colorado swarmMedia attention today focused like a laser on the 5.8 M Mineral, Virginia, earthquake that shook the Eastern seaboard from Atlanta, Georgia, to Maine.  And that is as it should be, the event rocked Washington D.C. and New York City and send thousands of people scurrying into the streets.  The USGS’s “Did you feel it” recorded nearly 14,000 reports from 3754 zip codes and 33 cities.  While strong shaking was localized, moderate shaking was reported widely.   For additional information and maps visit the USGS’s events page. 

The Mineral earthquake overshadowed what otherwise was a newsworthy seismic swarm of moderate- to low-magnitude events on the Colorado-New Mexico border 9 miles WSW of Trinidad, Colorado.  (Broadband seismometers of the Arizona Integrated Seismic Network nicely captured the Mineral event and the two largest Trinidad events.)AISN Snowflake seismogram

The Trinidad swarm began with a 4.6 M event at 5:30 p.m. (MDT) on 22 August and continued through 8:11 a.m. (MDT) the next morning.  In all, 11 events occurred.  The largest event was a 5.3 M that occurred at 11:46 p.m. (MDT) on the evening of 22 August;  this is also the largest event reported for the area since 1973.   Small to moderate magnitude earthquake are not uncommon here – unlike the rare 5.8 M near Mineral, Virginia – and eight M 4.x events have occurred over the past decade. 

The causative fault(s) lack any surface expression and thus are not included on the interactive USGS Quaternary fault map of Colorado-New Mexico.  Nonetheless, the past day’s events and didyoufeeit_5.3MTrinidadthose of a swarm in 2001 are consistent with a north-northeast trending fault system, congruent with east-west extension responsible for forming the Rio Grande Rift. 

Michael Conway (23 August 2011)