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Magnitude 5.3 earthquake in Colorado today. Why?
A magnitude 5.3 earthquake in California might not attract much national attention, but one of this magnitude in Colorado did so today near Trinidad, a town very close to the border with New Mexico. A precursor of magnitude 4.6 occurred about 6 hours before the main shock. The earthquake was the largest in Colorado since 1973 when an earthquake hit the northwestern part about 50 miles north of Grand Junction. It's about 300 miles as the crow flies between these two sites. Today's earthquake caused rock slides and damaged a few homes.
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. Colorado has only one major fault, the Sange de Christo fault. Scarps in late Quaternary deposits are common along the northern part of the fault indicating that it's an active fault. In this area, it bounds the sharp tall Sange de Christo range. However, in the southern part near the New Mexico border, the topography is more subdued, the fault is discontinuous and is composed of en echelon strands. Scarps in Quaternary deposits are less common, but can be seen at the mouths of a few major drainages. The difference is pronounced enough to suggest that the two parts probably have different behaviors over the long term.
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.