July was a 'doozy' of a month for Earthquake Country

A collection of quakes that occurred during July 2021. (USGS)

The U.S. Geological Survey's (USGS) Yellowstone Volcano Observatory clocked over 1,000 quakes during the entirety of July, and seismologists say most of these have roots not in magma but in water.

In its monthly update, the observatory's scientists reported that at least 1,008 earthquakes occurred in the national park during the entirety of July -- the most quakes recorded in Yellowstone since June 2017 when officials recorded over 1,100 events.

"It was a doozy of a month for earthquakes in Yellowstone during July," Michael Poland, USGS geophysicist and Scientist-in-Charge at the Yellowstone Volcano Observatory, said in a monthly update video for the observatory.

The activity last month was clumped together in swarms, mostly consisting of 12 to 40 events, though one swarm under Yellowstone Lake totaled an impressive 764 quakes. The swarm kicked off on July 16, although it tapered off in activity during the following days.

This flurry of activity doesn't reflect magmatic activity in the "earthquake country," however.

"These earthquakes are not associated with magma movement as might be assumed because, of course, the word is out that Yellowstone is a volcanic structure," Poland told AccuWeather. "Yellowstone also has a tremendous amount of groundwater, and that's from all the snow that falls in that area. That's the highest area in the Rockies on average, and it's absolutely full of preexisting faults."

As the underground pore space soaks up the snowmelt-turned-groundwater, it increases pressure on those nearby faults and can even cause them to fail.

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"We typically see lots and lots of earthquakes when the snow melts or it gets into the ground and interacts with these faults," Poland said. "That's what caused earthquakes the last real big month for earthquakes back in June of 2017."

Over 2,400 earthquakes fired off in a swarm that lasted for three months starting back in June 2017, and that was just the park's second largest swarm. The largest swarm was another three-month event during 1985 that included over 3,000 earthquakes.

FILE - In this Oct. 8, 2018, file photo, Emigrant Peak is seen rising above the Paradise Valley and the Yellowstone River near Emigrant, Montana. (AP Photo/Matthew Brown, File)

While they come in multitudes, these kinds of quakes are relatively small compared to their magmatic counterparts. The largest earthquake from the 2017 flurry of events that lasted from June into September reached an M4.4. Last July, the largest was a M3.6 quake under Yellowstone Lake on July 16.

Usually, the strength of these earthquakes range from M1 to the occasional M4, meaning the lower-end ones might not be felt at all.

Scientists are able to separate the earthquakes associated with the snowmelt from those with magma by looking at the environment around the site.

"If these were volcanic quakes ... we would see other changes," Poland pointed out. "We would see different types of seismicity, different frequencies of shaking, and we would also see other changes, for example, ground deformation."

If magma was moving toward the surface, it would move rock out of the way and be reflected at the surface, he added. Other changes that would indicate magmatic activity would be changes of thermal emissions or gas emissions. The observatory noted not finding any of these indicators that could suggest the quakes were associated with magma.

Visitors are seen at Grand Prismatic Spring in Yellowstone National Park, Wyoming on May 1, 2021. (AP Photo/Iris Samuels)

These quakes can occur in areas other than Yellowstone as well, such as the Sierra Nevada, for example. And since snowmelt is a large part of the cause, there can be almost a seasonality to them.

"We can see earthquakes that are sort of seasonal in nature and that tie in with snowmelt and the recharge that that snowmelt gives to groundwater systems," Poland said. "Same thing happens say in the Cascades. Mount Hood often sees earthquakes and earthquake swarms that are occurring at about the time that all that seasonal snowmelt is really getting into the groundwater system."

It's currently unclear whether climate change and the changing snowpacks would have an impact on the frequency of these kinds of earthquakes, he added, noting that while there is a change in the amount of snow falling in Yellowstone, there's a delicate balance regarding if a fault was ready to fail.

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