Updated | The ongoing earthquake swarm at the Yellowstone National Park supervolcano is now one of the longest ever recorded, having started on June 12. Over the past three and a half months, almost 2,500 earthquakes have been recorded in the western part of the national park. This puts it on a par with the biggest swarm ever recorded, where more than 3,000 earthquakes took place over three months.

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The swarm in no way signals an impending eruption, and it appears now to be coming to an end. However, experts at the U.S. Geological Survey (USGS) say it has been “fascinating” to monitor and are eager to learn more about it in their analysis of the event.

In a monthly update about activity at Yellowstone, the USGS said 115 earthquakes had been reported in the park during September. Of these, 78 were part of the ongoing swarm 6 miles north of West Yellowstone. The biggest event in the swarm last month was magnitude 2.3.

A swarm of almost 2,500 earthquakes has been recorded at Yellowstone National Park.JIM URQUHART/REUTERS

“Including the events from the prior three monthly reports beginning on June 12, total swarm seismicity includes one earthquake of magnitude 4.4, 12 in the magnitude 3 range, and 185 earthquakes in the magnitude 2 range,” the statement said.

Earthquake swarms account for about 50 percent of the seismic activity recorded at Yellowstone. Swarms occur when many earthquakes take place over several weeks or months, with no clear sequence—unlike traditional earthquakes, where there is a main event, then a series of aftershocks.

Mike Poland, the scientist-in-charge at the USGS’s Yellowstone Volcano Observatory, tells Newsweek that it is a “bit too soon” to say whether the swarm has ended. “But the activity has certainly waned drastically since August, and the swarm appears to be winding down, if not completely over,” he says. “It will probably take a little while longer to declare it ‘over.’”


Yellowstone earthquakes A time history of the Yellowstone caldera uplift and subsidence patterns, along with quarterly catalog earthquake counts.BOB SMITH AT THE UNIVERSITY OF UTAH

He says that the actual number of earthquakes that have taken place is difficult to ascertain because quakes can overlap or are too small to be recorded. However, there are ways to retrospectively locate some of these events, so it could be that there have been “many times more earthquakes” than initially reported.

“This is the sort of work that will happen in the months to come, as we gather up all of the available data and start crunching numbers,” Poland says. “What we can say now is that through the end of September, the University of Utah has located 2,475 earthquakes in the swarm. This puts the 2017 swarm on par with that of 1985, which lasted three months and had over 3,000 located events.  

“[This is] certainly a fascinating event and one that we hope to learn more about through some post-swarm analysis,” he adds. “There's a lot to work on this winter, for sure.”

This story has been corrected to reflect  that the current swarm is one of the longest on record, rather than the longest. The swarm in 1985 was longer if the tail end of seismicity is taken into account. 

Yellowstone supervolcano was recently hit with over 450 earthquakes, prompting scientists to closely monitor the area. The first of the swarm of earthquakes began on June 12 and continued in the following days, including up to a 4.5 magnitude earthquake.

It's important to note that despite Yellowstone supervolcano's ability to erupt catastrophically, there do not appear to be signs that this swarm of earthquakes is leading to an eruption.

Geologists have monitoring systems setup surrounding the Yellowstone supervolcano that can detect seismicity underground. This monitoring system both provides data for researchers to better understand the volcano and as an early warning system. In this case, the most recent swarm of earthquakes likely was a result of the magma within the supervolcano moving through channels and conduits as it continues to fill.

The recent 4.5 magnitude earthquake was the largest to hit Yellowstone since March of 2014 when a 4.8 magnitude earthquake struck the area. Most of the earthquakes were limited to magnitudes below 1 and within the upper 9 miles of the crust. Keep in mind the earthquake magnitude scale is logarithmic, so a magnitude 2 earthquake is 10 times more powerful than a magnitude 1 earthquake.

The USGS estimates the probability of a large eruption from Yellowstone is 1 in 730,000 over the coming year. The USGS published a report in 2017 on the estimated impact from a Yellowstone eruption. In that report, it is estimated that most of the continental United States would be covered in ash, with areas in the northern Rocky Mountains covered by several meters of ash.

Highlighted areas are where ash beds have been identified from previous Yellowstone supervolcano eruptions.USGS

Highlighted areas are where ash beds have been identified from previous Yellowstone supervolcano eruptions.

The Yellowstone supervolcano is a result of a hotspot similar to the one that formed the Hawaiian Island chain. The Yellowstone hotspot is a result of mantle convection of magma that upwells through the lithosphere (Earth's crust) at Yellowstone. As an analogy, imagine a match held underneath the middle of a piece of cardboard. Eventually, the match would burn its way through the cardboard, similar to the hotspot burning its way through the lithosphere.

The 2017 swarm is farther north, and would more likely be linked to pressurization around the Norris Geyser Basin, or even the Hebgen Lake Fault Zone north of West Yellowstone. Future studies will allow us to learn more about the details of the swarm, and its effect on the surrounding crust.
As with the other well-studied earthquake swarms near Yellowstone in the past 50 years, damage to facilities and infrastructure has been minimal, as is potential for related volcanic eruptions. Fortunately, these events do provide an incredible learning opportunity for earth scientists around the globe.

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