Deroussi, Sebastien

After 25 years of gradual increase, volcanic unrest at La Soufrière 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 1976–1977 phreatic eruptive crisis. This event marked the onset ofa seismic swarm(180 events, 2 felt) occurring after three previous swarms on 3–6 January (70 events), 1 st February (30 events, 1 felt) and 16–17 April (140 events, 1 felt). Many events were hybrid VTs with long-period codas, located 2–4 km below the vol- cano summit and clustered within 2 km along a regional NW-SE fault cross-cutting La Soufrière. 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 °C and gas exit velocity of80 m/s, before declining to ~95 °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 °C, H2S/SO2 ≈ 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 Soufrière 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 3–9mm/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 6–7 kmdeep crustal reservoir feeding La Soufrière and, therefore, with a potential evolution of the volcano's activity towards magmatic conditions.

Over the past two decades, La Soufrière 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 Soufrière 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 80–100 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 SE–NE 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 Soufrière in the future and to better forecast hazards from this dangerous andesitic volcano.

The 8th International Comparison of Absolute Gravimeters (ICAG2009) took place at the headquarters of the International Bureau of Weights and Measures (BIPM) from September to October 2009. It was the first ICAG organized as a key comparison in the framework of the CIPM Mutual Recognition Arrangement of the International Committee for Weights and Measures (CIPM MRA) (CIPM 1999). ICAG2009 was composed of a Key Comparison (KC) as defined by the CIPM MRA, organized by the Consultative Committee for Mass and Related Quantities (CCM) and designated as CCM.G-K1. Participating gravimeters and their operators came from national metrology institutes (NMIs) or their designated institutes (DIs) as defined by the CIPM MRA. A Pilot Study (PS) was run in parallel in order to include gravimeters and their operators from other institutes which, while not signatories of the CIPM MRA, nevertheless play important roles in international gravimetry measurements. The aim of the CIPM MRA is to have international acceptance of the measurement capabilities of the participating institutes in various fields of metrology. The results of CCM.G-K1 thus constitute an accurate and consistent gravity reference traceable to the SI (International System of Units), which can be used as the global basis for geodetic, geophysical and metrological observations of gravity. The measurements performed afterwards by the KC participants can be referred to the international metrological reference, i.e. they are SI-traceable. The ICAG2009 was complemented by a number of associated measurements: the Relative Gravity Campaign (RGC2009), high-precision levelling and an accurate gravity survey in support of the BIPM watt balance project. The major measurements took place at the BIPM between July and October 2009. Altogether 24 institutes with 22 absolute gravimeters32 and nine relative gravimeters participated in the ICAG/RGC campaign. This paper is focused on the absolute gravity campaign. We review the history of the ICAGs and present the organization, data processing and the final results of the ICAG2009. After almost thirty years of hosting eight successive ICAGs, the CIPM decided to transfer the responsibility for piloting the future ICAGs to NMIs, although maintaining a supervisory role through its Consultative Committee for Mass and Related Quantities.