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Part-time GPS – What's Up With That?

October 14, 2019

The backbone of Yellowstone's ground-based monitoring network consists of seismic stations, which detect earthquakes, and GPS (Global Positioning System) receivers, which track ground motion.

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Dan Dzurisin, geologist with the U.S. Geological Survey.

Map showing locations of GPS stations in the Yellowstone region as of 2019
Map of GPS stations in the Yellowstone region as of 2019. Continuous GPS stations are indicated by green circles, and temporary stations which are deployed in May and recovered in October each year, are red squares.

Yellowstone is one of the best-monitored volcanoes in the world. The backbone of the ground-based monitoring network consists of seismic stations, which detect earthquakes, and GPS (Global Positioning System) receivers, which track ground motion. The GPS network is especially noteworthy because it is made up of not just permanent installations, but also temporary deployments of equipment.

An earlier Caldera Chronicles article described the evolution of GPS from its original mission of tracking U.S. military assets with meter-scale accuracy to a multinational tool for monitoring ground deformation at millimeter scale. Today, thousands of stations around the globe collect information 24/7; more than a dozen such stations are located within Yellowstone National Park. Why, then, does YVO deploy a similar number of temporary GPS stations each year in springtime and pick them up them in the fall? The answer is more complicated than "more is better" (although, in this case, that's part of it).

Temporary GPS stations are smaller, less expensive, easier to deploy, and therefore less intrusive on the landscape than continuous stations. They produce nearly the same data accuracy while they are operating, and when the station is removed the only thing that remains is a small, threaded, stainless steel pin epoxied into rock (used as an antenna mount when the station is deployed). That makes them ideal for use in Yellowstone, where management policies emphasize reducing the visual and environmental impacts of research and monitoring activities. Recognizing those sensitivities and the need for a denser GPS network to monitor a complex deformation pattern, YVO has installed 16 temporary GPS stations at strategic locations in and near the Park.

Time series of vertical displacements during April–October 2017
Time series of vertical displacements during April–October 2017 at four GPS stations near Yellowstone Lake: continuous station LKWY and temporary stations SEDG, LAK2, and LAK1. Downward trends indicate subsidence and upward trends show uplift. Error bars are one standard deviation. Bottom plot shows water level measured by stream gage USGS 06186500, located at the outlet of Yellowstone Lake. High gage levels correlate to high lake levels. Note that subsidence of the lake shore occurs simultaneously with increased lake level.

The effort paid off in during an episode of rapid uplift centered near Norris Geyser Basin that began in late 2013 and continued at rates as high as 20 cm/yr until a magnitude 4.8 earthquake shook the area on March 30, 2014, triggering an abrupt reversal to rapid subsidence. The episode was well recorded by InSAR (satellite radar) and several GPS stations, including temporary stations GRZL and BRYL. By modeling both the InSAR and GPS datasets together, YVO scientists showed the deformation source was located about 3.2 km (2 miles) below the surface and probably resulted from accumulation and sudden escape of volatiles (gases) from an earlier magmatic intrusion. Without the temporary GPS stations, only one of the continuous stations would have been able to clearly "see" the deformation.

Another demonstration of the value of temporary GPS stations occurred in 2017, which was the first full year after stations LAK1 and LAK2 were added to the temporary network to study the loading effect of water level changes in Yellowstone Lake. Records from those stations clearly show that the lakeshore subsides during periods of high water level in early summer and rebounds when lake level falls later in the year. LAK1 moved down about 25 millimeters (1 inch) from late April through late June, while LAK2, farther from the shoreline, moved down about 20 millimeters. That information is useful for estimating the strength and temperature of the upper crust beneath the lake.

So it's true that more is better in this case, at least during summertime.

In the coming years, YVO will explore the deployment of additional temporary GPS stations in places where coverage by the continuous network is thin. The existing network will also be continued, with deployments every May and the stations recovered in October. Stay tuned to future editions of Caldera Chronicles for details about GPS results!