A map pinpoints the epicenter for two small earthquakes north of Glenwood Springs Tuesday morning, one along Cascade Creek and the other west of Mitchell Creek.

Courtesy USGS

Richter scale The Richter magnitude scale was developed in 1935 by Charles F. Richter of the California Institute of Technology as a mathematical device to compare the size of earthquakes. The magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs. Adjustments are included for the variation in the distance between the various seismographs and the epicenter of the earthquakes. On the Richter Scale, magnitude is expressed in whole numbers and decimal fractions. For example, a magnitude 5.3 might be computed for a moderate earthquake, and a strong earthquake might be rated as magnitude 6.3. Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude; as an estimate of energy, each whole number step in the magnitude scale corresponds to the release of about 31 times more energy than the amount associated with the preceding whole number value. Source: U.S. Geological Survey

Two small-magnitude earthquakes struck north of Glenwood Springs early Tuesday morning, but were not strong enough to cause any major damage, according to a U.S. Geological Survey scientist in Golden.

“These minor earthquakes are not really capable of causing any kind of significant damage,” said John Bellini, a geophysicist with the USGS Earthquake Center. “You might feel a jolt or a rumble, but that’s about it.”

The first quake was recorded about 3:02 a.m. Tuesday, just a little over a mile north of Glenwood Springs and at a depth of 3.1 miles beneath the earth’s surface. It registered at a magnitude 3.4 on the Richter scale.

The second quake came a little over an hour later, at 4:13 a.m., and occurred about 3.7 miles north of town at about the same depth. It registered 3.6 magnitude, according to the USGS earthquake information website.

There were no reports of any damage from the quakes, but there was evidence of the seismic disturbance inside the Fairy Caves on Iron Mountain just north of Glenwood, said Nancy Heard, general manager at the Glenwood Caverns Adventure Park.

“It shook enough to cause some dust and pebbles and a baseball-size rock to come down,” Heard said, explaining what’s called “moon milk” on the ceilings of the cave is sensitive to even the most-minor of tremors.

“None of the big features were damaged, and nothing structural,” she said. “We will go in and do a more extensive walk-through of the cave before we open in the spring to make sure there are no structural issues.”

Heard noted that, if a line were drawn between the two epicenters, the caverns are right along that line.

“The first thing I thought of this morning when I heard about it was the cave,” Heard said.

According to the educational website UPSeis, an earthquake of 2.5 to 5.4 magnitude is often felt, but only causes minor damage.

Small earthquakes are not uncommon in the mountainous region stretching from the Flat Tops north of Glenwood Springs to the west. Two similar-magnitude quakes were recorded earlier this year, one north of Parachute in August, and another about 18 miles north of Glenwood Springs in April.

“Colorado has some siesmicity, but mostly down in the southern part of the state near LaJunta,” Bellini said. “We probably have 10 or 12 in the state every year.”

However, Colorado is not nearly as active as California or Alaska, he said.

Several area residents, especially in Glenwood Springs and nearby New Castle, reported similar experiences Tuesday morning to the independent reporting website Earthquake-Report.com.

“Walls shook for 3 seconds at 3 a.m. and then two more tremors about a half hour apart. You could see walls move shake and the floor shake,” one person reported to the site from Glenwood Springs regarding the later quake.

“At around 3 a.m., I suddenly woke up to a strange shaking like sensation that was clearly felt around all my apartment,” another Glenwood resident wrote regarding the initial quake. “A second shake occurred about one hour later, this time stronger with smaller ones following it for about 5 seconds.”

And another commenter wrote, “House was shaking enough my china was clinking. Second quake was stronger and woke up everybody who all came running out of the bedrooms.”

Some Facebook commenters also suggested a link to hydraulic fracturing in Garfield County’s oil and gas industry, or the limestone quarry that’s situated just north of Glenwood Springs which has been drilling exploratory bores. Bellini quickly dismissed that notion.

The area north of Glenwood Springs does not have any natural gas wells. And, the quarry activity is not deep enough to trigger any seismic activity, he said.

The test bores that are being drilled are only up to 200 feet deep, where the quake epicenter was more than 3 miles beneath the earth’s surface.

jstroud@postindependent.com