Mount Rainier was experiencing "the most significant seismic activity" at the volcano in more than 15 years as an "earthquake swarm" hit the site starting on Tuesday.

There are typically only about nine earthquakes detected per month at Mount Rainier and swarms usually occur once or twice each year, the USGS said. Even so, swarms are generally

Scores of tiny earthquakes rattled Mount Rainier this week, marking the largest swarm of earthquakes at the mountain since 2009, according to the U.S. Geological Survey. Why it matters: Even though scientists say there's no immediate cause for alarm,