# Monitoring volcano deformation at La Soufrière, St Vincent during the 2020–21 eruption with insights into its magma plumbing system architecture

Michal Camejo-Harry, Karen Pascal, Pablo Euillades, Raphaël Grandin, Ian Hamling, Leonardo Euillades, Rodrigo Contreras-Arratia, Graham A. Ryan, Joan L. Latchman, Lloyd Lynch, et al.

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## To cite this version:

Michal Camejo-Harry, Karen Pascal, Pablo Euillades, Raphaël Grandin, Ian Hamling, et al.. Monitoring volcano deformation at La Soufrière, St Vincent during the 2020–21 eruption with insights into its magma plumbing system architecture. The Geological Society, London, Special Publications, 2024, 539, pp.41-62. 10.1144/SP539-2022-270. insu-05081600

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# Monitoring volcano deformation at La Soufrière, St Vincent during the 2020–21 eruption with insights into its magma plumbing system architecture

Michal Camejo-Harry<fnref n="1" />,<fnref n="2" />*, Karen Pascal<fnref n="2" />,<fnref n="3" />, Pablo Euillades<fnref n="4" />,
Raphaël Grandin<fnref n="5" />, Ian Hamling<fnref n="6" />, Leonardo Euillades<fnref n="4" />,
Rodrigo Contreras-Arratia<fnref n="2" />, Graham A. Ryan<fnref n="2" />,<fnref n="3" />, Joan L. Latchman<fnref n="2" />,
Lloyd Lynch<fnref n="2" /> and Minjeong Jo<fnref n="7" />

iD MC-H, 0000-0001-5979-7497

*Correspondence: michal.camejo@earth.ox.ac.uk

**Abstract:** Measurements of surface deformation provide valuable insight into sub-volcanic processes operating before, during and after eruptions. Here, we investigate the drivers behind the 2020–21 effusive–explosive episode at La Soufrière volcano in St Vincent using Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (InSAR) data between 2018 and 2021, and geodetic modelling. We observe inflation up to six months before the start to the effusive phase, which continued as the dome extruded. Once the eruption transitioned to the explosive phase, the volcano rapidly deflated, the bulk happening within the first three days of explosions. Our analytical modelling distinguishes three pressure source depth ranges contributing to this eruptive episode: 16–20, c. 6 and <1 km. Deformation data are therefore in line with a vertically extensive magmatic system being tapped pre- and syn-eruption with interaction between deep and shallow reservoirs by ascending magma batches. The combined use of GPS and InSAR proved to be instrumental for constraining the deformation field active during this eruptive episode. The direction of future geodetic monitoring at La Soufrière should therefore utilize both techniques with a view towards maximizing coverage while making up for shortfalls in station upkeep and variations in satellite overpass regularity.

**Supplementary material:** Supporting quantitative information related to the geodetic observations and modelling used in this study are available at https://doi.org/10.6084/m9.fishshare.c.6477358

La Soufrière, St Vincent is the most historically active subaerial volcano in the Lesser Antilles arc. Eruptions have been both explosive (1718, 1812, 1902–03, 1979) and effusive (1784, 1971–72), with the two most recent episodes exhibiting transitions between both styles. After 41 years of quiescence, La Soufrière re-awakened with an effusive (dome-forming) eruption on 27 December 2020. Dome growth continued until it was superseded by an explosive phase lasting from 9 to 22 April 2021. This fortuitous sequence of eruptive styles was instrumental in fast tracking the much-needed expansion of geodetic monitoring in St Vincent.

Geodetic data have been shown to be especially useful for detecting the accumulation of magma and volatiles as a harbinger to an eruption (Sparks et al. 2012) as well as assessing the geometry and volume of the active magma plumbing system (Fernandez et al. 2017; Magee et al. 2018). At La Soufrière, associated monitoring techniques began in the 1970s with the use of dry-tilt successfully recording the gradual inflation and rapid deflation preceding and following the 1979 explosive–effusive eruption (Fiske and Sigurdsson 1982; Fiske and Shepherd 1990). Global Positioning System (GPS) was introduced into routine geodetic monitoring in 2001,

From: Robertson, R. E. A., Joseph, E. P., Barclay, J. and Sparks, R. S. J. (eds) 2024. *The 2020–21 Eruption of La Soufrière Volcano, St Vincent*. Geological Society, London, Special Publications, **539**, 41–62.

First published online May 22, 2023, https://doi.org/10.1144/SP539-2022-270

© 2023 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/). Published by The Geological Society of London.

