 Graphic by Wes Thelen.
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.
By Garth Guibord/MT | 
