There were more than 1,000 earthquakes in Yellowstone National Park in July/USGS

Yellowstone National Park generated more than 1,000 earthquakes in July, a large number, but not a record and not greatly concerning to geologists. However, the park's seismic activity seems to be pointing to a decrease in Steamboat geyser's massive eruptions after a spectacular period of activity.

"It was a doozy of a month for earthquakes," said Michael Poland, the scientist-in-charge of the Yellowstone Volcano Observatory run by the U.S. Geological Survey. "The University of Utah seismograph stations, they maintain and monitor the Yellowstone Seismic Network, located over 1,008 earthquakes during the month, and they're not even done counting, yet."

Most of the temblors were part of seven distinct swarms, with five centered in the area between Hebgen Lake just beyond the park's western border and the Norris Geyser Basin, he said. There also was a small swarm recorded in the Pitchstone Plateau in the southern end of the park.

"All of these swarms were anywhere between one and three or four dozen events, magnitude one to two, so fairly typical swarms for the Yellowstone region," Poland said in his monthly report. 

However, there was a huge swarm of more than 750 tremors recorded beneath the middle of Yellowstone Lake, he said.

"The largest was a magnitude 3.6 and that occurred on July 16th, when most of the earthquakes of this swarm took place," said the scientist.

"Now, this may seem like a lot and it is," Poland said of the more than 1,000 earthquakes. "The month was quite a doozy for earthquakes, but it's not actually the most we've seen in any given month. Back in June of 2017, we saw over 1,100 earthquakes."

Back in 1985, he added, "there was a much larger earthquake swarm or more than 3,000 events located in the Madison Plateau area."

Poland attributed the activity not to movement of magma, which might indicate a stirring of the Yellowstone volcano, but of groundwater.

"These are existing faults that are moving, sometimes sort of goosed by increases in pore pressure, that's groundwater from snow melt that's getting into these faults and causing them to trigger," he explained. "One of the things we might look at to see if magma might be a cause of these sorts of earthquakes is ground deformation, and we haven't seen any changes in ground deformation during the month of July."

Poland did note that Steamboat geyser, which was given its name in 1878 by members of the Hayden expedition who thought its roaring sounded like that of a paddle wheel steamboat, seems to be retreating from a very active period that started in 2018. That year the iconic geyser erupted 32 times (a record for a single calendar year).

"We have seen one eruption during the month of July, and that was on July 8th," he said. "It very much seems like Steamboat might be, to pardon the pun, running out of steam. We have seen many fewer eruptions this summer than we have each of the past three summers. And this is what Steamboat does, it goes through cycles of more eruptions and less. So it appears that we may be coming to an end of the current cycle of Steamboat eruptions."