A Geophysicist's Perspective on Mount Hood Monitoring Stations and the Recent Earthquake Swarm

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A CVO geophysicist discusses how the monitoring stations installed at Mount Hood in 2020 provide insight into the recent Mount Hood earthquake swarm.

In late September 2020, the USGS Cascades Volcano Observatory, in cooperation with the U.S. Forest Service Mount Hood National Forest, installed three new volcano monitoring stations on the flanks of Mount Hood. Each station included seismic and GPS instruments. Dr. Wes Thelen, a geophysicist at CVO, commented on how the stations performed during the recent Mount Hood earthquake swarm.

Q: First, is the earthquake swarm at Mount Hood over?  

A: To recap, on January 17, 2021, there were over 100 individual earthquakes in an area south of the summit of Mount Hood. The maximum Magnitude was 2.7 and the depths were mostly around 5 km (3 mi) below sea level. Since our January 17 update, we have seen fewer earthquakes, and seismicity has returned to near background levels. We can’t say seismic activity will not resume, but since swarms at Mount Hood typically last hours to days, this fits the pattern and seems to indicate that the swarm is over. 

Q: Is the swarm a sign that Mount Hood (the volcano) is waking up?  

A: No. The swarm has all the characteristics of a "tectonic" swarm. The swarms can be located on or near the volcano but have the characteristics of earthquakes that respond to regional stresses and are not associated with magma movement or changes in local magmatic pressure. The location of the earthquakes is similar to other tectonic swarms in this area that occurred in 2009, 2012, 2014 and 2020. Since 1980, there have been occasional earthquakes beneath the summit, but most of the seismic action has taken place on faults close to the volcano.   

Q: Did it help to have new stations on the volcano?  

A: Yes. The stations have sensitive GPS instruments. We are able to say with confidence that there was no inflation associated with this swarm. Previously, the north side of the volcano was a bit of a blind spot – we had stations farther away but nothing high on the flank. Future eruptions are likely to include inflation of the volcano so we are well poised to see the earliest signs of it. 

Q: What about seismicity?  

A: The stations each have broadband seismometer that detect tiny earthquakes (smaller than Magnitude 1.0 and not felt by humans). The new stations helped us understand how the faults responsible for the earthquakes slipped. In the past, this type of information could only be calculated on larger earthquakes, and even then there was less certainty in the solutions. By having a more robust monitoring network at Mount Hood, the information helps us put the current swarm in context of past activity and is one of the main lines of evidence we have for a "tectonic" origin.  

Q: Did the new stations help pin-point where the earthquakes were occurring? 

A: Yes. The new stations contributed toward finding more precise locations compared to previous swarms. Subsequent post-processing will be able to use the new stations to better define the extent of faulting and any migration that occurred during the swarm. This will help us understand the process behind why this fault is so active.   

Q: Will the data provide any new information about Mount Hood as a volcano?  

A: Yes. Each new earthquake recorded on these new stations provides an opportunity to track the path of seismic waves to “see” what is inside the volcano and what has changed. Future studies will use this information to better understand the location, size and state of the magmatic plumbing system under Mount Hood.   

Q: Can we see the data?  

A: Yes. Data from the remote monitoring stations transmit in real-time data to the Cascades Volcano Observatory and its monitoring partner, the Pacific Northwest Seismic Network. You can view data from these new stations on the CVO webpage Mount Hood monitoring data (all monitoring data streams) or at PNSN (earthquakes only).