Brief earthquake swarm detected beneath Mount Rainier

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While the seismicity represents a temporary uptick in activity, Mount Rainier remains at normal, background levels of activity.

Mount Rainier Seismicity 2010-2020

Map shows earthquakes at Mount Rainier from 2010 to 2020. Circles show earthquake epicenters, sized by magnitude. The red circles are earthquakes from a swarm that occurred February 17-18, 2021, in an area that has seen many earthquakes since 2010. Seismic stations are shown as blue triangles. 

(Credit: Weston Thelen, USGS-Cascades Volcano Observatory. Public domain.)

On February 17, 2021 at 3:13 p.m. local time, a small swarm of earthquakes began at Mount Rainier. Activity associated with the swarm ended about five hours later (around 8:00 p.m. local time), although a single earthquake occurred the morning of February 18.  

In total, nearly 20 earthquakes were located by the Pacific Northwest Seismic Network (PNSN). The maximum magnitude was a M2.5 and depths were between 0 to 1 km (0.6 mi) below sea level, according to PNSN. There have been no reports that the earthquakes were felt at the surface. 

Earthquakes are part of the background activity at Mount Rainier, and swarms of this number of earthquakes typically occur once or twice a year. The most notable swarm occurred between September 20 and September 22, 2009, when over 1000 earthquakes were detected.  

Since the early 1980s, Mount Rainier seismicity has been monitored by PNSN and the USGS Cascades Volcano Observatory (CVO) via a network of seismic stations located within 20 km (12 mi) of Mount Rainier, including new stations installed late last year to help detect lahars. From the data, scientists believe that earthquakes at Mount Rainier occur by hydrothermal fluids moving along ("lubricating") existing faults within basement rock underlying the Rainier edifice. 

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