LOOKING FOR THE SOURCE OF LIGHT-TO-MODERATE EARTHQUAKES IN THE RATON BASIN WEST OF TRINIDAD, COLORADO

The purpose of this study is to try to find the fault(s) that produced seven M 3.3-5.3 earthquakes from 2005 through 2011 in the Raton Basin west of Trinidad, Colorado (latitudes 36.9°-37.2°N, longitudes 104.5°-104.9°W). There are not many faults shown on published geologic maps of this area, none that spatially correlate with the earthquakes we studied, and no active faults.

Major tributary streams in this area flow either NE or SE toward the Purgatoire River, which flows east and is the primary drainage. Prominent drainage lineaments that trend N-S are obvious because of their anomalous orientation within this stream network. From Trinidad to ~2 km beyond Weston, at least seven of these N-S composite lineaments are evident. The composite lineaments are typically continuous across the Purgatoire River, and extend from ~15 to ~30 km.

We used hypocenter and focal mechanism data compiled by the ISC (www.isc.ac.uk) for the seven earthquakes in our study. All seven earthquakes have normal or normal-oblique focal mechanisms, and some have multiple solutions. Most of the hypocenters had assigned (rather than computed) depths. Vertical uncertainties for these events were assumed to be 5 km. The average nodal-plane strikes for the seven earthquakes we considered are 355°±13° for east-dipping planes and 6°±21° for west-dipping planes: N-S on average. Average dip angles for all of the nodal planes are 48±5° and average rakes are -90°±21°, indicating normal faulting.

One or more of the prominent N-S drainage lineaments is inferred to have developed along an active normal fault, while others might have developed along a regional mode I joint trend. A road cut just east of Valdez, ~100 m east of mile-marker 57 along Colorado State Highway 12, exposes a down-to-the-east normal fault that is partially obscured by slope wash or shallow landsliding. Seismo-lineaments (Cronin et al., 2008; bearspace.baylor.edu/Vince_Cronin/www/SLAM/) associated with a M4.9 earthquake on 8/22/2011, and M5.3 and M4 events on 8/23/2011 encompass this fault exposure and have the same east-down sense of normal displacement. Another down-to-the-east normal fault was observed near County Road B (37.1342N, 104.6976W), with a mean orientation of 312°±10°, 47°±7°NE based on 5 measurements. Neither of these faults is on published geologic maps that we are aware of.