Physics-Based Volcano Geodesy with Application to Effusive Eruptions at Mount St Helens

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Non-technical summaryPrior to volcanic eruptions magma accumulates in shallow reservoirs in Earth?s crust. As a result, pressure increases in these magma chambers, which deforms or ?inflates? the Earth?s surface; in contrast, during eruptions, magma leaves these reservoirs, decreasing pressure and causing the Earth?s surface to ?deflate?. Better understanding of these signals could help improve societal responses to volcanic eruptions, such as possible evacuations and changes to airline routes near volcanoes like following the 2010 Icelandic eruption. This project is developing new physics-based models of volcano deformation, which can be coupled with deformation measurements from the EarthScope Plate Boundary to improve forecasts of the duration of an eruption and the volume of material that may be erupted. The project is investigating a data assimilation approach in which available data are used to develop probabilistic estimates for parameters that describe the state of the magmatic system. These are then used to initialize an ensemble of forward models that predict future behavior, including eruption duration and total erupted volume. Given improved forward models, this approach has the advantage of being consistent with both available data and realistic physics-based eruption models. Physics-based volcano eruption forecasts are similar in concept to numerical weather forecasts that assimilate satellite and other data into sophisticated weather models. Technical summaryThis project employs Markov Chain Monte Carlo (MCMC) inversion of continuous GPS positions, magma efflux, and other data using a physics-based forward model of an effusive eruption. Including a physically consistent eruption model allows the estimation procedure to constrain parameters of interest that are not resolved by traditional approaches, including the volume of the crustal magma chamber and the initial water content of the magma. These parameters influence the size and potential explosive potential of eruptions. Ongoing work is increasing the realism of the forward model by including: 1) equilibrium crystallization of the magma as it ascends and pressure decreases; 2) explicit consideration of the rheological transition from distributed viscous flow to solid plug flow with slip on bounding faults, based on a Bingham fluid model and 3) explicit consideration of gas loss (both H2O and CO2) through both lateral and vertical diffusion. Other goals include better models of the eruption onset and cessation. The method is being applied to the 2004-2008 dome forming eruption of Mount St. Helens (MSH), including GPS data from the Plate Boundary Observatory (PBO) and could be applied to other volcanoes, including Augustine in Alaska, Unzen in Japan, and the Soufriere Hills on Montserrat.