Minor quake rattles Cotopaxi area

Brief by Central Staff

Geology – March 2008 – Colorado Central Magazine

Despite our jagged terrain, which reflects many faults in the earth’s crust hereabouts, earthquakes are not common in Central Colorado. So it was something of a surprise when the ground shook a little at about 8:45 p.m. on Jan. 25 in the Cotopaxi area.

The U.S. Geological Survey has a seismic observatory in Golden, which put the quake’s center on BLM land a few miles north of the intersection of Colo. 69 and U.S. 50. The magnitude was 3.1 on the Richter scale, which means it had enough energy to be felt, but not enough to cause damage.

(The Richter scale is logarithmic; a 4.0 is not twice as powerful as a 2.0, but releases 1,000 times as much energy.)

One who did feel our January quake was Charlie Green of greater Cotopaxi, an occasional contributor to this magazine. He checked with his neighbors, who felt it too, and called the sheriff to be sure it wasn’t a plane crash or the like.

Charlie wondered what fault or faults might be in motion, since the major faults of the Rio Grande Rift (the mountain-flanked depression that runs from Leadville to El Paso where the two sides are slowly spreading apart) are at least 20 miles west of him.

Consulting various maps, he found a few. He reported that “Texas Creek follows one branch off the long fault that runs along the west side of the Wet Mountains. The Arkansas follows a branch of the one that runs along the east side of the Sangres (the Alvarado Fault), which explains why the Arkansas River turns here.

“This fault splits (or crosses) at Coaldale, creating Pleasant Valley (Howard, etc.) between them. The north fork is the bed for Badger Creek. Between these two, it is also possible Oak Creek and Bernard Creek are following an even older (or unmapped) fault line right through downtown Cotopaxi on the Arkansas. An even wilder speculation: Hayden Pass and Creek on the map make a continuation of the straight line of the Arkansas from its turn at Cottonwood Creek/Hidden Valley/ The Narrows to Parkdale; is this another ancient/unmapped fault line which the river followed? There is also a diagonal fault from the Texas Creek branch over to the fault on the east side of the Wet Mountains which passes very near us.”

So one can easily find fault (or faults) with (or within) our terrain.

One earlier quake in this area happened at 2:55 p.m. on March 16, 1985. It registered 3.3, and the center was about nine miles northeast of Salida, where some residents felt it. Our Martha was among them; she recalls the kitchen table bouncing a bit and wondering whether there’d been some kind of explosion in town.

Ed didn’t feel it. He was riding a bus back from Denver that afternoon, and it was stopped for a break in Fairplay. “When you’ve been sitting on a bus for nearly two hours on mountain roads and finally get a chance to stretch your legs, the bus would have to tip over before you’d feel the ground was moving.”