Swarms of small earthquakes happen frequently in Yellowstone. But Robert Smith, a professor of geophysics at the University of Utah, said it's very unusual for so many earthquakes to happen over several days.
"This is an active volcanic and tectonic area and these are the kinds of things we have to pay attention to," Smith said. "We might be seeing something precursory.
"Could it develop into a bigger fault or something related to hydrothermal activity? We don't know. That's what we're there to do, to monitor it for public safety."
Smith directs the Yellowstone Seismic Network, which operates seismic stations around Yellowstone National Park. He said the quakes have ranged in strength from barely detectable to one of magnitude 3.8 that happened Saturday. A magnitude 4 quake is capable of producing moderate damage.
The strongest of dozens of tremors Monday was a magnitude 3.3 quake shortly after noon. All of the quakes were centered beneath the northwest end of Yellowstone Lake.
"They're certainly not normal," Smith said. "We haven't had earthquakes in this energy or extent in many years."
A park ranger based at the north end of Yellowstone Lake reported feeling nine quakes over a 24-hour period last weekend, according to park spokeswoman Stacy Vallie.
Vallie said no damage was reported.
"There doesn't seem to be anything to be alarmed about," she said.
Smith said it's difficult to say what might be causing the tremors. He pointed out that Yellowstone is the caldera of an active volcano that last erupted 70,000 years ago.
He said Yellowstone remains very geologically active - and its famous geysers and hot springs are a reminder that a pool of magma still exists five to 10 miles underground.
"That's just the surface manifestation of the enormous amount of heat that's being released through the system," he said.
Yellowstone has had significant earthquakes as well as minor ones. In 1959, a magnitude 7.5 quake near Hebgen Lake just west of the park triggered a landslide that killed 28 people.