Publishing disclaimer: www.geolsoc.org.uk/pub_ethics

<footnote n="1">¹Department of Earth Sciences, University of Oxford, Oxford OX1 3AN, UK</footnote>

<footnote n="2">²Seismic Research Centre, The University of the West Indies, St. Augustine, Trinidad and Tobago</footnote>

<footnote n="3">³Montserrat Volcano Observatory, Flemmings, Montserrat, West Indies</footnote>

<footnote n="4">⁴CEDIAC Institute, Universidad Nacional de Cuyo, Argentina</footnote>

<footnote n="5">⁵Université de Paris, Institut de Physique du Globe de Paris, CNRS, F-75005 Paris, France</footnote>

<footnote n="6">⁶GNS Science, New Zealand</footnote>

<footnote n="7">⁷USRA, NASA-GSFC, Greenbelt, MD, USA</footnote>
with short occupation campaigns using a network of benchmarks (eGPS), later supplemented by permanent continuously operating GPS (cGPS) installations in 2007 (Dondin *et al.* 2019). The addition of Interferometric Synthetic Aperture Radar (InSAR) as a complementary monitoring technique was initiated internally (Ryan *et al.* 2021) and progressed through external collaboration in response to the 2020–21 eruptive crisis. This was accompanied by a rapid densification of the cGPS network and reoccupation of eGPS benchmarks (Joseph *et al.* 2022).

At La Soufrière, human resource limitations prevented the real-time identification of precursory inflation from the then 2-station cGPS network. Infrequent eGPS occupations, a restricted pre-2021 cGPS network geometry, combined with a lack of volcanic activity since the start of GPS monitoring, have impeded attempts at characterizing the volcano's plumbing system architecture before now. Here, we describe the expansion of geodetic monitoring following the onset of the 2020–21 eruption of La Soufrière volcano. This includes the

deployment of additional GPS stations and the incorporation of InSAR datasets. We then retrospectively analyse available cGPS and InSAR (ALOS-2, SAOCOM-1 and Sentinel-1) time-series from 2018 for possible precursory deformation, and assess their individual effectiveness for geodetic monitoring in this region. Finally, we attempt to model the volcano's magma source architecture and integrate available seismic data to provide tighter constraints on eruptive processes.

## Geodetic network expansion

Prior to the 2020–21 eruption, La Soufrière’s ground deformation monitoring network consisted of two cGPS stations: SVGB and SVGK (Fig. 1), operated by The University of the West Indies Seismic Research Centre (UWI-SRC), headquartered in Trinidad. Data are transmitted via the internet using File Transfer Protocol (FTP) to UWI-SRC for processing and curation. On-island station maintenance is

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Map of the Lesser Antilles arc. Territories monitored by UWI-SRC are labelled in red. Bathymetry is taken from GEBCO 2021. Inset map shows the GPS network operated on St Vincent during 2021.</figcaption></figure>
carried out by the Soufriere Monitoring Unit (SMU). Until 2010, 'regular' (every few years) eGPS measurements were made by UWI-SRC scientists, with local support from the SMU. Financial constraints (high inter-island travel, operational and instrument costs) have since curtailed this practice and the growth of the cGPS network. In an attempt to address this deficit, UWI-SRC embarked on projects with GNS Science, New Zealand and JAXA (Japanese space agency) to investigate the effectiveness of InSAR for detecting volcanic deformation at the volcanic systems in its area of monitoring responsibility (Fig. 1) (Dondin et al. 2019). This technique had not yet been applied to St Vincent prior to the recent eruption.

## GPS

From inception, cGPS monitoring in St Vincent has been done using dual-frequency receivers (Table 1). Data are collected using a 30 s sampling interval for

a 24 h per UTC day. Instruments also record data at a higher sampling rate (1 s intervals); however, these are not routinely stored. Earliest installations SVGB (Belmont Observatory) and SVGK (Kingstown), located c. 9 and c. 19 km away from La Soufrière respectively (Fig. 1), have their antennas mounted on the roofs of buildings, using stainless-steel pins secured in cement.

The first indicator of precursory unrest related to this eruption was elevated seismicity in November 2020 (Joseph et al. 2022). Due to scientific contractual changes, this timing also coincided with the resumption of daily cGPS data processing alongside the backfilling of processing from earlier months. Furthermore, on 14 December 2020, SVGK functionality was lost and not restored until 16 February 2021 (Fig. 2). January 2021 saw the installation of three more cGPS stations in Georgetown (SVGG, c. 9 km away from the vent), volcano summit (SVGS, c. 1 km from the vent) and Richmond (SVGR, c. 5 km away from the vent), followed by

