A pair of relatively small earthquakes that occurred within about 15 miles of Old Faithful geyser over the weekend were not caused by volcanic activity in the park, according to the U.S. Geological Survey.
The concern that any earthquakes in the park might be related to the activity of the large, powerful volcano atop which Yellowstone sits is a frequent one, according to Michael Poland, a USGS geophysicist and the scientist-in-charge of the Yellowstone Volcano Observatory.
Earthquakes, even when caused by the volcano, are a regular feature of the park, Poland said. Earthquakes in Yellowstone can be caused by volcanic activity, subsurface water, or tectonic plates.
Earthquakes caused by the volcano usually occur in “swarms” and are often related to the movement of magma. A swarm is when numerous small earthquakes, sometimes thousands, happen over a limited period of time that in some cases stretches over months. But not all swarms in the park are caused by the volcano.
“If it were really an intense swarm that would precede some sort of volcanic activity there would be thousands of events,” Poland said. “This is the kind of thing that would be pretty unmistakable.”
There have been about 1,100 earthquakes in Yellowstone region so far in 2019, and in an average year the area experiences between 1,500 and 2,000, Poland said. Earthquakes of at least 3.5 magnitude, like both of those that happened over the weekend, are less common but still not unheard of.
There have been six earthquakes of magnitude 3 or more in 2019. There were five in 2018, 14 in 2017 and none in 2016, Poland said.
Last weekend the first of the two quakes occurred Saturday at 7:15 p.m., about 14 miles northeast of the geyser, and was a magnitude 3.5.
The second earthquake happened at 5:30 a.m. Sunday. That earthquake was a magnitude 3.5 and occurred in Idaho about 15 miles west and southwest of Old Faithful, just outside the park’s border.
If one of these earthquakes happened below someone’s home, they might see plants or hanging lights sway. “It wouldn’t be expected to cause any damage,” Poland said.
Offering an example for comparison, Poland pointed to the Hebgen Lake earthquake of 1959 in Montana, which was measured at a magnitude 7.3 and is the largest in the U.S. Intermountain West in recorded history.
That earthquake dammed the Madison River, created a lake, displaced 50 million cubic yards of rock, mud and debris, damaged roads and even affected geothermal features in the park.
The USGS has reports of five people feeling the weekend earthquake in Idaho and one person feeling the earthquake northeast of Old Faithful.
“There are thousands of earthquakes every year at Yellowstone, and that’s normal,” Poland said.