If you've been paying attention to monitoring data from Yellowstone or reading the Yellowstone Volcano Observatory (YVO) monthly updates, you might have noticed a recent change in how the ground around the Norris Geyser Basin is moving.
A recent "hiccup" in deformation of the Norris Geyser Basin
This ground motion, called deformation, is one of the primary indicators of activity within Yellowstone's magmatic and hydrothermal systems. In December, deformation data indicate that the Norris Geyser Basin experienced a "hiccup," probably due to changes in hydrothermal fluids in the subsurface.
As last week's column reported, surface deformation can be monitored by many types of instruments, including borehole strainmeters, borehole tiltmeters and Global Positioning System (GPS) stations. GPS is one of the best ways of measuring long-term deformation and is used at volcanoes around the world. At Yellowstone, about 15 GPS stations are operating within the National Park, and many more are located in the surrounding region. These instruments track the ups and downs of the region in great detail, capturing numerous episodes of changing ground motion over the past several years.
Since 2015, deformation of Yellowstone has been largely consistent. GPS stations in the caldera indicate downward motion (subsidence), while stations near the Norris Geyser Basin show upward motion (uplift) of that area. Rates of subsidence and uplift have been small—approximately a few centimeters (about an inch) per year.
In early December, however, the pattern at Norris changed—the GPS station located closest to the geyser basin, named NRWY, suddenly began to record subsidence. Over the next 2-3 weeks, that station subsided by about 2 cm (almost 1 inch). By the end of December, the subsidence had stopped, and uplift resumed.
GPS stations outside the area could "sense" the Norris subsidence. Nearby stations began moving toward Norris, being drawn in by the subtle downwarping of the surface there.
This is not the first time a sudden change in deformation has occurred at Norris. In late 2013, the area began uplifting rapidly, accumulating 5 cm at the NRWY GPS station after just a few months. The uplift abruptly switched to subsidence on about March 30, 2014—the same day of a M4.8 earthquake in the area (the largest earthquake to have occurred in Yellowstone since 1980!).
By the end of 2014, the subsidence had returned Norris to its previous levels. Scientists believe that the sudden episode of uplift was caused by accumulation of hydrothermal fluids beneath the region, and that the earthquake represented the rupturing of a "seal" or other blockage. After the rupture, the fluids were able to drain from the system, and the surface subsided.
It is possible that the December 2017 subsidence represents a similar process. The uplift could be caused by hydrothermal fluids accumulating behind a blockage in the subsurface. This blockage was breached and allowed some fluids to drain, resulting in the subsidence, but then reestablished itself by the end of the month, and uplift resumed. Unlike the 2014 episode, however, there were no significant earthquakes in the Norris area at the time of the change in deformation.
Despite the recent "hiccup" at Norris, overall deformation of the caldera did not change. GPS data show that subsidence there continued at the same rates as have been measured since 2015. And the activity is not a signal of a potential eruption, but rather reflects the dynamic and ever-changing nature of Yellowstone's hydrothermal system.
If you would like to follow the Yellowstone deformation story yourself, all GPS data from the region are publically available. You can find plots by clicking on the stars that indicate GPS stations on the YVO Monitoring page, by zooming in to the region and clicking on GPS stations via the UNAVCO GPS station map interface, or by zooming in on the Yellowstone region and clicking on individual stations on the Nevada Geodetic Laboratory interactive map.