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Kitou, Gaƫtan-Thierry
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- PublicationOpen AccessThe 2018 unrest phase at La SoufriĆØre of Guadeloupe (French West Indies) andesitic volcano: Scrutiny of a failed but prodromal phreatic eruption(2020-01-11)
; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; After 25 years of gradual increase, volcanic unrest at La SoufriĆØre of Guadeloupe reached its highest seismic en- ergy level on 27 April 2018, with the largest felt volcano-tectonic (VT) earthquake (ML 4.1 or MW 3.7) recorded since the 1976ā1977 phreatic eruptive crisis. This event marked the onset ofa seismic swarm(180 events, 2 felt) occurring after three previous swarms on 3ā6 January (70 events), 1 st February (30 events, 1 felt) and 16ā17 April (140 events, 1 felt). Many events were hybrid VTs with long-period codas, located 2ā4 km below the vol- cano summit and clustered within 2 km along a regional NW-SE fault cross-cutting La SoufriĆØre. Elastic energy release increased with eachswarmwhereas inter-event time shortened. At the same time, summit fractures con- tinued to open and thermal anomalies to extend. Summit fumarolic activity increased significantly until 20 April, with a maximum temperature of111.4 Ā°C and gas exit velocity of80 m/s, before declining to ~95 Ā°C and ~33 m/s on 25 April. Gas compositions revealed increasing C/S and CO2/CH4 ratios and indicate hydrothermal P-T condi- tions that reached the critical point ofpure water. Repeated MultiGAS analysis of fumarolic plumes showed in- creased CO2/H2S ratios and SO2 contents associated with the reactivation of degassing fractures (T = 93 Ā°C, H2S/SO2 ā 1). While no direct evidence ofupward magma migration was detected, we attribute the above phe- nomena to an increased supply ofdeepmagmatic fluids that heated and pressurized the La SoufriĆØre hydrother- mal system, triggering seismogenic hydro-fracturing, and probable changes in deep hydraulic properties (permeability) and drainage pathways, which ultimately allowed the fumarolic fluxes to lower. Although this magmatic fluid injectionwasmodulated by the hydrothermal system, the unprecedented seismic energy release and the critical point conditions ofhydrothermal fluids suggest that the 2018 sequence ofevents can be regarded as a failed phreatic eruption. Should a similar sequence repeat, we warn that phreatic explosive activity could re- sult fromdisruption ofthe shallowhydrothermal system that is currently responsible for 3ā9mm/y ofnearly ra- dial horizontal displacements within 1 km from the dome. Another potential hazard is partial collapse of the dome's SW flank, already affected by basal spreading above a detachment surface inherited from past collapses. Finally, the increased magmatic fluid supply evidenced by geochemical indicators in 2018 is compatible with magma replenishment of the 6ā7 kmdeep crustal reservoir feeding La SoufriĆØre and, therefore, with a potential evolution of the volcano's activity towards magmatic conditions.163 58 - PublicationOpen AccessSpatio-Temporal Relationships between Fumarolic Activity, Hydrothermal Fluid Circulation and Geophysical Signals at an Arc Volcano in Degassing Unrest: La SoufriĆØre of Guadeloupe (French West Indies)(2019-11-15)
; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ;; ; ; ; ; ; ; ; ; ; ; ; ; ;Over the past two decades, La SoufriĆØre volcano in Guadeloupe has displayed a growing degassing unrest whose actual source mechanism still remains unclear. Based on new measurements of the chemistry and mass flux of fumarolic gas emissions from the volcano, here we reveal spatio-temporal variations in the degassing features that closely relate to the 3D underground circulation of fumarolic fluids, as imaged by electrical resistivity tomography, and to geodetic-seismic signals recorded over the past two decades. Discrete monthly surveys of gas plumes from the various vents on La SoufriĆØre lava dome, performed with portable MultiGAS analyzers, reveal important differences in the chemical proportions and fluxes of H2O, CO2, H2S, SO2 and H2, which depend on the vent location with respect to the underground circulation of fluids. In particular, the main central vents, though directly connected to the volcano conduit and preferentially surveyed in past decades, display much higher CO2/SO2 and H2S/SO2 ratios than peripheral gas emissions, reflecting greater SO2 scrubbing in the boiling hydrothermal water at 80ā100 m depth. Gas fluxes demonstrate an increased bulk degassing of the volcano over the past 10 years, but also a recent spatial shift in fumarolic degassing intensity from the center of the lava dome towards its SEāNE sector and the Breislack fracture. Such a spatial shift is in agreement with both extensometric and seismic evidence of fault widening in this sector due to slow gravitational sliding of the southern dome sector. Our study thus provides an improved framework to monitor and interpret the evolution of gas emissions from La SoufriĆØre in the future and to better forecast hazards from this dangerous andesitic volcano.174 21