<table><caption>**Table 1.** Details of GPS sites deployed in St Vincent during the 2020–21 eruption</caption><thead><tr><td>ID</td><td>Name</td><td>Location</td><td>Installation date</td><td>Equipment</td></tr></thead><tbody><tr><td>JCWL</td><td>Jacob's Well</td><td>13.3267° N, 61.1714° W</td><td>12 February 2021</td><td>Septentrio PolarRx5 receiver/ Trimble Zephyr 3 Geodetic antenna</td></tr><tr><td>TBRK</td><td>Table Rock</td><td>13.3278° N, 61.1783° W</td><td>20 February 2021</td><td>Septentrio PolarRx5 receiver/ Trimble Zephyr 3 Geodetic antenna</td></tr><tr><td>MESO</td><td>Mesopotamia</td><td>13.1781° N, 61.1672° W</td><td>21 July 2021</td><td>Septentrio PolarRx5 receiver/ Trimble Zephyr 3 Geodetic antenna</td></tr><tr><td>SDBY</td><td>Sandy Bay</td><td>13.3372° N, 61.1237° W</td><td>16 July 2021</td><td>Septentrio PolarRx5 receiver/ Trimble Zephyr 3 Geodetic antenna</td></tr><tr><td>SVGB</td><td>Belmont Observatory</td><td>13.2746° N, 61.2503° W</td><td>1 January 2007</td><td>Trimble NetR9 receiver/ Trimble Zephyr Geodetic antenna</td></tr><tr><td>SVGF</td><td>Fancy</td><td>13.3815° N, 61.1659° W</td><td>27 February 2021</td><td>Septentrio PolarRx5 receiver/ Trimble Zephyr Geodetic antenna</td></tr><tr><td>SVGG</td><td>Georgetown</td><td>13.2804° N, 61.1194° W</td><td>3 January 2021</td><td>Septentrio PolarRx5 receiver/ Trimble Zephyr Geodetic 2 antenna</td></tr><tr><td>SVGK</td><td>Kingstown</td><td>13.1622° N, 61.2284° W</td><td>24 December 2008</td><td>Trimble NetRS receiver/ Trimble Zephyr Geodetic antenna</td></tr><tr><td>SVGR</td><td>Richmond</td><td>13.3116° N, 61.2312° W</td><td>23 January 2021</td><td>Septentrio PolarRx5 receiver/ Trimble Zephyr 3 Geodetic antenna</td></tr><tr><td>SVGS</td><td>Volcano Summit ( Destroyed during 9 April 2021 explosions )</td><td>13.3229° N, 61.1826° W</td><td>18 January 2021</td><td>Septentrio PolarRx5 receiver/ Trimble Zephyr Geodetic antenna</td></tr><tr><td>SVSS</td><td>Volcano Summit</td><td>13.3229° N, 61.1825° W</td><td>1 September 2021</td><td>Trimble NetRS receiver/ Trimble Zephyr 3 Geodetic antenna</td></tr></tbody></table>
<table><thead><tr><th rowspan="2"></th><th rowspan="2">Site</th><th colspan="4">2018</th><th colspan="4">2019</th><th colspan="4">2020</th><th colspan="4">2021</th></tr><tr><th>J</th><th>F</th><th>M</th><th>A</th><th>M</th><th>J</th><th>J</th><th>A</th><th>S</th><th>O</th><th>N</th><th>D</th><th>J</th><th>F</th><th>M</th><th>A</th><th>M</th><th>J</th><th>J</th><th>A</th><th>S</th><th>O</th><th>N</th><th>D</th></tr></thead><tbody><tr><td rowspan="2">eGPS</td><td>JCWL</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>TBRK</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="9">cGPS</td><td>MESO</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>SDBY</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>SVGB</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>SVGF</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>SVGG</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>SVGK</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>SVGR</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>SVGS*</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>SVSS</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

* DESTROYED DURING EXPLOSIVE PHASE

<caption>**Fig. 2.** Gantt chart showing the operational periods of GPS monitoring sites between 2018 and 2021. Bars are coloured by network type following Figure 1. The stippled, red polygon delineates the effusive start and explosive end to the eruption.</caption>

a fourth in Fancy (SVGF, c. 6 km away from the vent) in late February (Figs 1 & 2). SVGG and SVGR antennas were mounted on the roofs of buildings (e.g. Fig. 3b), while concrete monuments equipped with stainless steel threaded rods were constructed for SVGF and SVGS (Fig. 3c, d). The latter were built to substitute for solid bedrock which is absent in these locations, by manually digging pits

and constructing reinforced foundations connected to concrete pillars.

