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Since Yellowstone National Park was founded, the iconic Old Faithful geyser has attracted millions of tourists every year. Although Old Faithful's activity on the surface is well observed and monitored, our current knowledge of the subsurface properties and processes, such as the depth of the plumbing system and how Old Faithful recharges over time, remains largely unknown. But new research is helping to shed light onto this problem by using seismology to image geysers in much the same way as an MRI is used to image the human body.

Seismic signals originating in volcanic systems have often been used to infer the status of magmatic activity. Similarly, active hydrothermal systems also generate observable seismic signals, called hydrothermal tremor, due to steam bubble formation and collapse. A better understanding of the origin of the hydrothermal tremor can lead to better understanding of the subsurface fluid movement. Most previous work, however, has used just a few seismometers and so does not have the ability to precisely determine the spatial and temporal characteristics of the tremor sources.

With the recent availability of low-cost, easily-deployed nodal seismometers, however, it is now possible to deploy dense seismic arrays close to hydrothermal features and record high-quality hydrothermal tremor signals. When data from these dense arrays, made up of tens to hundreds of stations, are analyzed together, we can image the subsurface with unprecedented spatial and temporal resolution—we can get a four-dimensional (4-D) view of a geyser system!

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In November 2016, the University of Utah, in collaboration with Yellowstone National Park and the University of Texas at El Paso, deployed a nodal array with 80 seismic stations on and around Old Faithful (this was done under Yellowstone research permit YELL-2016-SCI-0114). The stations recorded ground vibration in three directions (north-south, east-west, and vertical), which can be used to reconstruct the incoming directions of the observed hydrothermal tremor signals. With the dense array configuration, it is possible to identify the tremor source locations. By locating these tremor signals, the fluid pathway of Old Faithful geyser down to a depth of ~260 feet (80 meters) is illuminated. Moreover, the observation provides new constraints on the eruption dynamics and recharge process of Old Faithful.

Based on the results of this work, Old Faithful's deeper plumbing system is approximately vertical between ~65 and 260 feet (20 and 80 meter) depth and is offset by 65 feet (20 meters) southwest of the geyser vent. The top portion of this deeper conduit is in the same place as a bubble-trap structure that allows fluid and pressure to build up prior to an eruption. So the main source of the water feeding Old Faithful eruptions is not coming from directly beneath the geyser, but actually from off to the side! And with seismology, we can actually "watch" the boiling water rise towards the surface before Old Faithful eruptions!

With the 4-D imaging, we can probe Old Faithful's recharge evolution and further understand the driving physics of geysers. The methodology also provides new opportunities for exploring the deep plumbing geometry of other hydrothermal features including Steamboat Geyser, which has been quite active over the past year. In fact, the same nodal seismic array that was deployed at Old Faithful is currently in place around Steamboat! The instruments will be collected in late July, and hopefully those data will help to define Steamboat's plumbing system, just as it has with Old Faithful! Stay tuned for more details of the seismology of geysers!