Yellowstone Supervolcano Has Most Earthquakes Since 2017 in 'Doozy' of a July

Yellowstone had a "doozy" of a month for earthquakes in July, with the region experiencing the most tremors since June 2017, according to the U.S. Geological Survey (USGS).

Seismograph stations operated by the University of Utah detected more than 1,008 earthquakes during July and scientists have not even finished counting yet.

"It was a doozy of a month for earthquakes in Yellowstone during July," Michael Poland, scientist-in-charge of the USGS Yellowstone Volcano Observatory, said in a video update. "They still have some small earthquakes that they need to review that may have been missed by the computer."

Most of the earthquakes occurred as part of seven distinct swarms, most of which comprised of one and three or four dozen events ranging from magnitude one to two. According to Poland, these are fairly typical swarms for the Yellowstone region.

But the biggest swarm of the month occurred right beneath the center of Yellowstone Lake and comprised of at least 764 earthquakes. The largest of these was a magnitude 3.6 quake that occurred on July 16 around 11 miles beneath the lake, according to a USGS update. Most of the earthquakes of this swarm took place on this date.

One thousand and eight may seem like a high number of earthquakes for a single month—and it is above average. But the USGS said the situation is not unprecedented.

In June 2017, scientists recorded more than 1,100 earthquakes in the region, mostly due to a swarm that occurred in the Maple Creek area that lasted for three months. Over this time, the region experienced more than 2,400 earthquakes.

And in 1985, there was a much larger earthquake swarm—comprising over 3,000 events in the Madison Plateau area.

"So, [the July 2021 event] is a really impressive earthquake swarm, no doubt about it, and a big month for earthquakes," Poland said. "But this is not out of the range of what Yellowstone has done in the past."

Poland and the USGS said that the recent swarm is not a sign that magma is moving around beneath the surface.

#Yellowstone July update: “doozy” of a month for EQs (7 swarms, 1000+ EQs, largest a M3.6). Existing faults “goosed” by increased pore pressure (from snow melt) & not magma moving (no change in #deformation data). #DYK: Solitary geyser once fed a swimming pool near Old Faithful?

— USGS Volcanoes🌋 (@USGSVolcanoes) August 2, 2021

"There have been lots of quakes in the past two months, but this is how Yellowstone behaves, with cycles of more and less seismicity. It's not a sign of magma movement. An eruption is not imminent," the USGS said in a Tweet.

If magma activity was the cause of the quakes, scientists would expect to see other indicators, such as changes in ground deformation or gas emissions, but none were detected.

Yellowstone National Park sits atop a giant supervolcano that is still active, which explains the abundance of hydrothermal features and frequent seismic activity in the area. The most recent volcanic eruption at Yellowstone occurred around 70,000 years ago and scientists have not detected any signs that an eruption is imminent.

"Because of the 'supervolcano' mythos, there's a tendency among some to assume that any earthquake in Yellowstone is related to magma moving beneath the surface, but we know that this is not the case," Poland told Newsweek.

Earthquakes like those seen in the July, 2021, swarm are often caused by motion on pre-existing faults that can be triggered by the effects of rising groundwater levels due to snow melt.

"The earthquakes of July are all rock-breaking events typical of those we see in other places where active faults are moving," Poland said. "Extensive research into the large 2017 swarm identified existing faults as the sources of the earthquake activity, and the fault ruptures were aided by increases in groundwater pore pressure as water from snow melt percolated underground."

"It will take some time to research the most recent Lake swarm in detail, but that's a good starting hypothesis for the source of the July seismicity, as it is a common cause of earthquake activity in Yellowstone."

Poland said the Yellowstone region has several characteristics that make it particularly prone to earthquakes.

"There are a lot of existing faults in the Yellowstone region. Yellowstone is also weak because of the heat generated by the magmatic system beneath the surface. And finally, there's a lot of water in the subsurface—the area has the highest average elevation in the entire Rocky Mountain range, so it gets a lot of snow, and when that snow melts, it percolates into the rock to recharge the region's groundwater system," he said.

"Combine these elements and you have a recipe for lots and lots of earthquakes, including some that can be quite large."

Yellowstone National Park
A herd of buffaloes in Yellowstone National Park, Wyoming. The Yellowstone region experienced more than 1,000 earthquakes in July, according to the U.S. Geological Survey.

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