The expansion of the cGPS network from two to six stations dramatically improved its sensitivity. This was verified by estimations of variations in the minimum volume of an isotropic source (depth range 0–8 km) detectable by a 1 mm horizontal and 2 mm vertical displacement, which showed that the

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Photographic examples of GPS monitoring sites. (a) eGPS setup at JCWL, February 2021. (b) cGPS rooftop antenna at SVGG, February 2021. (c) cGPS station at SVGF, February 2021. The GPS antenna is mounted on a reinforced concrete pillar built into a foundation. To the right is the collocated posthole seismometer. (d) cGPS station at SVGS, January 2021. The GPS antenna is mounted on a cement-filled pillar.</figcaption></figure>
network had reasonably good geometry without
major azimuthal gaps, but suffered from a lack of
near-field stations to characterize shallow deforma-
tion sources (after Beauducel et al. 2006).
Campaign-style GPS benchmarks were therefore
reoccupied at two sites on the volcano's flanks
(Jacob's Well, JCWL and Table Rock, TBRK,
Fig. 1) during February and March of 2021 for at
least 7 days at a time (Figs 2 & 3a). The antennas
were mounted on stainless steel threaded rods drilled
and cemented into pre-existing lava flows. It should
be noted that these campaigns were conducted on
foot whilst the dome continued to grow on the west-
ern edge of the pre-existing 1979 dome. This guided
the choice of eGPS sites to the east, predicated upon
life safety assessments and the westerly direction of
the gas plume. The closest cGPS station to the vol-
cano, SVGS, stopped working on 16 March and
was subsequently destroyed during explosive erup-
tions in April (Fig. 2).

Following the end of the explosive phase to this
eruptive episode, cGPS station deployment contin-
ued in 2021. In July, two stations were installed in
Mesopotamia (MESO) and Sandy Bay (SDBY),
and in September the summit station was replaced
(renamed SVSS) in a slightly different location
(Figs 1 & 2). Up to the time of writing, automatic
data acquisition for MESO, SDBY and SVGF had
not been achieved due to a lack of internet connec-
tion at those sites (communication to SVGF was
lost during the explosive eruption). Data from these
stations are downloaded approximately every fort-
night by the SMU and sent to UWI-SRC via the
internet for processing and storage. St Vincent's
cGPS network now consists of 8 cGPS stations
(Fig. 1), a sizeable improvement from its sparse
pre-eruptive network.

InSAR

Despite the extensive spatial and temporal coverage
expected from monitoring deformation fields using
InSAR, this technique has not yet been incorporated
into routine volcano monitoring by UWI-SRC. Con-
tributing factors have ranged from budgetary to fea-
sibility. Historically, there has been a lack of
in-house specialist experience needed for interpret-
ing satellite datasets. The application of InSAR to
tropical volcanoes is also known to be inhibited by
dense vegetation, large variations of water vapour
and steep topography (Ebmeier et al. 2013), features
which prevail in the Eastern Caribbean. Coherence is
improved with increasing radar wavelengths, but the
corollary of compounded limitations to freely avail-
able images is also true. Long satellite repeat-times
(InSAR requires at least a pair of SAR images and
preferably a longer time-series) are an added

hindrance towards real time InSAR measurements
during unrest or eruptive episodes (Ebmeier et al.
2018).

The 2020–21 eruption crisis attracted an outpour-
ing of support from the InSAR community in
regional and international scientific organizations
and research institutions: GNS Science (New Zea-
land), Institut de Physique du Globe de Paris
(IPGP, France), National Aeronautics and Space
Administration (NASA, USA), University of Leeds
(UK), Universidad Nacional de Cuyo (Argentina)
and United States Geological Survey (USGS,
USA). This was a welcomed addition to UWI-SRC's
geodetic monitoring efforts given the assertion that
InSAR favours the detection of shallow (<5 km)
deformation sources (Ebmeier et al. 2018). The
first radar imagery used for observing eruption-
related deformation was acquired from the open-
source Sentinel-1 satellite (operating in C-band,
λ = 5.6 cm), with interferograms provided by
IPGP. Sentinel-1 repeat passes at that time occurred
every 12 to 18 days. By contrast ALOS-2 imagery
(operating in L-band, λ = 23.5 cm), originally
made available under an ALOS-2 6th Research call
project (Ryan et al. 2021) and subsequently through
an emergency collaboration with NASA, had consid-
erably longer repeat-times (approximately once per
year) in the year building up to the eruption.
UWI-SRC made formal requests to ESA (European
Space Agency) and JAXA for additional satellite
coverage that were subsequently granted, allowing
Sentinel-1 to acquire images every six days and
ALOS-2 every 14 days during the crisis. On the
tail end of the crisis to support monitoring efforts,
RCM (C-band, λ = 5.6 cm), COSMO-SkyMed
(X-band, λ = 3.1 cm) and SAOCOM-1 (L-band, λ
= 23.5 cm) interferograms were provided by
USGS, University of Leeds and Universidad Nacio-
nal de Cuyo respectively, the latter of which only
will be discussed here in detail.

