SEICHES IN LAKE YELLOWSTONE: A GEOPHYSICAL TOOL

In July of 2009, strong, persistent, high Q factor, multi-day signals with approximate periods of 11.5, 13.4 and 76 minutes were observed at a PBO borehole strainmeter installed within 200 m of the West Thumb basin of Lake Yellowstone. Coincident fluctuations in the Yellowstone River Lake Outlet stream gauge gave rise to the hypothesis that this was a barometrically-induced seiche. Recent installation of tide gauges have confirmed this hypothesis and allowed for direct calibration of existing geodetic instrumentation. In addition FEM models of the Lake Yellowstone basin find excellent agreement of these periods with the fundamental modes of the system.

The spatially and temporally dense geodetic network, combined with a known time varying load, allow for using Yellowstone lake as a tool to probe the structural and crustal properties of the caldera. Observed far field strain amplitudes are consistent with the seiche load on a two-layered viscoelastic model representing an elastic upper crust overlying a partially molten body deeper than 3–6 km with Maxwell viscosity less than 10^-11Pa s. These strain observations and models provide independent geodetic evidence for the presence of partially molten material in the upper crust, consistent with seismic tomography studies that inferred 10%–30% melt fraction in the upper crust.

Other properties of this seiche also provide insight into the structure and evolution of the caldera. The 76 minute strain signal is consistent with the principal eigenmode of the Lake and surprisingly consistent, even when the Lake is ice covered eliminating the only observed physical excitation. Higher order modes switch on and off with ice but remain consistent with the conservation of volume. It is hypothesized that interaction with the caldera related heat flow and hydrothermal could excite this seiche.

This work was supported by the NSF, NPS and UNAVCO. In particular thanks to Henry Heasler at the NPS for logistical support and physical insights.