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Graphic by Wes Thelen.
Earthquake swarms bring a deeper understanding posted on 04/01/2021

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




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