Counterfactual Analysis of Runaway Volcanic Explosions

Willy Aspinall,Gordon Woo

doi:10.3389/feart.2019.00222

Willy Aspinall, Gordon Woo

Open Access

https://doi.org/10.3389/feart.2019.00222

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Abstract

If your volcano is erupting, “The past is a nice place to visit, but certainly not a good place to stay…”. One of the major challenges in volcano crisis decision-making arises when unrest escalates and the threat could exist of a massive sudden eruption, despite information suggesting past eruptive activity had been more limited. Even a small chance of this happening may warrant an urgent call for evacuation. For most active volcanoes, there is only a very limited geological and historical record upon which to base an estimate of the chance of a massive eruption. However, this database may be expanded by stochastic modelling of past notable volcanic crises that had the dangerous potential but did not ultimately result in a massive eruption. The conceptual framework for the counterfactual analysis of runaway volcanic explosions is presented here, with reference to other extreme geohazards and georisks. This innovative type of probabilistic analysis has widespread application and is illustrated with the example of the well-documented 1997 Montserrat Vulcanian explosions sequence. An alternative possible mode of eruptive behaviour might have substituted this sequence with fewer but larger explosive eruptions or even by a single runaway extreme event. This latter contingency was considered at the time, and motivated a brief, temporary evacuation of the building then housing the Montserrat Volcano Observatory; in response to the escalating violence of explosions, the base for observatory operations was subsequently re-located much further away from the volcano.

Highlights

Our understanding is continually evolving about the internal form and processes that take place within a volcano producing an eruption, whether explosive or effusive
Evidence has been emerging that is inconsistent with that simple, traditional conceptual model, with the realization that magmatic processes can progress dynamically faster, or slower, than previously thought, and the recognition that explosive volcanism can develop over a range of timescales, sometimes very quickly (Cashman et al, 2017)
RUNAWAY EXPLOSION SCENARIO ANALYSIS FOR MONTSERRAT. We address this question retrospectively in relation to the latent hazard of a pyroclastic density current (PDC) flow reaching the site of Montserrat Volcano Observatory (MVO)[S], due to a violent explosion of the nearby volcano

Summary

Introduction

Our understanding is continually evolving about the internal form and processes that take place within a volcano producing an eruption, whether explosive or effusive. In our drawdown model we calculate the depth to which magma is evacuated in an explosion from a conduit of given (but uncertain) radius, and restrict discussion of our findings by relating them to the assumed depth of the top of the upper reservoir.

Results

Conclusion