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Did you feel that?

Like April’s tremor, earthquakes hit periodically where mountains dissolve into the Western Slope

On April 11, a magnitude-2.8 earthquake was centered about 7 miles north of Aspen and 5.6 miles east of Woody Creek. Broadband channels BHZ, BHN, and BHE are sampled 40 times per second to record higher frequencies, which are useful for viewing local or regional earthquakes and P-wave events, according to the USGS.
Courtesy USGS

Every few years, the Roaring Fork Valley gets a reminder of how all the beauty around us was geologically formed through millions of years.

Earthquakes hit periodically here, where the mountains dissolve into the Western Slope. By the time the minor shake is over, residents are still trying to figure out what just happened — and whether it really did just happen.

On April 11, some in the upper valley got a gentle reminder of this phenomenon when a magnitude 2.8 earthquake was centered about 7 miles north of Aspen and 5.6 miles east of Woody Creek.



Since Jan. 1, 2021, there have been at least 74 earthquakes in Colorado, according to U.S. Geological Survey data. The quake near Aspen ranks second-strongest this year; a magnitude 2.9 on Feb. 17 in south-central Colorado just north of the New Mexico border holds the top spot.

Nearly two dozen people went to the U.S. Geological Survey website to fill out a “Felt Report” — a 16-question survey — about the April event in the Roaring Fork Valley.



The tremor was a “relatively small quake,” said Paul Caruso, a geophysicist with the National Earthquake Information Center office based in Golden. “We do get earthquakes in the mountains from time to time.”

But there have been three other small quakes in the valley this year registered by the USGS, and the trio came in the early morning hours of Jan. 23 in the Fryingpan Valley. They were clustered in an area about 3.6 miles east of Basalt and started at 4:38 a.m. with a magnitude 1.6, followed about 3 hours later by a 2.0. Ten minutes after that, a mag 0.8 hit and ended the shifting.

A map from the U.S. Geological Survey shows the April 11, 2021, earthquake east of Woody Creek (blue dot) as well as the three quakes that happened Jan. 23 in the morning along the Fryingpan Valley east of Basalt.
Courtesy USGS

Experts concluded it was a naturally occurrence, unlike earthquakes caused by humans that over the past decade have increased in frequency apace with oil and gas production, especially in Oklahoma and Texas.

While there are thousands of old mines in our mountains and hundreds of oil and gas operations on the Western Slope, those are not the cause of quakes in the area, researchers have said.

Longtime local geologist and Colorado Mountain College professor Garry Zabel said the likely cause of the April 11 event, and others in the valley, is the layer of gypsum and soft minerals that make up part of the area’s underground.

“The one that was reported in April between Ruedi Reservoir and the Roaring Fork Valley, there are quite a bit of outcroppings of gypsum in the area of Basalt and up the Fryingpan and a little way up the Roaring Fork River,” he said.

HUMANS CAUSING QUAKES

There are 100 potentially active faults in Colorado, and there have been nearly 500 tremors of magnitude 2.5 or higher in Colorado since 1870, according to the Colorado Geological Society. But while those faults are the cause of many quakes, in Colorado humans have amplified the frequency.

The largest tremor recorded in Colorado was a magnitude 6.5 earthquake west of Fort Collins on Nov. 7, 1882. Damage to structures was reported in Denver and Boulder.

In August 1967, the state’s most economically damaging earthquake happened near Commerce City at the site of the now-shuttered Rocky Mountain Arsenal weapons-making facility. The magnitude 5.3 quake caused an estimated $1 million-plus in damages around the Denver area. It was precipitated by humans.

“This earthquake is believed to have been induced by the deep injection of liquid waste into a borehole at Rocky Mountain Arsenal,” according to a state report. “It was followed by an earthquake of magnitude 5.2 three months later in November 1967.”

It was scientists’ first evidence in the world that human activity underground was causing tremors.

Justin Rubinstein is a seismologist and now the chief of USGS’s “induced seismicity project.“ He studied the recurring series of quakes started by oil and gas production near Trinidad in the Raton Basin that began in 2001. There was a 5.3 there in 2011.

