More than 20 small quakes have struck under the volcano in the past week, a rate that’s higher than usual but not unprecedented.

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Nearly two dozen small earthquakes have rattled Mount Rainier over the past week, but seismologists say there’s no cause for worry.

“In the past, these swarms last a couple of days to a week or so and then die out,” said Paul Bodin, of the Pacific Northwest Seismic Network at the University of Washington.

The first of the 23 quakes struck on Sept. 11 near the volcano’s summit. The largest of the quakes registered magnitude 1.6.

The top map shows the location of historical earthquakes at Mount Rainier, with the recent quakes in red. Seismic stations are marked by arrows. The cross-section below shows the depth of the quakes. (Source: USGS)
The top map shows the location of historical earthquakes at Mount Rainier, with the recent quakes in red. Seismic stations are marked by arrows. The cross-section below shows the depth of the quakes. (Source: USGS)

Earthquake swarms are common at volcanos, and usually don’t signify any threat of eruption, Bodin said in an email. “So I’m treating this as a single eyebrow raised halfway,” he wrote. “Yeah, I see you and will be watching, but I don’t think you’re going to attack.”

Most volcanic quake swarms originate in the hydrothermal plumbing system, related to slight changes in temperature or groundwater pressure that cause cracking of the rocks, Bodin explained. The recent quakes are shallow, which also suggests they are not connected to the deep movement of magma.

Rainier experienced similar upticks in the past two years, and a more sustained episode of seismicity in 2009.