Dozens of earthquakes occur over 45 minutes at Oregon’s Mount Hood. What caused them?
A 45-minute “earthquake swarm” reported this week near Oregon’s Mount Hood has the U.S. Geological Survey offering explanations — and reassurance the volcano is not becoming more active.
Dozens of earthquakes, most not felt on the surface, came in succession around 12:13 p.m. Monday, centered about a half mile southwest of the volcano, geologists say.
Data show the quakes came from 3 to 4 miles beneath the summit, officials said Tuesday. The volcano is about 50 miles east of Portland
“The Pacific Northwest Seismic Network located eight events so far with a maximum magnitude of M 1.3; dozens of other events were too small to locate,” the USGS posted.
“Although not a common occurrence, short-lived swarms have occurred near the summit in the past, including November 2013, September and October of 2014, and May 2016.”
So what caused the 45 minutes of seismic rattling?
It was likely “pressurized” super-heated water — not bubbling hot magma, experts say.
“The recent earthquakes at Mount Hood are interpreted to be the movement of hydrothermal fluids along preexisting faults,” the USGS said.
“What we are seeing is slip on a fault or faults. It is easiest to achieve such earthquakes if you push a little water in there and ‘jack’ the fault apart just enough to induce slip,” the statement read. “It may be possible to do the same thing with magma, but ... water is much less viscous and able to penetrate any weakness in the pre-existing rock.”
Experts also said they do not believe the swarm is evidence Mount Hood is becoming more active, but it does represent “a temporary uptick in activity,” the USGS says. The volcano last erupted “in the mid-1800’s (when) local residents reported minor explosive activity.”
The same “burst-like swarms” have been seen at other volcanoes around the world, including Mount Rainier and Mount St. Helens in the northwest, experts say.
A “robust network of instruments” has been deployed around Mount Hood “to catch the subtle signals beneath,” the USGS says.
Concerns in the region about the volcano’s status have been elevated since a series of “100 individual earthquakes” were recorded south of the summit on Jan. 17. All were mild, with the maximum magnitude at 2.7, and they were centered about 3 miles down, the USGS said.
This story was originally published March 3, 2021 at 8:36 AM with the headline "Dozens of earthquakes occur over 45 minutes at Oregon’s Mount Hood. What caused them?."