An “earthquake swarm” has produced more than 330 minor seismic events under Mount Rainier in Washington state since Tuesday, the U.S. Geological Survey said.
As the name suggests, an earthquake swarm refers to a cluster of seismic events happening quickly in succession in the same area. The current swarm, the USGS Cascades Volcano Observatory said, started at 1:29 a.m. local time on Tuesday. As of 9 a.m. Thursday, there had been 334 earthquakes observed as part of the swarm.
The quakes are happening 1.5 to 4 miles beneath the summit of Mount Rainier and cannot be felt on the surface because they are small, CVO said. The largest registered quake so far, a magnitude 2.3, happened at 1:26 p.m. on Tuesday.
The swarm has also slowed down over time; there were around 26 seismic events per hour at the outset but, as of Thursday, the rate had dropped to just a few per hour.
CVO said that there are normally around nine quakes at Mount Rainier each month, with swarms happening once or twice a year. The ongoing earthquake swarm is larger than normal and has outpaced a swarm observed in 2009 that included a magnitude 2.5 quake.
Officials are not worried yet about the swarm heralding a possible eruption. Mount Rainier has not had a significant eruption in the past 500 years, USGS said.
“Currently, there is no indication that the level of earthquake activity is cause for concern, and the alert level and color code for Mount Rainier remain at GREEN / NORMAL,” CVO said.
