On Monday, March 1, a 45-minute-long swarm of earthquakes occurred to the southwest of Mount Hood’s summit at a depth of approximately one to two kilometers below sea level. This was after more
than 100 individual earthquakes hit the south side of the mountain’s summit
on Sunday, Jan. 17, with a maximum magnitude of 2.7 on the Richter scale
and at a similar depth.

Residents around Mount Hood need not fear an imminent eruption, as these
swarms are characteristic of earthquakes related to regional stresses and not
associated with movement in magma. Scientists are watching, though.

“Anything that happens near the summit of any volcano is going to get our
attention,” said Wes Thelen, Research Seismologist with the Cascade Volcano
Observatory (CVO), adding that they did not see any other activity that would
indicate a lead up to some “broader unrest.”

And thanks to three volcano monitoring stations installed on Mount Hood in
September 2020, swarms such as these can help our understanding of its
volcanic secrets and the seismic activity in the area. The sites each include
seismic and GPS instruments, and scientists now have an opportunity to
better understand the factors involved with Mount Hood’s volcanic activity.
Thelen, who has worked at CVO since 2016, assesses seismic data on a daily
basis for volcanoes from Mount Baker to Crater Lake to determine their
volcanic hazard. He noted the location of the three new equipment sites are
far from ski resorts and other developments where previous equipment exists,
such as at the top of the Palmer Lift at Timberline and a waste processing
plant at Mount Hood Meadows, eliminating much of the noise that occurs at
times.

“These sites are in areas that are very very quiet,” he said. “These are very
good observations of the same events.”

That could help scientists determine what orientation the fault plane is in or
how an earthquake slips, thanks to the next level of details.

Thelen also noted the GPS sensors offer data that was not previously
available, which could help reveal any deformation in the land (such as
inflation or deflation) associated with an earthquake. That type of deformation
would be expected if a volcano was building toward significant unrest or an
eruption.

Thelen added that while the recent earthquake swarms may not have been
missed if the stations were not in place, he likened the added equipment to

having more witnesses at a crime scene, giving observations a higher degree
of reliability.

“What’s different about these stations is that we’re seeing these things much
better now,” Thelen said. “We’re getting now at least three stations, quite
close, up on the volcano.”

Swarms such as these are not common, but have happened in the past,
including in November 2013, September and October 2014 and May
2016. The swarm to the south (in January), Thelen noted, is fairly typical on
Mount Hood, typically occurring once a year, while he added that there is a
regional stress present in the crust around the mountain and even if Mount
Hood wasn’t there, he’d expect similar earthquakes to occur.

In time, enough earthquakes will be recorded offering a new data set that can
reveal some of the hidden secrets of Mount Hood, perhaps including the size
of the magma chamber and how deep the chamber is, thanks to being able to
track the path of the seismic waves.

“When we start to get a picture like that, we can build some conceptual
models ... of what might link these swarms together,” he said, adding that the
new data will also offer a better assessment of what hazards might be
associated with the activity.

And the new stations should add momentum to interest in Mount Hood’s
volcanic activity, spurring studies to look deeper into the mountain than we
have seen before.

“We don’t know a lot about what’s happening inside the volcano,” Thelen said.
Mount Hood is a challenging volcano to study, he added, in part because there
is no record of its eruptive cycle. Most volcanic earthquakes will occur under
the summit and remain so small that people are unlikely to feel them.

“It really dissipates energy quickly,” Thelen said, adding that an earthquake
approaching a magnitude of 3 at the summit of Mount Hood would get their
attention.

And our understanding of Mount Hood will grow more later this year, as
scientists received permits from the U.S. Forest Service (USFS) to add

instruments and monitoring equipment around Government Camp for long-
term study of thermal water features and infrasound instruments at Mt. Hood
Meadows.

Thelen also noted that their work would not be possible without the efforts
from different partners on Mount Hood, including the USFS, Timberline and
Mt. Hood Meadows.

“We’re really appreciative of the different partners we have in the area to
keep these stations going,” he said.

Data from the remote monitoring stations transmit in real-time data to the
CVO and its monitoring partner, the Pacific Northwest Seismic Network. To
view data from these new stations on the CVO webpage,
https://www.usgs.gov/volcanoes/mount-hood/monitoring.