Other techniques

Electronic Distance Measurements were conducted
in February and March 2021 to monitor the stability
of the SW crater wall, against which the dome was
growing. No deformation was recorded during this
period. The reader is directed to Joseph et al.
(2022) for details of these measurements. Such exer-
cises were discontinued just before the vulcanian
explosions of April 2021.

Data processing

GPS

In preparation for processing, binary format receiver
data collected from GPS sites are converted into
standard Receiver Independent Exchange (RINEX) format using the program TEQC (Estey and Meertens 1999). Raw receiver data and converted RINEX observation and navigation files are archived by UWI-SRC. Daily positions are calculated with GAMIT/GLOBK software (version 10.71) (Herring et al. 2018) using over 100 global (International Global Navigation Satellite Systems Service, IGS) reference stations with final orbits. To minimize tectonic influences, time-series of station positions are calculated relative to the ITRF2014 Caribbean Plate motion model as defined by Altamini et al. (2017).

## InSAR

For this study, we processed Sentinel-1 (C-band) radar imagery acquired between 11 December 2017 and 30 April 2021 from descending track 156 and ascending track 164 (Table S1a, Fig. S1). During the eruptive crisis, continuous processing of Sentinel-1 interferograms acquired through effusive and explosive phases (27 December 2020–8 April 2021) was conducted by IPGP and shared with UWI-SRC. Processing techniques include the application of Small Baseline Technique (Doin et al. 2011), correction of atmospheric delays (Jolivet et al. 2011), adapted filtering and a tailored unwrapping strategy. Due to poor coherence outside the crater, detection of any deformation at the scale of the edifice was impossible with Sentinel-1, providing useful results only within the crater region. In other words, Sentinel-1 was essentially insensitive to any deep-seated signal (i.e. taking place at depth >2–3 km under the crater).

In order to provide constraints on deeper processes, we include images acquired by L-band satellites ALOS-2 and SAOCOM. Contrary to C-band, SAR data acquired in L-band have the capability of penetrating through vegetation, which gives access to deformation at the edifice scale, gaining resolution on processes taking place at greater depth. We processed ALOS-2 images between October 2014 and April 2021 (Table S1a) using GAMMA version 2.1 (Werner et al. 2000) and performed topographic corrections using the 30 m ASTER GDEM. Because of limited coherence in many of the interferograms due to long temporal baselines and dense vegetation across large parts of St Vincent, we used StaMPS (Hooper et al. 2007) to identify and extract stable scatterers to be used in forming an PS-SBAS time-series. In total we used 78 interferograms to form the time-series providing reasonable coverage around the edge of the island and summit area. Topospheric delays were corrected assuming a linear correlation with elevation. In addition, we solved for and removed the best-fitting quadratic ramp from each interferogram.

We processed 12 SAOCOM ascending pass scenes acquired in stripmap mode S5 between 14 September 2019 and 14 March 2022 (Table S1a) applying software and methods used as for Roa et al. (2021), Boixart et al. (2020) and Velez et al. (2016). Due to a lack of coherence at the volcano edifice during the explosive phase, we did not compute a deformation time-series. We computed individual interferograms spanning the onset of the effusive phase centred at the crater and surroundings and co-eruptive interferograms spanning the explosive phase targeting the whole island. Topographic corrections were applied using the STRM 30 m DEM and interferograms were unwrapped using the MCF networks approach (Costantini 1998).

## Geodetic observations

This section outlines the deformation signals detectable by both GPS and InSAR before (1 January 2018–26 December 2020), during (effusive phase: 27 December 2020–8 April 2021 and explosive phase: 9–22 April 2021) and after (23 April–31 December 2021) this eruption. We identify five distinct periods of deformation, each identified according to the technique used ('G' for GPS, 'I' for InSAR and 'GI' for GPS and InSAR).

