Mr Smith, director of the Yellowstone Seismic Network, which operates seismic stations around the park, said the earthquakes have ranged in strength from barely detectable to Saturday's 3.9. A magnitude 4 earthquake 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," he said. "We might be seeing something precursory.
http://www.telegraph.co.uk/earth/environment/4032098/Yellowstone-Park-shaken-by-hundreds-of-earthquakes.html