YVO scientists were busy in October 2017. During that month, a number of field measurements were completed in Yellowstone National Park, and some instruments that had been recording data throughout the summer were recovered before the onset of the harsh Rocky Mountain winter. This included 12 semipermanent Global Positioning System (GPS) stations that had been in place since the spring—part of an annual campaign, now 10 years and still running, aimed at improving our understanding of how the ground deforms due to magmatic and hydrothermal activity in the Yellowstone region.
Together with frequent earthquakes, vigorous hydrothermal activity, and abundant emissions of volcanic gases, relatively slow movements of the ground surface attest to ongoing activity in the vast magmatic-tectonic system beneath Yellowstone National Park. Imperceptible to the naked eye, episodes of uplift, subsidence, and stretching or contraction of the surface are monitored by repeated surveys and networks of sensitive instruments operated by YVO and its partners. The process is called deformation because it changes the shape of the land surface in subtle but detectable ways. Since the first leveling survey along Park roads in 1923, the central part of the caldera floor has moved up nearly 1 meter (about 3 feet). But the uplift hasn't been steady. Annual leveling surveys from 1983 to 2007 revealed periods of subsidence lasting up to a decade. More recently, scientists using a satellite radar technique called InSAR have discovered that both the pattern and rate of surface deformation change over time. Such movements are part of normal background activity at one of Earth's largest active magmatic systems.
One of the tools that scientists use to keep track of surface deformation is the same GPS technology that you might use to navigate while driving your car. With specialized equipment and data processing techniques, the location of a GPS monitoring station can be measured very precisely, to within about 1 millimeter (less than the thickness of a dime). YVO and its partners, the University NAVSTAR Consortium (UNAVCO) and University of Utah, have installed a network of more than two dozen such stations in and around Yellowstone National Park to track surface deformation. The stations operate continuously, and data are processed at several locations to obtain daily positions. Small movements of the stations with respect to one another reveal the pattern of surface deformation over time.
The continuous GPS network cannot cover all areas in Yellowstone National Park, so YVO scientists have devised a less invasive GPS technique called semipermanent GPS (SPGPS). As the name implies, SPGPS stations are temporary. A network of SPGPS stations is installed each year from spring through autumn, avoiding months with heavy snowfall that can bury GPS antennas. This arrangement allows SPGPS stations to be smaller than continuous GPS installations—a distinct advantage in ecologically sensitive areas like Yellowstone. The trade-off, of course, is that SPGPS stations only acquire data when they are deployed. Despite the winter data gaps, SPGPS data have been useful for tracking movements of the caldera floor and an active area along the north caldera rim centered near Norris Geyser Basin. For example, on March 30, 2014, a magnitude 4.8 earthquake—the largest Yellowstone quake in 34 years—shook the Norris area. Nearby SPGPS stations that had been deployed only a few days earlier recorded a sudden reversal from uplift to subsidence that coincided with the timing of the quake. Combined with data from the continuous GPS network and InSAR observations, the SPGPS results helped to identify the location and depth of the deformation source. SPGPS may also prove useful for investigating the summer 2017 earthquake swarm north of West Yellowstone, MT. A SPGPS station at Horse Butte is only 15 km west of the swarm's center. Analyzing the data from the station, which was recovered in late October, might prove useful to understanding the cause of the earthquakes and any associated ground motion. Stay tuned—we'll post details about the 2017 SPGPS results to the YVO website over the winter!
Seismic networks locate earthquakes by comparing the arrival times of seismic waves emanating from the earthquake location, or hypocenter. Very small earthquakes (less than a Magnitude 1) cannot be detected on distant seismometers and sometimes even well-located earthquakes may have horizontal and depth uncertainties of more than 0.5 km.
In order to more fully study a 2010 earthquake swarm on the Madison Plateau, in the northwest part of the Yellowstone Caldera, USGS seismologist David Shelly used a specific timeframe of seismic data and mathematical algorithms to detect and relocate tiny earthquakes. Shelly and his colleagues were able to recognize 8710 events, including many small events with magnitude as low as -1. By including these events, Shelly and his colleagues were able to understand more about the relative location of all the earthquakes and how the earthquakes migrated along the crustal fault during the 2010 swarm. The research was published in 2013 in the Journal of Geophysical Research, co-authored with colleagues form the University of Utah. Read more in the web article Taking a closer look at a Yellowstone earthquake swarm.