### G1: pre- and syn-eruptive (effusive) inflation far field

Pre-eruptive uplift can be observed on the then 2-station cGPS network from as early as July 2020 (Fig. 4). During the period 1 July to 9 December (between 10 December 2020 and 15 February 2021, SVGK was out of commission), sites expanded away from the volcano in an overall horizontal displacement of 5.8 (±5.1) mm south and 6.9 (±6.9) mm west for SVGB, and 7.3 (±9.8) mm south and 3.7 (±10.7) mm west for SVGK, with upward vertical motions 11.6 (±6.0) mm for SVGB and 7.6 (±15.8) for SVGK (Table 2). Inflation continued even as the volcano entered its effusive phase until late March 2021, which is clearly illustrated on SVGB's time-series (most apparent on the north-south component, Fig. 4a). SVGB's measured displacement over the period 1 July 2020 to 30 March 2021 was 13.7 (±5.1) mm south, 7.1 (±3.1) mm west and 10.4 (±17.1) mm north (Table 2). Similar observations were not immediately obvious or ubiquitous across the rest of the cGPS network due to recording interruptions and new station installations. For example, the north-south component of SVGG showed an apparent southern trajectory from around February 2021 (Fig. 5c), while comparable movements were not observed on any other component or station.
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Raw time-series of cGPS measurements for (a) SVGB and (b) SVGK stations from 2018 to 2021 (relative to the ITRF2014 Caribbean Plate motion model as defined by Altamini *et al.* 2017). Data are collected using a 30 s sampling interval for a 24 h per UTC day. Stippled green lines denote the observed start of pre-eruptive deformation. White lines illustrate the linear regression used to calculate displacements in Table 2. Yellow and red bars correspond to effusive and explosive eruptive phases respectively. Included on the bottom panels are daily volcanic earthquake counts for the same period, as recorded by UWI-SRC. Volcanic earthquakes above 50 per day are not shown.</figcaption></figure>

## 12: near-field deformation

Near-field deformation was detected by Sentinel-1, ALOS-2 and SAOCOM and radar using images collected during 19–31 December 2020, January 2020–January 2021 and October 2020–February 2021 (Table S1a & b) respectively. The shorter frequency of Sentinel-1 acquisitions allowed for the detection of a deformation signal occurring between two consecutive acquisitions (both in ascending and descending passes), likely associated with the dome breaching the surface on 27 December 2020

(Joseph *et al.* 2022). The deformation pattern is amplified within the crater and amounts to approximately 4 cm peak-to-peak displacement in the line-of-sight (LOS). The interferograms and preliminary models were communicated to UWI-SRC by IPGP on 11 January 2021, and are reported in Joseph *et al.* (2022).

Using ALOS-2 and SAOCOM data, negative LOS displacements of c. 80 mm were measured on the western edge of the crater consistent with uplift and/or westward motion (Fig. 6). Due to the timing of these acquisitions, both datasets contain the
<table><caption>**Table 2.** Estimated displacements from cGPS time-series pre- and syn-eruption</caption><thead><tr><th>Site</th><th>dN (mm)</th><th>Propagated error (mm)</th><th>dE (mm)</th><th>Propagated error (mm)</th><th>dU (mm)</th><th>Propagated error (mm)</th></tr></thead><tbody><tr><td colspan="7"><strong>1 July to 9 December 2020 (G1)</strong></td></tr><tr><td>SVGB</td><td>-5.8</td><td>5.1</td><td>-6.9</td><td>6.9</td><td>11.6</td><td>6.0</td></tr><tr><td>SVGK</td><td>-7.3</td><td>9.8</td><td>-3.7</td><td>10.7</td><td>7.6</td><td>15.8</td></tr><tr><td colspan="7"><strong>1 July 2020 to 30 March 2021 (G1)</strong></td></tr><tr><td>SVGB</td><td>-13.7</td><td>5.1</td><td>-7.1</td><td>3.1</td><td>10.4</td><td>17.1</td></tr><tr><td colspan="7"><strong>31 March to 8 April 2021 (G3)</strong></td></tr><tr><td>SVGF</td><td>9.6</td><td>1.5</td><td>7.3</td><td>2.4</td><td>-3.7</td><td>4.3</td></tr><tr><td colspan="7"><strong>9-22 April 2021 (G4)</strong></td></tr><tr><td>SVGB</td><td>42.5</td><td>2.8</td><td>37.1</td><td>3.5</td><td>-81.0</td><td>13.8</td></tr><tr><td>SVGF</td><td>-119.9</td><td>3.9</td><td>-41.0</td><td>4.3</td><td>-146.3</td><td>20.0</td></tr><tr><td>SVGG</td><td>67.9</td><td>2.5</td><td>-43.6</td><td>3.0</td><td>-65.1</td><td>12.6</td></tr><tr><td>SVGK</td><td>23.9</td><td>3.0</td><td>3.3</td><td>3.7</td><td>-27.3</td><td>15.1</td></tr><tr><td>SVGR</td><td>61.8</td><td>2.6</td><td>63.8</td><td>3.1</td><td>-97.5</td><td>13.0</td></tr></tbody></table>

dN, dE and dU denote estimated displacements for north, east and up components respectively. Offsets for G1 and G3 are estimated from a least-squares linear regression of time-series position data (shown in Figs 4 & 5). Propagated errors therefore consider residuals of the final and initial values of the linear regression. For G4, displacements are estimated using the tsfit program of GAMIT/GLOBK. Connectivity was lost to SVGF after 20 April 2021, therefore displacement for this station is calculated during 9–19 April.

