rio Grande Rift system. The Rio Grande Rift runs through Colorado and New Mexico and south into Mexico and Texas. It is effectively a split opening in the North American Continent, with the potential to form a new ocean (though most rifts of this kind do nothing of the sort), that has been drawing apart for about 35 million years. The Sangre de Cristo Fault forms part of the eastern boundary of this rift. As the fault opens, surface rocks in the central part sink downwards, tearing away from the rocks on either side.
Part of the Sangre de Cristo Fault, showing how the sediments to the west (right) are sinking down into the Rio Grande Rift.
Southern Colorado has a history of earthquakes, with some historical quakes having caused damage to towns and cities across the state. However, while it is clear that some of these earthquakes are natural, many have been attributed to drilling and blasting by the mining industry, which is extensive (and extremely financially important) within the state. Colorado produces coal, gold, molybdenum, silver, gypsum, limestone and uranium, amongst other resources. This quake is thought to be of natural origins.
See also Virginia Earthquake shakes Washington, New York, Earthquake in South Dakota and Earthquakes on Sciency Thoughts YouTube.