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Another Earthquake Swarm Under Yellowstone Lake?

December 6, 2020

Have you noticed that there has been an increase in the number of earthquakes happening in Yellowstone over the last week?  This is because there is an active earthquake swarm occurring beneath Yellowstone Lake!  Although it looks impressive, it pales in comparison to past sequences, including the 2008–2009 Yellowstone Lake swarm.

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Jamie Farrell, assistant research professor with the University of Utah Seismograph Stations and Chief Seismologist of the Yellowstone Volcano Observatory.

Map of Yellowstone Lake seismic swarms in 2008-2009 and 2020
Map of earthquakes that occurred beneath Yellowstone Lake as parts of seismic swarms in 2008-2009 (green) and 2020 (red).  Orange line is the boundary of Yellowstone Caldera, which formed 631,000 years ago.

Since December 1 and up to the time of this writing (on December 7), an earthquake swarm has been ongoing beneath the eastern part of Yellowstone Lake.  Earthquake swarms are clusters of earthquakes in time and space.  Seasoned Yellowstone watchers know that earthquake swarms are exceedingly common, with about half of all Yellowstone earthquakes occurring as part of swarms.  The current swarm is interesting in that it is slightly larger than the average swarm in terms of numbers of earthquakes, and it is located beneath Yellowstone Lake.  This naturally brings comparisons to the 2008–2009 Yellowstone Lake swarm, one of the most famous swarms to occur since the formation of the Yellowstone Volcano Observatory!

The 2008–2009 swarm began on December 27, 2008, in northern Yellowstone Lake and lasted for around 11 days until January 7, 2009.  The swarm contained over 800 located earthquakes, with the largest being a magnitude 4.1 event that occurred towards the beginning of the sequence.  The 2008 swarm is notable in that the seismicity slowly migrated north over time at about 1 km per day.  In addition, there was recorded surface deformation at nearby GPS stations as the swarm migrated by.  A migrating swarm with accompanying surface deformation is indicative of fluids moving through the shallow crust.  As the pressurized fluid moves through the crust, it breaks the rocks and displaces them, causing earthquakes and surface deformation.  It is hard tell for sure what fluids are moving (water, gas, magma, etc.) given the small amount of total deformation.  This remains the only time that GPS stations in Yellowstone have recorded deformation related to an active earthquake swarm.

Earthquake rates over time for Yellowstone Lake 2008-2009 and 2020 seismic swarms
Rate of earthquake occurrence for the 2008-2009 Yellowstone Lake swarm (green) and 2020 swarm (red).

In comparison, the current swarm is much weaker in terms of total numbers of events and the energy they have released—known as the seismic moment.  Unlike the 2008 swarm, the seismicity in this current swarm doesn’t seem to be migrating with time.  However, one interesting aspect of this swarm is that it is occurring directly on the boundary of Yellowstone Caldera—a fault which formed due to collapse of the surface during the most recent large explosive eruption 631,000 years ago. 

Could this swarm be a reactivation of that boundary fault?  Only further research will be able to address that question, but swarms on caldera-bounding faults are relatively common.  For example, earlier this year, on September 10, 2020, another swarm occurred with about 100 located earthquakes in 24 hours (the largest was magnitude 2.8).  This swarm was located where the southern part of the caldera-bounding fault intersects a regional fault zone.  Existing faults and cracks are weak zones that are frequently prone to slipping, especially in the presence of hydrothermal fluids like those that are found in abundance at Yellowstone!

At this point, it doesn’t look like the current swarm beneath Yellowstone Lake will approach the size of some of the larger swarms that have occurred in the past few decades, but one never knows how things are going to go, especially when it comes to earthquakes.  We will continue to keep an eye on this swarm and see how it progresses—you can follow along by checking the seismicity map at the University of Utah Seismograph Stations website!.  One thing is for certain, Yellowstone will continue to have earthquake swarms like this in the future.  They are as much a part of the life of Yellowstone as the geysers and hot springs, and they are all a manifestation of the incredibly dynamic nature of the region!

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