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.

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Not all earthquakes are the same.  Obviously, there are larger magnitude and smaller magnitude events.  However, even two earthquakes with the same magnitude can be very different.  Some of it depends on what exactly is happening to cause the earthquake.  Most earthquakes in Yellowstone are from brittle failure on a fault.  These types of earthquakes are typically called “tectonic” earthquakes—or “Volcano-Tectonic” earthquakes (VTs) when in a volcanic setting—and produce easily identified P- and S-waves that can be “picked” and used for location and magnitude determination.  Tectonic earthquakes also produce energy across a wide range of frequencies, typically ~1–20 Hz, although the exact range depends on the magnitude and depth of the earthquake as well as the distance at which it is recorded.

It is well known, however, that some volcanic areas host different types of earthquakes that lack the higher frequencies. These events are typically called “Long-Period” earthquakes (LPs) or “Very-Long-Period” earthquakes (VLPs) and tend to produce energy at frequencies below 5 Hz—much gentler and resonant “shaking” than is produced by VT earthquakes. These longer-period earthquakes are not caused by brittle failure, but rather are thought to be caused by fluids moving through cracks beneath the ground. These fluids can be hydrothermal (water) or magma.

Although some LP earthquakes may have easily identifiable P- and S-wave arrivals, they most commonly have what are called emergent arrivals. This means the energy slowly builds in amplitude, and it is harder to identify the onset of the arrival due to background seismic noise.  For this reason, LPs are typically harder to detect and locate than the much more common VTs.  In addition, LP earthquakes usually last longer than VT events.  LP earthquakes typically occur at greater depths (10–35 km, or about 6–22 miles) than VT events and are often located near or below what is called the “brittle-ductile transition”, which is the area where the crust stops behaving in a brittle manner (fracturing) and starts to flow (like Silly Putty).

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LP seismicity has been documented at many volcanic centers throughout the U.S. and the world, including at Long Valley caldera in California. Even though Yellowstone is a silicic caldera system like Long Valley, LP earthquakes have never been identified at Yellowstone and seismologists have always wondered why.

Recently, two events that may be considered LP earthquakes were located in Yellowstone by the University of Utah Seismograph Stations (UUSS).  One of these events occurred on August 26, 2021, near the Norris Geyser Basin at ~16 km (10 mi) depth, and the other occurred on October 25, 2021, near Gibbon Falls at ~19 km (12 mi) depth.  As expected, energy in the < 5 Hz frequency range dominates the seismogram from these events.  In comparison, a VT event located in the same region and at a similar depth shows more broadband energy in the 1–15 Hz range.

The identification of possible LP events in Yellowstone is exciting not only because it shows us what we expected all along (Yellowstone is an active volcanic system with lots of fluids moving around in the crust!), but also because we can use these LP events to hopefully find other similar earthquakes.  We can use a technique called “template matching,” where we take the known recordings of LP events and use them as templates. The templates are scanned through all the seismic data that we have recorded in Yellowstone, and we find data that match the templates. These are investigated as potential new LP events.  Hopefully, using this technique, we can build up a catalog of LP seismicity in Yellowstone and have an additional source of data to help us better understand how the Yellowstone volcanic system works at depth.