Craig — The earth made some rumbles at about 11 a.m. on Earth Day, Saturday when a minor earthquake originating roughly 3 miles west, northwest of Rangley left some people feeling shaken.

“It was the most bizarre thing. There was this boom. The house seemed to be spinning. I swear I saw it moving side to side. There was this loud noise like a siren. There was a crashing sound. Then it was over, completely over,” said Rangley resident Mickey Allen.

The earthquake registered a moment magnitude (Mw) of 3.7 at a depth of about 4 miles (6.4 kilometers).

“Typically a magnitude 3.0 to 3.9 event would be only slightly felt as a vibration, maybe similar to that of a passing truck. People on upper floors of buildings would notice it more,” said Matt Morgan, deputy director and senior research geologist for the Colorado Geological Survey.

However, the relatively shallow depth of the tremor may explain why it was short and sharp and people in Rangely felt rattled.

“I don’t think it lasted more than two seconds. I have been through other minor earthquakes that lasted much longer,” Allen said.

And that’s typical of earthquakes of this type as “shallower, crustal earthquakes, have more surface effects,” said Julie Dutton, geophysicist for the national earthquake center in Golden.

The Rangely earthquake was the smaller of the two measured last week.

At about 2:45 a.m. last Thursday an earthquake with a 3.9 magnitude was measured at a depth of about 3 miles (5 kilometers) roughly 22 miles northwest of Craig.

“The bed started shaking and the pull cord on the lamp started hitting the lamp and they woke me up. It kind of scared me because I didn’t know what was happening it’s actually the first one I’ve felt,” said Becky Plummer of Craig who was lying in bed asleep when she felt the quake.

The Craig earthquake was roughly equivalent to the detonation of 11 tons of TNT “or enough energy to power a household for half a year,” Morgan said.

Minor earthquakes like these are not unusual in Western Colorado, said Dutton and though it is unusual to have two so close together, she does not believe they are related as a cluster.

“At this point (and there is still ongoing research) we do not yet know the cause of the events, but there has been both natural and induced seismicity in the Rangely area,” Morgan said.

Drilling and wastewater injection by the oil and gas industry have been linked to minor earthquakes.

Satellite imagery shows provided by SkyTruth — a nonprofit with a mission to protect the environment by making more of it visible — show both types of activity close to the Rangely epicenter.

However, “we can’t conclude there is a relationship between these two quakes and drilling, just because of the proximity of these quakes to fracking and wastewater injection operations… It’s complicated,” said SkyTruth President John Amos.

In contrast the earthquake north of Craig was in an area with relatively sparse drilling activity.

“The Craig area has a history of small earthquakes as well, which have been considered natural… including a magnitude 4.3 south of Craig in 2002. So will earthquakes continue? That is likely, given the past, albeit infrequent and relatively light, seismic activity in the region,” Morgan said.

To better understand and monitor earthquakes the Colorado Geological Survey is in the process of installing additional seismometers across Colorado.

“Our next site will be near Hayden, hopefully this summer and in south Colorado near the Cheraw fault,” Morgan said.

A map of seismometers in Colorado can be viewed here: coloradogeologicalsurvey.org/geologic-hazards/earthquakes/seismometer-networks/

Contact Sasha Nelson at 970-875-1794 or snelson@CraigDailyPress.com or follow her on Twitter @CDP_Education.