Environment
Hikers stand along Burroughs Mountain Trail with a view of Mount Rainier on Saturday, July 20, 2019.
Enlarge Icon
Hikers stand along Burroughs Mountain Trail with a view of Mount Rainier on Saturday, July 20, 2019.
Credit: KUOW Photo/Megan Farmer

What do the earthquakes at Mount Rainier mean?

Twenty small earthquakes shook near Mount Rainier this past week.

They were all within 8.2 miles of the volcano, most of them closer.

Mount Rainier is one of Washington's active volcanoes, but earthquake experts say this swarm of quakes is not cause for alarm.

This isn't the type of earthquake activity that would lead up to an eruption, said Harold Tobin, director of the Pacific Northwest Seismic Network, which is based at the University of Washington. They track earthquakes across the Northwest.

The quakes have been small and fairly shallow. For example, a 0.4 magnitude Monday afternoon and a 1.2 magnitude that morning.

"For us to start to be concerned about an imminent eruption we would have to see a sort of persistent swarm of earthquakes," Tobin said. "We would tend to see them extending up and down in depth, suggesting that there's magmatic movement."

He said a cluster like that happens several times a year.

"All of our active volcanoes have occasional swarms of this kind of seismic activity," Tobin said, "so small swarms of earthquakes like this at Mount Rainier are tremendously common."

Tobin said there were a few fairly large quakes toward the beginning of December but not near the volcano. A magnitude 4.1 shook Tofino, Canada; a magnitude 3.4 at Goat Rocks, Washington; and a 4.5 at Coos Bay, Oregon.

These earthquakes are a reminder that we're in earthquake country, but do not indicate volcanic activity or a "big one" — an 8- or 9-pointer — ahead.