The shaking signal of far-away earthquakes at Yellowstone

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Beth Bartel, Michael Gottlieb, Dave Mencin, Glen Mattioli, and Tim Dittman, from the non-profit UNAVCO consortium in Boulder, CO.

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Yellowstone is an incredible place to record earthquakes—not only earthquakes that occur within Yellowstone's caldera, but also those that occur elsewhere in the world. This is because Yellowstone is very well instrumented. For example, a look at the distribution of NOTA GPS stations near the March 31 Stanley, Idaho, earthquake shows that most of the stations are around Yellowstone!  This is because monitoring Yellowstone deformation—its ups and downs—is of great interest to geoscientists.

While NOTA's GPS stations stretch from Alaska all the way through the Caribbean, there are only a few clusters of borehole geophysics stations—places where scientific instruments are installed hundreds of feet deep in holes drilled into the ground—and Yellowstone is host to one of them. There are six borehole sites within Yellowstone National Park, and they yield very precise measurements of how the ground is deforming, with multiple instruments installed in some of the boreholes.

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Borehole seismometers measure shaking, and although there are lots of other seismometers at Yellowstone and around the world, borehole seismometers are particularly sensitive because they are installed well below the surface, isolating them from many noise sources. You may have read reports of seismic readings being unusually "quiet" because of COVID-19 stay-home orders; this means not that there are fewer earthquakes, but that there is less "cultural noise"— the vibrations caused by movement along the ground, like road traffic, that sometimes hide small earthquakes. Borehole seismometers like the ones in Yellowstone are almost always quiet, because they are so far below the ground surface. This means the earthquakes stand out and are easily detected.

Borehole strainmeters, typically installed slightly below the borehole seismometers, measure small changes in the borehole shape and are incredibly sensitive. They can measure a change in the 10 cm (4 in) diameter of the borehole to less than the width of a single helium atom! When seismic waves pass through, these sensors measure any push, pull, or shear. Similar to the borehole seismometers, the earthquakes are recorded loud and clear as the seismic waves temporarily change the shape of the borehole in very small ways.</p>

All of this means that there are a lot of earthquakes recorded at Yellowstone, including ones that happen quite far away, if they are strong enough, because seismic waves from large earthquakes travel around and through the planet! But this doesn't mean that these earthquakes will have any effect on Yellowstone. Wherever you are reading this, whether in Florida or Minnesota or the Netherlands, a seismometer or strainmeter will record a magnitude 8 earthquake on the other side of the world, even though you won't be able to feel it. The shaking is just too small; it takes really sensitive instruments to record the passing of the seismic waves when the earthquake is so distant. (In fact, some seismometers are so sensitive that they can't measure earthquakes that happen too close by because the recordings are literally off the charts!)

Measurements at Yellowstone help us understand the earth and also improve research methods that we can apply elsewhere. Historically, borehole strainmeters have been primarily used to study slower changes in the earth, like fault creep events that are similar to normal earthquakes but occur on time scales of days to weeks instead of seconds to minutes. Using the borehole strainmeters to study earthquakes is still somewhat new, scientifically speaking, and we continue to learn what these instruments can do. Scientists at the USGS are developing ways to use strain measurements to quickly determine an earthquake's magnitude. When we tried this new method for the March 31, 2020, Idaho earthquake, Yellowstone strainmeters overestimated the earthquake's size. This is probably because of the geology around Yellowstone.  Volcanic areas tend to amplify near-surface ground motion, making the earthquake appear to be larger than it is. Like with all data and calculations, it's important to understand the factors that affect the outcomes when interpreting the results.

All NOTA geophysical data from Yellowstone are free and open-access, which means they are available to the general public and to researchers working around the world. What's more, the data are provided as soon as they are received, usually with a maximum delay of 24 hours. Some geophysical stations, including many of the GPS sites, provide their data in real-time, as a constant stream of one measurement per second. And this doesn't only apply to Yellowstone&mdash;this is true for all NOTA stations! When there is a large earthquake within the network, UNAVCO creates an event response page, which includes links to the available data and shows how the instruments moved. For the recent Utah and Idaho earthquakes, plots showing how the shaking was measured in Yellowstone were posted, not because the earthquakes affected Yellowstone, but because Yellowstone was the closest place with a high-sensitivity, low-noise borehole geophysics network. The next closest instrument clusters are along the west coast.

You can learn more about borehole strain measurements through UNAVCO's earthquake event response data explainer for dynamic strain, or see what the March 18 magnitude 5.7 Utah earthquake, the March 31, 2020 magnitude 6.5 Idaho earthquake, and more looked like in the NOTA borehole seismic and strain data on UNAVCO's event response page.