Research paperReconstruction and analysis of sub-plinian tephra dispersal during the 1530 A.D. Soufrière (Guadeloupe) eruption: Implications for scenario definition and hazards assessment
Introduction
La Soufrière de Guadeloupe is a dangerous subduction zone andesitic composite volcano part of the Grande Découverte–Soufrière volcanic complex and characterized over the last decade by moderate seismic and fumarolic unrest (Komorowski et al., 2005a). In the last 15 000 years it has experienced phreatic and magmatic eruptions and unusually numerous flank-collapse events sometimes associated with magmatic eruptions (Komorowski et al., 2002, Komorowski et al., 2005a, Boudon et al., 2007). Forecasting its future behaviour and assessing associated hazards must rely on a detailed reconstruction of the volcano's eruptive past. Explosive open-vent eruptions that result in formation of a high convective eruption column sustained over a few hours result in a large mass flux of solid particles and gases injected in the atmosphere that can be dispersed by winds over large areas. Such eruptions thus constitute one of the scenarios for La Soufrière which could have a significant adverse often severe impact upon human settlements, the environment and civil aviation.
Plinian eruptions are characterized by sustained magma-discharge rates and the development of steady high convective columns for several hours before column collapse occurs and generates pyroclastic pumice-rich density currents (PDCs). Sub-plinian eruptions are characterized by more unsteady magma-discharge rates and the development of oscillating convective columns of lesser height that collapse repeatedly to generate PDCs. Fluctuations in the dynamics of sub-plinian eruption columns are associated with important changes in eruptive styles that produce contrasted hazards. Sub-plinian deposits are often less extensive and well-preserved than those of plinian eruptions. Thus, the recurrence rate of explosive eruptions of smaller intensity (VEI 2–3) is likely under-estimated as well as the risk they represent for populations that typically live on small volcanic islands close to the vent area within 5–10 km distance. Quantitative volcanic risk assessment and scenario definition require the collection of all data available on the eruption types most likely to occur at a specific volcano. This is particularly important in order to undertake even simple simulations of potential hazards which then serve as the foundation for quantitative risk and vulnerability assessment.
The 1530 A.D. Soufrière eruption is one of the most important event in the history of the volcano for which this analysis can be performed. The eruption which shares some similarities with the ongoing Soufrière Hills eruption on Montserrat (Sparks and Young, 2002), was documented by Boudon et al. (2008-this issue). The eruption began with phreatic explosions followed by partial collapse of the edifice that emplaced a debris-avalanche. The eruption then evolved into a sub-plinian phase (Phase-1) which produced coarse pumice and scoria tephra fallout rich in lithic clasts as well as pumice and scoria-rich pyroclastic density currents (PDCs) from column collapse. A short-lived period of violent strombolian activity (Phase-2) occurred before the final phase of the eruption which produced an andesite lava dome (the highest point of the Lesser Antilles at 1467 m) with a volume of ca. 0.05 km3 within the depression left by partial edifice collapse (Komorowski et al., 2002, Komorowski et al., 2005a, Boudon et al., 2008-this issue).
The ability to simulate and forecast the dispersal of tephra dispersal in areas exposed to volcanic activity is of fundamental importance in order: (1) to refine reconstructions of past eruptions and thus provide constraints for predicting expected tephra accumulation from future eruption; (2) to improve risk assessments and crisis management with respect to the impact on the populations, the environment, and aviation safety; and (3) to provide guidance for the implementation of mitigation measures and long-term land-use management. Over the past decades, significant progress has been achieved in numerical models of tephra dispersal that belong to two general classes: (1) diffusion–advection–sedimentation models (e.g. Macedonio et al., 1988, Hurst and Turner, 1999, Connor et al., 2001, Macedonio et al., 2005, Pfeiffer et al., 2005); or (2) particle-tracking models (e.g. Heffter and Stunder, 1993, D'Amours, 1998, Searcy et al., 1998, Barsotti et al., 2006). Sophisticated numerical developments of these earlier diffusion–advection models have been published (Bonadonna et al., 2005, Folch and Felpeto, 2005, Costa et al., 2006, Connor and Connor, 2006).
