Constraints on the Upper Crustal Magma Reservoir Beneath Yellowstone Lake Inferred From Lake-Seiche Induced Strain Observations

Periodic seiches with an ~80-minute period and wave heights up to 10 cm are observed in Yellowstone Lake. Though small, these water level variations act as a load on the solid earth that has been observed on highly sensitive borehole strainmeters throughout Yellowstone National Park. In particular, we observe no time delay (five minutes or less) between the appearance of a seiche signal at a close strainmeter (100 m from the lake edge) and a distant strainmeter (20 km away). The amplitude, timing, and spatial distribution of the observed strain response relative to the observed water load can be used to infer the mechanical characteristics of the subsurface. We calculate the strain field induced by the load of Yellowstone Lake seiches using an elastic half-space model that represents the upper few kilometers of solid crust. We determine that the predicted far-field strain response is much smaller (by more than an order of magnitude) than the measured strain at locations that are distant from the lake. This indicates that far-field strain induced by lake seiches may be sensitive to the location and composition of the crustal magma reservoir. To constrain the depth and viscosity of this reservoir, we calculate the strain field induced in a layered viscoelastic half-space using a semi-analytic model. We find that a magma reservoir must be at least 4 km deep with a Maxwell viscosity of less than 10¹¹ Pa s. The best-fitting depth to the top of an idealized magma chamber is 5-6 km. These new strain observations provide independent evidence for the likely presence of a partially molten body at shallow depth. Continuing observations can provide a valuable contribution to the monitoring and improved understanding of the Yellowstone magmatic system.