Responding to eruptive transitions during the 2020-2021 eruption of La Soufrière volcano, St. Vincent

Abstract

A critical challenge during volcanic emergencies is responding to rapid changes in eruptive behaviour. Actionable advice, essential in times of rising uncertainty, demands the rapid synthesis and communication of multiple datasets with prognoses. The 2020-2021 eruption of La Soufrière volcano exemplifies these challenges: a series of explosions from 9-22 April 2021 was preceded by three months of effusive activity, which commenced with a remarkably low level of detected unrest. Here we show how the development of an evolving conceptual model, and the expression of uncertainties via both elicitation and scenarios associated with this model, were key to anticipating this transition. This not only required input from multiple monitoring datasets but contextualisation via state-of-the-art hazard assessments, and evidence-based knowledge of critical decision-making timescales and community needs. In addition, we share strategies employed as a consequence of constraints on recognising and responding to eruptive transitions in a resource-constrained setting, which may guide similarly challenged volcano observatories worldwide.

Fig. 1. Volcano monitoring network for La Soufriere volcano, St. Vincent.

Hazard zones illustrate the potential for ground-based volcanic impacts such as pyroclastic flows and surges, tephra fall, ash fall and lahars that may impact the defined areas. The boundaries of the zones are based on the past incidence of the hazards and areas of maximum projected extent, in addition, experience of these hazards at similar volcanoes is combined with theoretical considerations of mass discharge rates of magma, wind direction and morphology. The effects of effusive eruptions have had little impact on the determination of hazard zones.

Fig. 2. Timeline of monitoring data for the 2020–2021 eruption at La Soufrière volcano, St. Vincent.

Seismicity data: daily (bars) and cumulative (teal line) seismicity observed and a marker on the first important increase (17 January). The bars in blue represent seismic events related to fluid dynamics (low-frequency and dome emplacement) and the bars in dark red represent VT events. RSAM values: calculated with 1 min windows and no overlap; they correlate with VT swarms and explosive phase. Moreover, it shows evidence of lahar signals after the explosive phase. Deformation: radial extension from the vent observed at station SVGB, 9 km away from the crater, and associated uncertainties computed with GAMIT/GLOBK. It shows a total movement of 62 mm towards the crater at the end of the explosive phase. C/S_tot (CO₂/H₂S) concentration ratios (ppm) in the plume from MultiGAS measurements: First two data points evidence only H₂S content, the remaining are a combination of H₂S and SO₂. The arrow shows the onset of explosive activity. Dome extrusion data: cumulative volume extruded in black line with an extrapolation until 6 April, extrusion rate in teal dots; the arrow marks the onset of rapid dome inflation as observed by a remote camera. The lowest bar shows the corresponding alert level for each day. In addition, vertical dashed lines show the onset of the effusive phase (orange) and the red area corresponds to the explosive phase.

Fig. 3. Seismicity located during 23–24 March and 5–6 April VT swarms.

a Epicentres calculated showing concentric distribution. b Temporal evolution of depths during 23–24 March, most seismicity is shallower than 5 km. c Temporal evolution of depths during 5–6 April, seismicity is shallower than 5 km until 18:00 UTC on 5 April, when locations transitioned to deeper levels.

Fig. 4. Main features of seismicity during explosive phase.

a Vertical time series at station SVV, RSAM and spectrogram showing the main features of the signal during the first 3 days: banded tremor, continuous and discrete explosions with exponential decays. b Vertical time series at station SVV, RSAM and spectrogram showing the main features of the whole explosive phase: increasing inter-explosion time, tremor build up to the last explosion and an abrupt end of the low-frequency tremor two hours after the referenced explosion. It also shows a lahar signal around 8 h after.

Fig. 5. Deformation associated with the 27 December 2020 extrusive eruption.

These visualisations use the Grandin and Delorme (2021) DSM. a Interferograms for descending (2020/02/25–2021/01/26, path 131, frame 3350) and ascending (2020/01/15–2021/01/13, path 36, frame 250) ALOS-2 radar. One fringe represents 11.4 cm in the line of sight; b Interferograms for descending pass (2020/12/07–2020/12/31, track 156, incidence 44.5°) and ascending (2020/12/07–2020/12/31, track 164, incidence 43.9°) Sentinel-1 radar. One fringe represents 2.8 cm in the line of sight; c Synthetic deformation predicted by an Okada dislocation dipping vertically (along-strike length: 600 m, along-dip width: 700 m, upper edge depth: 15 m), opening with a potency of 63,000 m³.

Fig. 6. Plume composition during the extrusive phase from MultiGAS measurements.

Ternary diagram of SO₂*3-CO₂/2-H₂S*5 showing the plume compositions obtained during the extrusive phase. HD hydrothermal dominated, DHM deep hydrothermal magmatic, SHM shallow hydrothermal magmatic, DM deep magmatic, SM shallow magmatic. Boundaries were obtained from the Central American Volcanic arc.

Fig. 7. Composition of the 2020–2021 dome and other explosive products.

TAS classification diagram (‘Total Alkalis vs Silica’) to compare the composition of the 2020–2021 dome and other explosive products with other eruptions of La Soufrière, St. Vincent. Results were obtained by XRF at the University of Plymouth.

Fig. 8. Backscattered electron images of the 2021 lava dome.

Five full thin section image maps, acquired on a Zeiss Gemini 300 SEM at the University of East Anglia, UK, were generated from clasts sampled in January 2021 from the growing dome and analysed to identify common textures as displayed in a–d. a Image showing vesicular nature of the dome sample (vesicles in black) with examples of the crystal population labelled: plagioclase (fsp), clinopyroxene (cpx), Fe-Ti oxides (ox) and gabbroic clots (outlined by yellow dashed line). b Example of heavily degraded olivine (ol) (blue box) shows close-up of this texture. c Examples of localisation of degradation of orthopyroxene crystals to oxides (dashed turquoise line in c). Higher levels of degradation observed close to larger oxides (e.g. image c and ci, x) comprises ~5% of each thin section. Elsewhere, orthopyroxenes are un-degraded (cii, y). d Example of a plagioclase phenocryst (fsp) with a fracture that has been later filled by melt and crystals, this is observed several times across the five sections analysed. Red box shows a close-up of this structure showing clinopyroxene, oxides and glass infilling the fracture.

Fig. 9. Footprints of the 2020–2021 lava dome of La Soufrière volcano, St. Vincent.

A map of the summit crater of La Soufriere, St Vincent, showing full and partial footprints for the new lava dome that first appeared on 27 December 2020. Footprints were extracted from multispectral and radar satellite imagery, oblique aerial and terrestrial photographs and drone surveys. Background is pre-eruption imagery that shows the 1979 lava dome inside the summit crater.

Fig. 10. UWI SRC communication and eruption response strategy.

Summary of UWI SRC communication strategy and response throughout the 2020–2021 eruption of La Soufrière, St. Vincent eruption.