William Kronholm, AP
Yellowstone National Park, as seen here in February, remains very active geologically, as evidenced by its famous geysers and hot springs.
Swarms of small earthquakes happen frequently in Yellowstone, but it's very unusual for so many earthquakes to happen over several days, says Robert Smith, a professor of geophysics at the University of Utah.
"They're certainly not normal," Smith said. "We haven't had earthquakes in this energy or extent in many years."
"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 said it's difficult to say what might be causing the tremors. He pointed out that Yellowstone is the caldera of a volcano that last erupted 70,000 years ago.
Yellowstone has had significant earthquakes as well as minor ones in recent decades. In 1959, a magnitude 7.5 quake near Hebgen Lake just west of the park triggered a landslide that killed 28 people.