“Colorado really has some of the longest history of (fluid) injection-induced seismicity of anywhere in the world,” Rubinstein said earlier this month from his San Francisco-area office. “The first observation of injection-induced seismicity was at Rocky Mountain Arsenal just outside of Denver. That’s really where we figured things out in the 1960s.”

The arsenal opened in 1942 and for 50 years was where the U.S. Army made chemical weapons. A 12,000-foot deep injection well for waste disposal was bored in 1961 but closed in 1966 because people were feeling earthquakes in the area.

The next big thing, Rubinstein said, was in Rangely in the early 1970s. A group of his predecessor USGS scientists convinced Chevron, which had seen an increase in quakes in the area related to injections, to let the USGS team take over a handful of wells in northwest Colorado to conduct an 18-month experiment. Their hypothesis was that the fluid pressure in the underground reservoirs impacted the probability of earthquakes.

He described the Rangely project as a “watershed moment” for science and a clear demonstration “that the hypothesis was reasonable.”

“They pressured up the reservoir as much as they could and caused a bunch of earthquakes, then they sucked everything out and returned the reservoir to its native pressure and the earthquakes turned off almost completely,” Rubinstein said. “That was a really nice and clean demonstration of this hypothesis. So those are two of the really landmark, historical studies.”

Earthquakes caused by humans picked up with oil and gas production, especially in Oklahoma and Texas, but in Colorado those human-caused shakes have been mostly in the eastern side of the state in the past decade, including near Greeley in the Denver Basin and before that in the Raton Basin near Trinidad. Rubinstein has spent years studying the industry-related quakes near Trinidad.

In Colorado, human-caused shakes have been mostly in the eastern side of the state in the past decade, including near Greeley in the Denver Basin and before that in the Raton Basin near Trinidad.

“When earthquakes start happening, I start looking into things,” he said.

Of the factors that control the likelihood of oil and gas operations causing earthquakes, Rubinstein said the biggest ones are well depth — if they’re shallow, they’re probably not going to cause earthquakes — and how much fluid are they putting into the ground? The more fluids operators put into the ground the more likely they are to cause earthquakes.

“That said, the big unknown is what are the geological conditions there? We have these absolutely massive operations in North Dakota in the Williston Basin that basically cause no earthquakes,” he said. “Then you look at operations of comparable size in Oklahoma and they are causing tons of earthquakes. There’s really where the geology comes in. Oklahoma 10 years ago was a surprise to everybody.”

The earthquake rate in Oklahoma, which sits on Crystalline Basin made of up incredibly solid rock, peaked in 2015 with nearly 900 magnitude 3.0 or larger quakes that year, USGS data shows. Since then, companies have adjusted operations with waste-water disposal and other injection procedures and Oklahoma’s earthquake rate has dropped to 40-50 annually, Rubinstein said.

He is currently working on reports of more earthquakes in southeast New Mexico near drilling operations on the Texas border.

On April 11, a magnitude 2.8 earthquake was centered about 7 miles north of Aspen and 5.6 miles east of Woody Creek.
Courtesy USGS

GYPSUM IS LIKELY CULPRIT

What shakes the ground around Aspen and the Colorado mountains has everything to do with geology, but not necessarily fault lines.

Zabel, the Colorado Mountain College professor emeritus in geology and adjunct professor for more than 40 years, moved to Colorado soon after getting his Master’s degree in geology from the University of Houston in 1977.

“This area is so attractive in many, many ways geologically. There is more geology here that you can see in two hours compared to University of Houston where we had to drive two hours to Austin just to find rocks,” Zabel said. “This is a fantastic area to turn students on to our surroundings.”

He started at CMC in 1977 with small, day-long field trips around the area. They expanded to Utah and then Arizona, and by 1996 he was leading a 10-day course that went to across southwest U.S. That trip is now up to 13 days, but it’s been on hold because of the pandemic; he hopes it returns for fall 2022.

A group explores the local geology during a field trip with Colorado Mountain College professor Garry Zabel, who has been taking students out on trip for more than 40 years.
Courtesy photo

Zabel wasn’t here too long before he heard of the first quake in the area, a magnitude 3.0 that was located outside of Redstone in May 1978. The faults created in the area when the Rocky Mountains were uplifted are inactive, Zabel said, so the local quakes aren’t often associated with stress along the formations.

