Poster of the Colorado Earthquake of 23 August 2011 - Magnitude 5.3
The magnitude 5.3 southern Colorado earthquake of 23 August 2011 occurred in a region that has experienced numerous small and moderate shocks in the last decade. Since August 2001, eight shocks of magnitude greater than 4.0 have occurred, with the previous largest having been a shock of magnitude 5.0 that occurred in August 2005. Many of the shocks have occurred in episodes of activity known as seismic swarms, in which more than one earthquake occurred at the nearly the same location within a period of several days and in which the largest shock occurred after the beginning of the sequence and was not greatly larger than preceding or following events of the swarm. The USGS conducted a detailed study of an earthquake swarm in this region that occurred in 2001; a report is available on-line. The shock of 23 August 2011 also occurred within a swarm of smaller shocks that began on the previous day. The 2001 and 2011 swarms notwithstanding, it should be noted that some shocks that have occurred within the southern Colorado source region in the last decade have not been part of seismic swarms.
The shock of 23 August 2011 occurred as the result of normal faulting, at a shallow depth of focus. The preliminary location, depth, and style of faulting for the 2011 earthquake are very similar to the earthquakes in the previously-cited 2001 swarm. The 2001 swarm did not occur on a mapped geologic fault. The north or north-northeast strike of the causative faults of the largest 2001 and 2011 earthquakes are consistent with the east-west extension that has formed the Rio Grande rift to the west of the epicentral region.
Prior to the onset of small earthquake activity in 2001, earthquakes had occurred in the Trinidad region in 1966 and 1973. A widely felt magnitude 4.6 earthquake occurred on October 2, 1966, and was felt over a 38,400 km² (15,000 mi²) area. The published location for the 1966 event is northeast of Trinidad, and detailed analysis of this event indicates that it likely did not occur in the same area as the 2001 swarm or the 2011 M 5.3 earthquake. However in September 1973, a swarm of six earthquakes within a period of five days was felt in and around Segundo, Colorado (which is near the epicenter of the M 5.3 quake), and the two largest events had magnitudes of 3.1 and 4.2. The published locations of these two earthquakes are directly northwest of Trinidad Lake, but their exact locations are uncertain by at least ±10 km (±6 miles). Considering the location uncertainty and the felt report information, the 1973 earthquake swarm could have originated from the same source area as the 2001 and 2011 swarms.
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