displacement associated with the onset of the effusive eruption detected by Sentinel-1. The longer wavelength of ALOS-2 and SAOCOM radar facilitated higher coherence away from the dome exposing a slightly broader deformation signal than what was resolved using Sentinel-1 data. This points towards an additional shallow deformation source occurring sometime between October 2020 and January 2021 based on image acquisition dates.

### G3: pre-explosive deformation

From 31 March to 8 April 2021, one of the closest operational cGPS stations to the volcano SVGF (Fig. 1) recorded small changes in deformation rates prior to the explosive phase (Fig. 5b). The most conclusive was observed on the horizontal components moving 9.5 (±1.5) mm north and 7.3 (±2.4) mm east.

### G4: syn-eruptive (explosive) deflation

The start to the explosive phase of this eruption was accompanied by a rapid contraction of the entire cGPS network towards the volcano (Figs 4 & 5) consistent with depressurization of the magma reservoir. The overall horizontal and vertical station displacements measured during the explosive phase are shown in Table 2. The bulk of the deflation occurred within the first three days of explosions, coinciding with a drop in the daily volcanic earthquakes recorded from >300 per day just prior to 9 April to <124 per day between 9 and 12 April (Fig. 5). This deflation was also detected by SAOCOM

radar using interferograms covering the whole island (Fig. 7a, Table S1b). A LOS lengthening (subsidence) of up to 100 mm is observed affecting the western shore, whereas no significant LOS deformation is observed in the eastern shore due to the SAR slanted geometry. Inside the island, decorrelation did not allow us to characterize the whole deformation pattern, although deformation clearly increases towards the volcano. Such extensive deformation is not observed using images outside the explosive phase (Fig. S2). These results fit very well with the deformation time-series observed at different cGPS stations (Fig. S3) confirming that the SAOCOM signal is not an artefact of orbital uncertainties. Deformation then slowed down considerably during the remainder of the explosive phase.

### G5: post eruption

Following the explosive phase, minimal deformation was observed, albeit modest northward movements in all stations suggestive of continued deflation (Figs 4 & 5). As the only station located north of the volcano, SVGF’s movement steadily away from the volcano from May to August, before slowing down towards the end of 2021, deviates from this deflationary trend.

### Modelling

To constrain the sources of deformation, we applied analytical models covering a range of source geometries where possible to three deformation periods
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Raw time-series of cGPS measurements for (a) SVGR, (b) SVGF and (c) SVGG stations for 2021 (relative to the ITRF2014 Caribbean Plate motion model as defined by Altamini *et al.* 2017). Data are collected using a 30 s sampling interval for a 24 h per UTC day. Yellow and red bars correspond to effusive and explosive eruptive phases respectively. Stippled orange lines on SVGF denotes possible deformation before start to explosive phase. White lines illustrate the linear regression used to calculate displacements in Table 2. The initial upward motion recorded by SVGG during the explosive phase is attributed to noise brought on by significant ash venting towards the east of the island. Included on the bottom panels are daily volcanic earthquake counts for the same period, as recorded by UWI-SRC. Note difference in y-axis scale compared to Figure 4.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Observed (a) ALOS-2 (15 January 2020–13 January 2021, ascending track, path 36, frame 250) and (b) SAOCOM (2 October 2020–23 February 2021, ascending track, path 29, frame 472) unwrapped interferograms illustrating near-field pre-eruptive (effusive) deformation. Negative LOS indicates motion towards the satellite.</figcaption></figure>

(G1, I2 and GI4), assuming an isotropic, homogeneous elastic half-space, no topography and a Poisson's ratio $\nu = 0.25$. Different modelling approaches are used each period, which will be detailed in each section.

Due to the fact that the models are conducted without accounting for topography, the estimated depths should be considered as relative to the average altitude of observations (c. 1 km above sea level). Periods G3 and G5 are excluded from analysis.

