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GPS monitoring

GPS monitoring station P709 is located on The Promontory between the South Arm and Southeast Arm of Yellowstone Lake. It was installed in 2005 as part of the Yellowstone component of the National Science Foundation's Plate Boundary Observatory.

Yellowstone is an incredibly dynamic place — there are frequent earthquakes, geysers erupt and go dormant and the ground moves up and down. As seasoned Yellowstone watchers know, all of the monitoring data recorded by the agencies that make up the Yellowstone Volcano Observatory is public. But how does one use this data to understand what is happening in and around the caldera?

In the year to come we will devote occasional Yellowstone Caldera Chronicles articles to explaining where to access Yellowstone monitoring data and how to understand them. This is increasingly important in this day and age of ever-growing Internet misinformation — if you know how to interpret the data yourself, you won’t have to rely on anyone else to tell you what the data means.

We’ll start with Global Positioning System, or GPS, data — the backbone of monitoring ground movements at Yellowstone and many other volcanoes across the planet.

In Yellowstone, continuous GPS stations operate year round and send data via radio to the UNAVCO facility for processing and online posting. There is also a network of semi-permanent GPS stations that are deployed in May and collected in October (before the onset of winter) each year. These sites do not include radios, so data is only available once the sites are recovered and downloaded each fall.

The continuous and semi-permanent GPS data is processed and made available by several independent institutions. If you want to see this data yourself, you have several options to choose from.

UNAVCO maintains a map interface. Just zoom in to the Yellowstone region, click on any of the station icons, then click on the station name in the popup box to go to the station page.

USGS maintains separate pages for continuous GPS and semi-permanent GPS stations. Click on a map symbol and then on the plot in the popup window to go to the station page.

The Geodesy Laboratory at the University of Nevada, Reno, processes all freely available GPS data in the world. Go to their map interface, click on one of the icons that indicate a GPS station, and then click on the plot in the popup box to go to the station page.

OK, so now you can get to the data. But what do the plots mean? We’ll use plots from UNAVCO as examples.

First of all, GPS data is plotted relative to some reference. Sometimes the reference frame is global, and will show not only how the Yellowstone area is moving, but also how the entire North American continent moves. We can also look at the data with the motion of the North American plate removed, which is usually best, since this will highlight changes specific to Yellowstone. You have to be careful when examining data from some of the sources above to make sure you are looking at data plotted in the North America reference frame. Fortunately, UNAVCO data plots are automatically done that way (some plots from USGS and the University of Nevada, Reno, also include reference lines that are approximations of long-term trends in the data, but these can be ignored).

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GPS graphs

Let’s look specifically at station NRWY, at the Norris Geyser Basin. GPS data is shown as points that are daily average positions in three separate time series — north, east, and up. The horizontal plot axis is time, and the vertical is motion (usually measured in millimeters or centimeters). A station that has an upward trend in the “north” plot is moving north, and a downward trend would be movement to the south. For the “east” plot, an upward trend over time is motion to the east, and down is to the west. And in the “height” (or “up” or” vertical”) plot, an upward trend is uplift and a downward trend is subsidence.

Since 2015, the NRWY site has been moving down in the north plot (in other words, south) by about 20 millimeters, up in the east plot (in other words, east) by about 20 millimeters, and up in the height plot (in other words, uplifting) by about 100 millimeters. Put it all together and you can see that NRWY has been uplifting and moving to the southeast since 2015. In contrast, between 2004 and 2006, the station was subsiding and moving to the northwest.

You can contrast the behavior of NRWY with that from a station in the caldera, like OFW2 near Old Faithful. Since 2015, that site has been subsiding and moving to the southwest.

A quick glance at a GPS plot can tell you quite a lot about how dynamic, or changing, a station is. The height change plot, usually at the bottom of any series of graphs, is best if you just want a quick assessment of whether a station is uplifting, subsiding, or not moving up or down over time.

Try it out for yourself. We invite you to explore the wealth of GPS data from Yellowstone. Have fun tracking deformation changes at one of the most dynamic places on Earth.

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Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week’s contribution is from Michael Poland, geophysicist with the U.S. Geological Survey and Scientist-in-Charge of the Yellowstone Volcano Observatory.

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