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‘Small swarm’ of earthquakes reported at Mount Rainier

Nearly 20 small earthquakes were detected at Mount Rainier on Wednesday. Experts said earthquake swarms like this “typically occur once or twice a year.”

Editor’s note: The video above was originally published in October 2020 when scientists were adding more sensors to detect lahars on Mount Rainier.

MOUNT RAINIER - A “small swarm” of earthquakes was reported at Mount Rainier Wednesday. While the swarm was an increase in activity, experts said there is no reason to sound the alarm.

"While the seismicity represents a temporary uptick in activity, Mount Rainier remains at normal, background levels of activity,” the United States Geological Survey (USGS) said.

The Pacific Northwest Seismic Network (PNSN) detected the first earthquake around 3:15 p.m., and the activity ended around 8 p.m. In all, nearly 20 earthquakes were detected in about 5 hours. A single earthquake was also detected Thursday morning.

The largest earthquake was a magnitude 2.5, and the quakes were between 0-0.6 miles below sea level, according to the PNSN. There were no reports the earthquakes were felt at the surface.

RELATED: How a Mount Rainier eruption could impact Seattle vs. the South Sound

The Washington Emergency Management Division said earthquake swarms like this “typically occur once or twice a year.”

The USGS Cascades Volcano Observatory and the PNSN have been using a network of seismic stations to monitor volcanic activity in the national park since the early 1980s. New seismic stations were installed last year by the USGS to help detect lahars, which are volcanic mudflows that include a mixture of water and volcanic debris like boulders.

Using the data from the seismic stations, the USGS said scientists believe the earthquakes stem from hydrothermal fluids "lubricating" faults within rock under Mount Rainier.

The most notable earthquake swarm, according to the USGS, happened in 2009 when over 1,000 earthquakes were detected between September 20-22.

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