What is “normal” seismic activity in Yellowstone National Park?

In monthly reports on earthquakes in Yellowstone, it is common to read the statement, “Seismic activity in Yellowstone remains at background levels.”

In fact, in the United States Geological Survey's monthly activity reports, there is an alert level assigned to each volcano, including Yellowstone. The “normal” alert level is associated with “a non-errupting volcano showing typical background activity.” The “advisory” alert level is associated with “a volcano showing signs of unrest elevated above known background activity.”

So, the question is, how do we know what is normal versus something that might be associated with a potential eruption?

This is a map of Yellowstone National Park and earthquakes that occurred there from 1973 to 2023. Red circles are earthquakes occurring in the Yellowstone region, and blue circles indicate swarm seismic activity. The size of the circle is proportional to the size of the earthquake. (Courtesy of Yellowstone Caldera Records)

Regarding earthquake activity in the Yellowstone region, the Yellowstone Volcano Observatory typically reports that the Yellowstone region experiences 1,500 to 2,500 earthquakes per year.

These numbers are based on seismic activity over the past 30 years, from 1995 to the present. During that time period, the year with the highest number of earthquakes was 2017, with 3,427 events, and the year with the fewest earthquakes was 2011, with 708 events.

As with most natural systems, Yellowstone does not behave in a linear or predictable manner, so taking the average number of annual earthquakes would not represent the characteristic range of such a dynamic system.

One reason the annual number of earthquakes is so wide is that nearly half of Yellowstone's earthquakes occur as part of earthquake swarms — groups of seismic events that cluster in time and space. Individual swarms can contain as few as 10 events and last less than an hour or as many as thousands of events and last for several months (the 2017 Maple Creek swarm is an example). Typically, years with large numbers of earthquakes, such as 2017, are characterized by large swarms.

Another thing you might wonder: Yellowstone has been monitored seismically since the early 1970s, so why is the range of “normal” seismic activity based on 1995 to present?

This is because the seismic network has grown and improved over the years. If you look at the seismogram over time, you can see a clear increase in seismic activity around 1995. Before 1995, the Yellowstone Seismic Network had fewer stations. In about 1995, the network was significantly upgraded with not only additional stations, but also stations with newer technology (including data transmission) that allowed the identification and location of more earthquakes (usually smaller events). These earthquakes were always happening, but before this upgrade in the network we couldn't pinpoint their location.

So the apparent increase in seismicity around 1995 is not real; This is simply because a better network allowed us to identify and locate more earthquakes that were occurring.

This is a histogram, or visual representation of the distribution of numerical data, showing the number of earthquakes for each three-month period in the Yellowstone National Park area from 1973 to 2023. Red bars represent all earthquakes occurring in the Yellowstone area, and blue bars indicate swarm seismic activity. (Courtesy of Yellowstone Caldera Records)

Although we have continued to improve the Yellowstone seismic network to this day (including a new station installed just last summer!), since 1995 the network has the coverage needed to detect most earthquakes that occur in the region. Therefore, seismic rate changes since 1995 likely represent the true variability in the system.

Volcanic systems like Yellowstone are naturally dynamic; However, there are patterns over time that can be identified through careful observation and study. The longer we have monitoring equipment that records seismic activity (and other tectonic, hydrothermal, and volcanic signals), the more likely we are to be able to identify periods of “unusual” behavior. But just because there is a high (above average) earthquake, doesn't mean an eruption is imminent, no matter what you may see online.

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators at the Yellowstone Volcano Observatory.

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