USGS: Periodic ‘Small Swarm’ of Earthquakes Reported Wednesday at Mount Rainier

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The United States Geological Survey recently reported that a “small swarm” of earthquakes occurred Wednesday at Mount Rainier. The activity represents a typical increase of seismic activity that’s annually reported in the area.

“While the seismicity represents a temporary uptick in activity, Mount Rainier remains at normal background levels of activity,” the USGS wrote on Facebook Thursday afternoon.

The uptick is a reminder of the rich zone of volcanic activity that exists in the Cascade Mountain Range. It’s being driven by the divergent Pacific Plate with North America. The range today contains roughly 2,900 volcanic features, according to USGS.

The small swarm at Rainier — which totaled 20 quakes, as recorded by the Pacific Northwest Seismic Network — first started Wednesday, Feb. 17, at about 3:13 p.m. and lasted about five hours. A single earthquake was reported the next day.

The maximum magnitude of the quakes were reported at M2.5 at down to a kilometer below the surface (for comparison, the 2001 Nisqually Earthquake measured in at M6.8). The Feb. 17 quakes weren’t felt at surface level, USGS says.

“The most notable swarm occurred between Sept. 20 and Sept. 22, 2009, when over 1,000 earthquakes were detected,” USGS wrote. “Since the early 1980s, Mount Rainier seismicity has been monitored by PNSN and the USGS Cascades Volcano Observatory via a network of seismic stations located within 20 km (12 miles) of Mount Rainier, including new stations installed late last year to help detect lahars (volcanic mudflow).”

Recent data has led scientists to believe the earthquakes stem from hydrothermal fluids “lubricating” faults within rock underneath the mountain.

Seven Cascade volcanoes have erupted since the start of the 18th century, according to USGS, with the most well-known one — and most extreme — being the eruption of Mount St. Helens in the spring of 1980.

Due to its proximity to high populations, Mount Rainier is listed among nine volcanoes in the range with a threat that is “very high,” according to a 2018 USGS updated study.

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