Earthquakes are common at Mount Rainier, happening about nine times every month, the agency said. Swarms of earthquakes happen once or twice per year. Larger swarms are more rare – the last major event was in 2009, when a “vigorous swarm” of more than 1,000 earthquakes, reaching up to magnitude 2.3, shook the mountain for three days.

Hundreds of earthquakes have been detected at Mount Rainier so far Tuesday morning in the largest such swarm at the active stratovolcano since 2009, according to the U.S. Geological Survey.