Earthquake patterns over time in Yellowstone highlight the complexity of seismic swarms

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators at the Yellowstone Volcano Observatory. This week's contribution is from David Shelley, a seismologist with the USGS.

The Yellowstone region generates a lot of earthquakes. Although most of these earthquakes are too small to be felt by humans, they can be studied using data from Yellowstone's dense, high-quality network of seismometers. This network, located in and around the national park, records extremely precise ground motions from up to thousands of (mostly small) earthquakes each year. Many of these earthquakes are routinely detected and located by the University of Utah Seismic Stations (UUSS), but others are poorly recorded and not detected at enough stations to be included in the earthquake catalog.

Earthquakes in Yellowstone are not evenly distributed in space and time, but can occur in clusters and are often focused on specific areas. The patterns of how earthquakes occur can tell us a lot about active processes beneath the surface. Although routine real-time seismic monitoring does an excellent job of detecting many of these patterns, others only become apparent when specialized analyzes are applied to previously recorded data. Such techniques can detect a greater number of earthquakes than routine processing and pinpoint the location of these earthquakes more precisely, enhancing our insight into their causes.

While some individual time periods with particularly high seismic rates (such as during earthquake swarms) have been examined previously, a study ( https://doi.org/10.1126/sciadv.adv6484 ) published earlier this year in the journal Science Advances took a longer-term view. Rather than focusing on a time period of just a few days, weeks or months, the new research analyzed 15 years of seismic data, from 2008 to 2022. Starting with raw, continuous seismic waveforms (ground motion records) from the network of seismometers around Yellowstone, this effort took advantage of recent advances in artificial intelligence, applying a machine learning approach to sift through this massive data set and determine the arrival of the seismic phase (P and S waves) of nearby earthquakes. This technology detects very small earthquakes that are not recognized by the routine system.

View map of transmitted Yellowstone earthquakes recorded during 2008-2022, colored by time, at left. The white line gives the outline of the Yellowstone caldera. Cross section AA' across Yellowstone Lake is shown on the right and shows how seismic swarms that vary over time relate to each other. Adapted from Florez et al., 2025 (https://doi.org/10.1126/sciadv.adv6484). Map of seismic stations in the Yellowstone region, with the number of channels indicated by number and sensor type by color. Inverted triangles indicate stations operated by the University of Utah Seismological Stations (UUSS), and squares indicate stations operated by other agencies.

After accounting for the magnitude of the earthquakes, the resulting earthquake catalog for 2008-2022 contained 86,276 events, more than 10 times the number listed in the routine earthquake catalogue. This isn't really a surprise, because there are obviously a lot more earthquakes occurring in the Yellowstone region, and around the world, than can be located by current real-time seismic analysis, but these events are all very small – typically less than M1.5. The locations of the earthquakes were then refined by using a recently produced model of 3D subsurface seismic velocities and by applying precise relative transport techniques that carefully compare the timing of seismic waves between nearby earthquakes. In total, 67,433 earthquakes were successfully transmitted.

This catalog provides a new window into the long-term behavior of earthquakes in Yellowstone. One striking feature is the long-term correlation between short-term earthquake swarms. Although individual swarms typically last days to weeks (and sometimes months), this study found that swarms separated by many years may occur next to each other in space. Understanding this behavior is still a subject of ongoing research, but one possible explanation relates to fluids (primarily water) present at depth. The movement of fluids in the Earth's interior may cause a seismic swarm. But what if the liquid moves a certain distance and then stops? Neighboring swarms separated by years may indicate reactivation of dormant fluids. An example of this behavior is swarm activity near the northern end of Yellowstone Lake in 2020-2021, which was located just south of the swarm in 2008-2009. Other areas, such as the Maple Creek area outside the Yellowstone caldera to the northwest, also showed spatially contiguous swarm activity with significant discontinuities between individual swarms.

Earthquakes in Yellowstone are generated by a complex and fascinating interplay of tectonic, hydrothermal, and volcanic processes. These new findings help us see the bigger picture of seismic activity in Yellowstone, correlating smaller snapshots of activity from previous detailed but short-duration studies of individual swarms. Thus, we now have a better understanding of the relationships between earthquake swarms that may be many years apart.

Although Yellowstone certainly has more surprises up its sleeve, long-term investments in monitoring are beginning to pay off. Seismic data, along with other valuable geophysical and geochemical data sets, continue to reveal active volcanic, hydrothermal and tectonic processes hidden deep below the surface. As we build a longer and longer record of this behavior, our understanding of the Yellowstone system will continue to deepen.

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