Cheyenne, Wyo. ? Yellowstone National Park was jostled by a host of small earthquakes for a third straight day Monday, and scientists watched closely to see whether the more than 250 tremors were a sign of something bigger to come.

Swarms of small earthquakes happen frequently in Yellowstone, but it’s unusual for so many earthquakes to happen over several days, said 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.”

Smith directs the Yellowstone Seismic Network, which operates seismic stations in the 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.

“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.”

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.