|Map of faults in Colorado that have last moved in|
Quaternary (red) and Cenozoic (yellow) time.
Map produced by Colorado Geological Survey.
The description of the fault in the main text is
from the USGS earthquake site here.
The cause of the earthquakes in southern Colorado has been the subject of much heated debate. Earthquakes are not new in Colorado, a M6.6 having been recorded in 1882, at the peak of Colorado's mining era. It was probably in the Front Range near Rocky Mountain National Park, caused damage in Denver, and was felt as far as Saline, Kansas, and Salt Lake City, Utah. However, extensive monitoring of earthquakes in this area did not begin until the early 1960's.
It is often speculated that coalbed methane operations have been the cause. In these operations, wells are drilled and water is injected. One well, for example, in the Raton Basin extends down to 4100 feet and in one year 2.5 million barrels of water were injected. Natural gas operators point out, however, that the injection activity is limited to 1-2 kilometers depth, much shallower than the likely depth of the earthquake focus. Colorado has had several episodes of earthquake onset that are directly associated with injection; more detailed information is here, from which I obtained the information above. The site referenced is the American Association of Petroleum Geologists; here is a NYTimes overview published 3 days ago.