Joint Inversions of Ground Deformation, Extrusion Flux, and Gas Emissions Using Physics‐Based Models for the Mount St. Helens 2004–2008 Eruption

Abstract

AbstractWith the increasing quantity and quality of data collected at volcanoes, there is growing potential to incorporate all the data into analyses of magmatic systems. Physics‐based models provide a natural and meaningful way to bring together real‐time monitoring data and laboratory analyses of eruption products, with the goal of better understanding volcanic processes. We develop a framework for joint inversions of diverse time series data using the physics‐based model for dome‐forming eruptions from Wong and Segall (2019, https://doi.org/10.1029/2019JB017585). Applying this method to the 2004–2008 eruption at Mount St. Helens, we estimate essential system parameters, including chamber geometry, pressure, volatile content, and material properties, from extruded volume, ground deformation, and carbon dioxide emissions time series. The model parameter space is first sampled using the neighborhood search algorithm (Sambridge, 1999b, https://doi.org/10.1046/j.1365-246X.1999.00876.x). The resulting ensemble of models is resampled to generate posterior probability density functions on the parameters (Sambridge, 1999a, https://doi.org/10.1046/j.1365-246x.1999.00900.x). We find models that fit all three data sets well. Posterior probability density functions suggest an elongate chamber with aspect ratio less than 0.55, located at 9–17 km depth. Since the model calculates pressure change during the eruption, we can constrain chamber volume to 64–256 km3. Volume loss in the chamber during the eruption is 20–66 million m3. At the top of the chamber, total (dissolved and exsolved) water contents are 5.0–6.4 wt% and total carbon dioxide contents are 1,600–3,900 ppm, giving a porosity of 5%–16% depending on the conduit length. Compared to previous inversions using a steady‐state conduit model, we obtain a lower magma permeability scale, radius, and friction coefficient.