Brief by Central Staff
Geology – September 2002 – Colorado Central Magazine
There’s drought, and there’s wildfire, and if that doesn’t give you enough to worry about, consider earthquakes.
Not that there have been any hereabouts lately — the most recent on record was in the Nathrop area at 2:55 p.m. on March 16, 1985. It registered 3.3 on the Richter Scale (just enough to be noticed without instruments) and caused no known damage; it was felt in Salida.
But last fall, there was a swarm of minor earthquakes along a geologic fault west of Trinidad, and that has inspired the Colorado Geologic Survey to do more research.
It was long thought that Colorado was fairly immune to major earthquakes. That seems odd, since earthquakes happen at faults in the earth’s crust, and our mountains are lined with faults. But there was a logic involved; almost all the known faults were rather old, and hadn’t really moved in the past 40 million years.
Few recent faults, the kind associated with quakes, had been found. But that was because nobody was looking, according to the spring edition of the Survey’s quarterly publication, Rocktalk (available free for the asking from the survey at 303-866-2611, or on the web at geosurvey.state.co.us).
In 1970, only eight recent faults were known; by 1998, more than 90 had been cataloged. That’s enough for the Survey to compile a map of “maximum credible earthquakes” that could happen any day.
Of the 13 possibilities on its map, five are in Central Colorado or the San Luis Valley. The map is pretty small, so it’s hard to be precise, but the general locations and potential Richter readings run like this: Leadville, 7.0; Mt. Princeton, 7.2; Cochetopa Pass, 6.75; Poncha Pass, 6.0; Crestone, 7.5.
This would probably come as no surprise to our friend Paul Martz, a geologist in Poncha Springs. The town slogan is “Crossroads of the Rockies,” which is accurate, but he has observed that to someone looking for potential earthquake zones, “Crosshairs of the Rockies” fits pretty well, too.
Most Colorado earthquakes occur along the Front Range; the largest in state history was about 10 miles north of Estes Park on Nov. 7, 1882, and it caused damage as far away as Denver. Denver was also shaken in the 1960s by many small quakes that were caused by deep injection of waste fluids into a fault under Rocky Mountain Arsenal.
In our part of the world, a quake on Nov. 15, 1901 cracked windows and rolled boulders onto the road in Buena Vista. On Jan. 5, 1978, a tremor centered near Florrisant was strong enough to cause minor damage at the Royal Gorge. And Summitville, west of Alamosa, experienced a minor quake a decade later, on Jan. 15, 1988.
If this makes you think you should be prepared, the Colorado Office of Emergency Management has produced a free publication, Colorado Earthquake Hazards, and you can get it from the Survey.