| Summary: | Seiche waves in Yellowstone Lake at ~78, ~51, and ~25 minute periods and heights <10 cm can cause measurable strain (< 40 ns) as observed on borehole strainmeters both near (<300 m) and far (~20 km) from the lake. Observations of the correlation between the seiche waves and the associated strain response can be used to constrain the rigidity of the upper crust, the depth to magma bodies in the subsurface, and the viscosity (related to melt percentage) of the magma bodies in place. Lake level was directly measured with campaign absolute pressure gauges deployed in the West Thumb basin, Breeze Channel, and central basin of Yellowstone Lake, and 13 large seiche events (¡Ý7 cm) were identified from these observations. Periodic strain transients were measured on borehole strainmeters in the caldera during the 13 large seiche events. The peak-to-trough amplitude and phase of the seiche wave and strain response were estimated by fitting a multi-frequency sinusoid to the time series over a 4-hour window during each seiche event. It was found that the relationship between an applied seiche load and the associated strain response is self-consistent and linearly proportional at each borehole strainmeter in the caldera. Over multiple seiche events the observed strain response is consistent with a modeled strain-field produced by a seiche load on a two-layered viscoelastic model defined by free parameters Young¡¯s modulus (E1), plate thickness (H), and shear modulus ratio (¦Ì2/¦Ì1). The two-layered viscoelastic model represents a solid upper crust overlying a partially molten body which may be small pockets of melt (<1 km thickness) or a larger magma reservoir. Results suggest crystallizing melt beneath Yellowstone caldera at depths (H) of ~4¨C8 km in the south-southeast and ~3-5 km in the north-northwest sections of the caldera. Temporal observations between strain meters, coupled with constraints of Young¡¯s and shear moduli suggest that melt in the shallow crust has a viscosity ¡Ü1013 Pa s.
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