Updated: August 23, 2011 at 12:00 am
The midnight earthquake that shook Trinidad and sent shocks all the way to Fort Collins is not as rare as Colorado residents might suppose.
The quake, which was reported at 11:46 p.m. Monday and originated nine miles southwest of Trinidad, had a magnitude of 5.3.
In a typical year, Colorado is shaken by 15 to 25 smaller quakes of 2 to 3 magnitude, according to Don Blakeman, a geophysicist with the National Earthquake Information Center in Golden.
It is unusual for Colorado to have an earthquake of this magnitude, Blakeman said, because the state is not on a tectonic plate boundary, the cause of larger quakes in California and Alaska.
These small-scale Colorado quakes are particularly common in Trinidad, Fort Collins and Woodland Park. Geophysicists believe that the pressure of slowly moving tectonic plates causes these small "intra-plate" quakes.
The use of fracking, creating fractures in rocks and rock formations by injecting fluid into cracks, to extract oil and gas has also been known to cause small earthquakes. Fracking was most likely not a factor in the Trinidad quake, Blakeman said.
On the National Earthquake Information Center's web page, 268 Colorado Springs residents reported that they felt tremors from Trinidad. The largest number of reports, totalling 45, were in the 80906 ZIP code area, just west of the Broadmoor Golf Course.
The quake was the largest in Colorado since a magnitude 5.7 was recorded in 1973, U.S. Geological Service geophysicist Amy Vaughn said. That one was centered in the northwestern part of the state — about 50 miles north of Grand Junction, she said.
A few homes were damaged in the overnight quake Monday and rockslides were reported along a highway in Las Animas County, a Sheriff's Office dispatcher said.
"I thought maybe a car had hit my house," 70-year-old Trinidad resident Nadine Baca said. "Then I called to my son and he said it was the third (quake) today."
A magnitude 4.6 quake was felt in the same area at 5:30 p.m. on Monday, and a magnitude 2.9 quake was recorded just before 8 a.m. Two aftershocks — one recorded at 3.5 and another at 3.8 — followed early Tuesday, more than an hour after the 5.3 quake.
The last time the area received such a series of earthquakes was in August and September 2001, when about a dozen smaller-sized temblors were recorded.
The first recorded earthquake in Colorado was on Nov. 7, 1882, and was probably centered near what is now Rocky Mountain National Park and caused damage in Denver, according to the USGS history of earthquakes in the state.
It remains the strongest earthquake in Colorado's state history at an estimated magnitude of about 6.6.
Colorado also has had one quake that may have been caused by drilling.
In 1961, a 12,000 foot well was drilled northeast of Denver at the Rocky Mountain Arsenal. The well was intended for waste disposal, but a series of unusual earthquakes occured in the area shortly after.
From 1962 to 1967 over 1,000 small quakes rocked the Denver area. A year and a half after the drilling at the arsenal stopped, a 5.3 magnitude quake hit Denver.
The Associated Press contributed to this report.