The general consensus back then was it was caused by a formation call the Eagle Valley Evaporite, which is mainly composed of gypsum, he said. Because it is a soft mineral, it can be dissolve by groundwater and swept away causing the surrounding rocks to shift. Each day, the Yampah Hot Springs (near Glenwood Springs) adds about 240 metric tons of dissolved halite and gypsum into the Colorado River, according to Zabel’s research.

“We have evidence that because of the hot springs in the area and all the other springs, there is a heck of a lot of gypsum that is removed underground by the flow of groundwater,” he said. “Back in 1978 or ’79 that’s what several people were saying, and that’s held true as far as I know throughout my career here. The location of earthquakes tends to be where the gypsum is present in the Eagle Valley Evaporite.”

According to mapping by the Colorado Geological Survey, scientists found there is a diapiric salt anticline centered under the Roaring Fork Valley. This has caused the highest density of sinkholes in Colorado to be located in the valley from Glenwood Springs to Carbondale, according to Zabel’s research. It’s known as the Carbondale Collapse Center.

A map shows the extent of the Eagle and Carbondale regional evaporite collapse centers (black dotted lines), Pennsylvanian evoporate rocks (tan shading) and karst feature (red crosses).
Courtesy Garry Zabel

DUE FOR A BIG ONE?

Earthquakes in the 2.5 to 5.4 magnitude range are often substantial enough for people to feel, but they only cause minor damage, according to UPSeis, a Michigan Tech website dedicated to information about seismology.

Those crop up often around the Roaring Fork Valley. There have been swarms of earthquakes around Mount Sopris for decades. Along Thompson Creek and the Jerome Park area in May 1984, a cluster of more than a dozen quakes hit north and west of Sopris, according to USGS data. Prior to that in April 1984, a 3.1 magnitude quake was registered at the base of the mountain on the north side. One in September 1944 knocked bricks from chimneys and walls in Basalt, according to USGS data and reports.

“I’m not a geophysicist, but that certainly agrees with location of the gypsum, which is right on the flanks of Mount Sopris, especially on the west side,” Zabel said. “And you can see it up in Thompson Creek, the lighter color, yellowish-colored rocks of Highway 133.”

More recently, there was a series of tremors near Marble in January 2017 — 11 of them in about a 12-hour span. People might recall the swarm in December 2018 north of Glenwood Springs, with the largest at magnitude 3.6.

According to Caruso, the geophysicist in Golden, there have been 19 earthquakes within a 30-mile radius of Aspen since 1986. The most significant was a magnitude 3.1 earthquake in 1993, centered about 6 miles east of Aspen. In 2015, there was a 2.8 magnitude quake that was about 3.1 miles northeast of Aspen.

That’s not quite enough to make an impact on the developed landscape here. According to UPSeis, quakes in the 5.6- to 6.0 magnitude ballpark shake up things enough to cause slight damage to buildings and structures; 6.1 to 6.9 mag shakes can cause a lot of damage in very populated areas; and 7.0 to 7.9 mag events can cause serious damage. (Magnitude 8.0 or higher quakes are pretty rare anywhere but can destroy communities near the epicenter.)

So are we ever due for a major quake here in the valley? Maybe not by California metrics, where the anticipated “Big One” would clock a magnitude of 7.8 or higher.

Still, there’s a chance Colorado could feel a significant shakeup at some point, according to the state’s department of Public Safety: “Seismologists predict that Colorado will again experience a magnitude 6.5 earthquake at some unknown point in the future.”

As for Aspen and the valley, though, the odds are fairly slim.

The largest earthquake within 30 miles of Pitkin County was a 3.5 magnitude in 1986 northwest of Crested Butte, and the USGS database predicts there is a 4.79% chance of a major earthquake within 30 miles of Pitkin County within the next 50 years.

Until that unknown point in time, valley residents will just be left to wonder every time there’s a hint of a ground shake: was that an earthquake, or is the washing machine still on the spin cycle?


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