Section snippets
Geologic setting
Guadeloupe, one of France's overseas departments, is situated in the central region of the Lesser Antilles volcanic arc formed by subduction of the Atlantic plate beneath the Caribbean plate. The activity of the Grande Découverte–Soufrière (GDS) calc-alkaline composite volcanic complex that began about 0.2 Ma. ago (Boudon et al., 1988, Carlut et al., 2000, Samper et al., 2007) can be divided in three stages related to construction of the Grande Découverte, Carmichaël, and Soufrière composite
Physical eruption parameters
Modeling of hazards associated with the most likely future magmatic eruptions of La Soufrière requires the determination of the realistic ranges for eruptive parameters that serve as input values for probabilistic tephra fallout and pyroclastic flow numerical modeling. In their study, Komorowski et al. (submitted for publication), have determined that the explosive Phase 1 of the 1530 A.D. eruption can be characterized by: (1) a total of 8.94 × 10−3 km3 of magma Dense Rock Equivalent (DRE)
Parametrization of tephra dispersal simulation
HAZMAP is a simple semi-analytical computer code that was designed to model in 2-D the dispersal of tephra from discrete point sources produced by explosive convective eruption columns. It is based on a simplified 2-D solution of the 3-D model developed by Armienti et al. (1988) and Macedonio et al. (1988) for the solution of equations for the diffusion, advection, and sedimentation of solid particles through a regional wind field based on an initial analytical solution by Suzuki (1983). The
Sensitivity analysis
We have performed 88 different HAZMAP runs for sub-plinian scenarios and 7 runs for plinian scenarios in the deterministic “deposit” mode assuming constant values for the Suzuki parameters (i.e. A = 4, λ = 1) and for the lithological distribution but taking different combinations of values for the other 5 main parameters such as total (1) column height HT (m), (2) erupted mass (kg), (3) diffusion coefficient K (m2 s− 1), (4) seasonal wind profile (dry, rainy), and (5) grain-size distribution.
Results
We have reconstructed the dispersal of tephra from the last sub-plinian eruption of La Soufrière using a combination of field data, theoretical interpretation of deposit characteristics and simulations using the diffusion–advection–sedimentation HAZMAP code described previously. The program was run both in a deterministic mode («deposit mode») to obtain specific isomass maps for a particular eruptive scenario or in a probabilistic mode («probability mode») using a statistically representative
Scenario definition and volcanic event tree
In recent years, the concept of volcanic event trees has become a fundamental tool for the quantitative assessment of the probability of occurrence of specific hazards and their associated risks at a volcano showing signs of unrest (Newhall and Hoblitt, 2002, Aspinall et al., 2002, Marzocchi et al., 2004). An event tree is a graphical representation of the succession of alternative events that can logically occur following some prior initiating event. Event trees are used to describe all
Conclusions
The sub-plinian phase of the 1530 A.D. eruption produced a column that reached between 9.7 and 11 km and erupted about 1.72 × 1010 kg in less than 2 h. This eruption is the only sub-plinian eruption of Soufrière of Guadeloupe for which we have relict outcrops. It is also the most recent one. We have reconstructed the potential dispersal of airfall tephra and associated mass loadings generated by this eruption to a first approximation using the simple first-order model HAZMAP for tephra dispersal
Acknowledgements
We thank A. Ansault-Masloup, E. Coudret, and V. Alaimo for assistance with laboratory and field analyses and discussions. We particularly thank A. Costa and G. Macedonio for advice on the HAZMAP code as well as N. Houlie, A. Nercessian and R. Gunasekera. T. Hincks and W. Aspinall provided advice related to probabilistic risk assessment. We acknowledge discussions with R. Spence, K. Saito, and A. Pomonis on vulnerability issues. We thank S. Brun and the ESRI ArcGis Software support team in
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Now at: Dipartamento di Scienze della Terra, Universita di Pisa, Italy.