Modeling tephra dispersal in absence of wind: insights from the climactic phase of the 2450 BP Plinian eruption of Pululagua volcano (Ecuador)

Speaker: Alain Volentik
The determination of eruptive parameters is crucial in volcanology, not only to document past eruptions, but also for tephra fallout hazard assessments. In most tephra fallout studies, eruptive parameters have been determined either by empirical techniques or analytical models, but the uncertainty of such parameters is usually not well described. We have applied both empirical and analytical models to characterize the climactic phase of the 2450BP Plinian eruption of Pululagua (BF2 layer) and explore the variations in the total erupted mass, column height and total grain size distribution. Both approaches yield comparable results in the total mass of tephra erupted (4.5 ± 1.5 x 1011 kg), while they show some discrepancies for the determination of the column height (36-20 km from empirical techniques and 30-20 km from analytical techniques). The total grain size distribution of the BF2 layers varies with the different techniques used for the calculation and significantly affects the outputs of analytical models. Inversion on individual grain size classes show that large diffusion coefficients are necessary to model the BF2 layer and might be required to model proximal tephra deposit in order not to overestimate the total erupted mass. Furthermore, we used a statistical method (smoothed bootstrap approach) to quantify the uncertainty in eruptive parameters such as column height and total erupted mass. Finally, the numerical models used in this study cannot infer some other processes in the plume, such as: sedimentation from the plume margins vs. sedimentation from the umbrella cloud, position of the plume corner, presence of secondary maximum in accumulation for fine grain size, or the influence of particle shape on tephra sedimentation. As a matter of fact, we still need to rely on direct detailed observations of grain size to infer such parameters in the plume dynamics.