SUBSURFACE SEISMIC IMAGING OF THE YELLOWSTONE UPPER GEYSER BASIN HYDROTHERMAL SYSTEM

The Upper Geyser Basin (UGB) in Yellowstone National Park contains one of the highest concentrations of hydrothermal features on Earth including the iconic Old Faithful Geyser (OFG). Although this system has been the focus of many geological, geochemical, and geophysical studies, the shallow (<200 m) subsurface structure remains poorly characterized. To investigate the detailed subsurface structure related to the hydrothermal plumbing of the UGB, we deployed dense arrays of three-component 5-Hz nodal geophones in both November of 2015 and 2016, composed of 133 stations with ~50 m spacing, and 519 station locations, with an average spacing of 20 m, respectively.

Using Rayleigh-wave signals between 1-10 Hz via seismic signals excited by nearby hydrothermal features (e.g. geysers and pools), we observe a distinct velocity boundary at 3.3 Hz frequency that delineates a higher phase velocity of ~1.6 km/s in the NE and a lower phase velocity of ~1.0 km/s to the SW of the OFG. This velocity boundary follows a geologic contact between mapped rhyolitic and glacial deposits. We therefore suggest this reservoir is likely controlled by the local geology with the rhyolitic deposit in the NE acting as a relatively impermeable barrier to vertical fluid ascent. We also image a relatively shallow (20-60 m deep) large reservoir/recharge volume with an estimated porosity of 30% located ~100 m SW of the OFG from spatially dependent waveform distortions and delays between 5-10 Hz frequency. Furthermore, we have also explored the viability of using autonomous three-component nodal geophones to calculate teleseismic Ps receiver functions. While a persistent converted phase likely associated with the upper boundary of the Yellowstone magma reservoir is observed across the UGB seismic array, a small-scale receiver function anomaly in the southwest is also observed and is likely related to the shallow OFG hydrothermal plumbing system.