Brief Communication: The effect of submerged vents on probabilistic hazard assessment for tephra fallout

J Selva,A Costa,L Sandri,R Tonini

doi:10.5194/nhess-15-409-2015

Abstract

Abstract. Many volcanic systems are partially or entirely submerged, implying that vents may open underwater. The effect of submerged vents on probabilistic volcanic hazard assessment (PVHA) for tephra fallout has always been neglected, introducing potentially uncontrolled biases. We present a strategy to quantify the effect of submerged vents on PVHA for tephra fallout, based on a simplified empirical model in which the efficiency of tephra production decreases as a function of the water depth above the eruptive vent. The method is then applied to Campi Flegrei caldera, comparing its results to those of two reference end-member models and their statistical mixing.

Highlights

Several very hazardous volcanic systems are located very close to seas, oceans or lakes worldwide and their vents can be partially submerged by water
The high risk associated with volcanic activity at some of these partly submerged volcanoes motivated many efforts to estimate the hazard posed on the surrounding high-density populated areas, for different possible hazardous volcanic outcomes (e.g. Sandri et al, 2012, at the Auckland Volcanic field)
On one hand, we propose a new model consisting of a statistical mixing (Selva et al, 2014) of the two probabilistic volcanic hazard assessment (PVHA) based on two endmember assumptions on the efficiency of submarine vents to produce subaerial tephra: (i) the effect of the sea as null, i.e. as if every possible vent behaves as subaerial, and (ii) the effect of the sea as a cap that totally inhibits the injection of tephra into the atmosphere

Summary

Introduction

Several very hazardous volcanic systems are located very close to seas, oceans or lakes worldwide and their vents can be partially submerged by water. In all of the studies on tephra fallout, the effect of potentially submerged vents on the computed hazard has never been explored This motivated us to propose two new possible strategies to analyse the tephra fallout probabilistic volcanic hazard assessment (PVHA), so as to take into account the effect of the water above a submerged vent on subaerial tephra production. We propose an empirically based simple model in which the efficiency of submerged eruptions in producing subaerial tephra linearly decreases as a function of the water depth above the eruptive vent, up to a maximum depth (Dmax) at which such production is totally suppressed

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Results

Conclusion