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Old Faithful Geyser in Yellowstone is one of the most well-known hydrothermal features in the world. Despite abundant geophysical studies, the structure of Old Faithful's plumbing system beneath similar to 20-m depth remained largely elusive. By deploying a temporary dense three-component geophone array, we observe 1-5 Hz low-frequency hydrothermal tremor originating from Old Faithful's deeper conduit. By applying seismic interferometry and polarization analyses, we track seismic tremor source migration throughout the eruption/recharge cycle. The tremor source drops rapidly to similar to 80-m depth right after the eruption and gradually ascends vertically back to similar to 20-m depth, coinciding with the previously inferred bubble trap location. Likely excited by the liquid/steam phase transition, the observed tremor source migration can provide new constraints on the recharge process and deeper conduit geometry. Combined with the shallow conduit structure from previous studies, these results provide constraints on the major fluid pathway down to 80-m depth.

Plain Language Summary The fluid pathways beneath a geyser exert direct control over its eruption behavior. The conduit geometry not only serves as a pathway for fluid and mass transportation but also creates a distinct pressure gradient from the deep reservoir to the surface vent. Understanding the complete conduit geometry and the physical state of the hydrothermal fluid within it directly constrains the recharge and eruption dynamics. The deep plumbing system, however, is extremely challenging to probe based on in situ geophysical methods. Despite its fame and well-known nearly regular eruption intervals, the plumbing structure of Old Faithful Geyser in Yellowstone below 20-m depth remains largely elusive. In this study, we use data from a dense array of three-component seismometers to track the time-lapsed locations of low-frequency hydrothermal tremor, which is likely the result of pressure perturbations from the steam/liquid phase transition within the fluid column, throughout Old Faithful's eruption cycles. The results illuminate the fluid pathways of Old Faithful between similar to 20- and 80-m depth and provide critical constraints on the eruption dynamics of Old Faithful.