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Modelling the transport and deposition of ash following a magnitude 7 eruption: the distal Mazama tephra
Bulletin of Volcanology, Volume: 84, Issue: 9
Swansea University Author: Hannah Buckland
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Volcanic ash transport and dispersion models (VATDMs) are necessary for forecasting tephra dispersal during volcanic eruptions and are a useful tool for estimating the eruption source parameters (ESPs) of prehistoric eruptions. Here we use Ash3D, an Eulerian VATDM, to simulate the tephra deposition...
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Volcanic ash transport and dispersion models (VATDMs) are necessary for forecasting tephra dispersal during volcanic eruptions and are a useful tool for estimating the eruption source parameters (ESPs) of prehistoric eruptions. Here we use Ash3D, an Eulerian VATDM, to simulate the tephra deposition from the ~ 7.7 ka climactic eruption of Mount Mazama. We investigate how best to apply a VATDM using the ESPs characteristic of a large magnitude eruption (M ≥ 7). We simplify the approach to focus on the distal deposit as if it were formed by a single phase of Plinian activity. Our results demonstrate that it is possible to use modern wind profiles to simulate the tephra dispersal from a prehistoric eruption; however, this introduces an inherent uncertainty to the subsequent simulations where we explore different ESPs. We show, using the well-documented distal Mazama tephra, that lateral umbrella cloud spreading, rather than advection–diffusion alone, must be included in the VATDM to reproduce the width of the isopachs. In addition, the Ash3D particle size distribution must be modified to simulate the transport and deposition of distal fine-grained (< 125 µm) Mazama ash. With these modifications, the Ash3D simulations reproduce the thickness and grain size of the Mazama tephra deposit. Based on our simulations, however, we conclude that the exact relationship between mass eruption rate and the scale of umbrella cloud spreading remains unresolved. Furthermore, for ground-based grain size distributions to be input directly into Ash3D, further research is required into the atmospheric and particle processes that control the settling behaviour of fine volcanic ash.
Data availability:The Mazama tephra thickness and grain size, and the input parameters used in each model run, are found in the electronic supplementary material. The wind data use are available online https://doi.org/10.5066/F7SQ8XKT. The input, log and output files from the simulations are also available online https://doi.org/10.5066/P9PVNY06.
Volcanic ash cloud; Atmospheric dispersion; Eruption source parameters; Umbrella cloud spreading;Tephra
Faculty of Science and Engineering
H.M.B was supported by a NERC GW4+ Doctoral Training Partnership studentship from the Natural Environment Research Council (NERC; NE/L002434/1) with additional support from the British Geological Survey. S.L.E. publishes with permission of the CEO, British Geological Survey, and acknowledges support from British Geological Survey NC-ODA grant NE/R000069/1: Geoscience for Sustainable Futures. K.V.C. acknowledges an AXA Research Fund and a Royal Society Wolfson Merit Award.