Imaging the Hydrothermal Plumbing Architecture of Steamboat Geyser Using a Dense Nodal Array and Seismic Interferometry

The plumbing architecture of a hydrothermal feature (e.g., geyser and spring) exerts direct control over its eruption and recharge dynamics. During an active geyser’s recharge, in response to the evolution of temperature and hydrostatic pressure within the plumbing, this two-phase-flow system experiences intensive steam bubble nucleation and collapse throughout the eruption cycle. Such steam-liquid phase transitions generate seismic signals observed as hydrothermal tremor, thus the spatiotemporal pattern of its origin can depict the plumbing architecture and illuminate how the geyser operates internally. Steamboat, the tallest active geyser on Earth, is thought to have a complex architecture and dynamics owing to the hydrologic interaction with the nearby Cistern Spring, ~100 m SW of Steamboat. To study the system, in 2019 we deployed a dense array across the Steamboat-Cistern area with an aperture of ~250 m. The array was composed of 50 three-component geophones and had a spacing of 15–35 m. During the deployment, 6 eruption cycles with intervals ranging from 3 to 8 days were recorded. We observe distinct 1–5 Hz tremor emitted from Steamboat and Cistern, which are persistent and show no isolated events and discernable arrivals. To simultaneously locate the tremor from both features, we perform multicomponent cross-correlation to isolate and enhance the coherent signals of interest with each station as the virtual source. We apply the same normalization to the 3-component data so that the particle motion excited by each virtual source is retained. We observe prevalent seismic P waves at receivers near the source, with complex wavefield transition and interference at distant receivers. Using the P wave linearity, we back project the polarized directions to constrain the 3D source location. The results provide the first 4D view of the tremor throughout the eruption cycles with hourly resolution.

 

The 4D view reveals the conduit beneath Steamboat is vertical and extends down to ~120 m depth and the plumbing of Cistern includes a shallow vertical conduit connecting with a deep, large, and laterally offset reservoir ~60 m southeast of the surface pool. No direct connection between Steamboat and Cistern plumbing structures is found above ~120 m. The temporal variation of the tremor combined with in situ temperature and water depth measurements of Cistern, do reveal the interaction between Steamboat and Cistern throughout the eruption/recharge cycles. The observed delayed responses of Cistern in reaction to Steamboat eruptions and recharge suggest the two plumbing structures might be connected through a fractured/porous medium instead of a direct open channel, consistent with our inferred plumbing structure.