<figure><img src="image_9.png" /><figcaption>**Fig. 7.** (a) Observed, (b) modelled and (c) residual unwrapped interferograms for syn-eruptive (explosive) deformation phase using SAOCOM images obtained between 6 January 2021 and 14 March 2022 (ascending track, path 29 and frame 472). The forward model is computed using spherical source parameters (McTigue 1987) as derived from cGPS data (Table 4) only. Ground-based GPS stations are labelled. These data are not used in the inversion model for GI4 deformation as stated in the text.</figcaption></figure>
## G1: pre-eruptive inflation far field

We ran a set of Mogi (1958) forward models to approximate source parameters (depth and volume change) responsible for the precursory inflation over the 1 July to 9 December 2020 (period shortened from the start of the effusive phase since connectivity was lost to SVGK from 10 December, Table 2, Fig. 2). Model input parameters include local source coordinates fixed to the vent (due to the restricted geometry of the 2-station cGPS network, both located west of the island and south of the vent (Fig. 1)), depth range of 0–30 km and volume change $10^{-5}$–$0.05 \text{ km}^3$. We ran 100 000 forward models randomly sampling from the parameter space and assessing the RMS misfit for each model to the GPS observations. We found the best fit was at a depth of 18 ($\pm$1.9) km with a volume change of 29 ($\pm$5) $\times 10^6 \text{ m}^3$, providing a reasonable fit to the estimated displacement with an RMS misfit of 1.2 mm. The absence of detected deformation by Sentinel-1 in the same time interval (December 2017–December 2020) indicates that this deep source of deformation was not associated with a substantial counterpart at shallow depth.

## I2: pre- and syn-eruptive (effusive) deformation near field

Modelling by Joseph et al. (2022) of Sentinel-1 acquisitions (19–31 December 2020) is compatible

with a shallow vertical dyke, starting at a depth of 0.7 km and reaching the surface along a 0.6 km long north–south-oriented crack, with a volume of $0.06 \times 10^6 \text{ m}^3$. This has been ascribed to syn-ruptive dome extrusion. The longer wavelength and time period (one year) covered by ALOS-2 radar provide an opportunity to investigate whether an additional deformation source can be unpicked from InSAR data pre-eruption. In this regard, we first remove the expected deformation caused by the shallow intrusion using a forward model and the dyke parameters detailed in Joseph et al. (2022) (Fig. 8). Due to the limited spatial extent of the InSAR data and high noise levels, rather than inverting for the deformation source directly we generate a suite of forward models to simulate the observed deformation testing three sources: point (Mogi 1958), dyke (Okada 1985) and penny-shaped crack (sill) (Fialko et al. 2001). We constrain the geometries using input parameters shown in Table 3. As above, for each source we ran 100 000 forward models randomly sampling from the parameter space and assessing the RMS misfits for each model to the ALOS-2 observations (Fig. S4). To estimate the range of parameters, we extract the standard deviation of all models that provide a fit to the data within 10% of the best-fit solutions. For the dyke, the best-fit solution (Fig. S5) yields an RMS misfit of 11.7 mm with 54 modelled intrusions falling within 10%. The best-fitting dyke is c. 500 m long, dipping to the west at 69° with a thickness of c. 0.45 m (Fig. 9). The preferred models tend to favour

<figure><img src="image_10.png" /><figcaption>**Fig. 8.** Comparison of (a) ALOS-2 interferogram using images acquired between 15 January 2020 and 13 January 2021 (same as Fig. 6a), (b) predicted deformation from shallow dyke intrusion derived from Sentinel-1 data (Joseph et al. 2022) and (c) residual interferogram after removal of dyke intrusion.</figcaption></figure>
<table><caption>**Table 3.** Modelling input parameters and results for pre-eruptive inflation near field (I2) using different sources</caption><thead><tr><th>Dyke</th><th>Strike</th><th>Dip</th><th>Top depth (m)</th><th>Length (m)</th><th>Width (m)</th><th>Opening (m)</th><th>RMS (mm)</th></tr></thead><tbody><tr><td>Input range</td><td>160°–200°</td><td>50–90° (both east and west dipping)</td><td>5–1000</td><td>500–3000</td><td>500–6000</td><td>0–1</td><td>11.7</td></tr><tr><td>Best fit</td><td>178° ± 9.8</td><td>69° west ±11.3</td><td>648 ± 213</td><td>545 ± 463</td><td>1519 ± 463</td><td>0.5 ± 0.5</td><td></td></tr><tr><td>Point (Mogi, 1958)</td><td>Location (east and north, m from origin)</td><td></td><td>Depth (m)</td><td>Volume change (km³)</td><td></td><td></td><td>RMS (mm)</td></tr><tr><td>Input range</td><td>1 km radius around vent</td><td></td><td>0–5000</td><td>1 × 10⁻⁵–0.05</td><td></td><td></td><td>13.0</td></tr><tr><td>Best fit</td><td>103 ± 305, 312 ± 315</td><td></td><td>874 ± 289</td><td>1.4 × 10⁻⁴ ± 9.9 × 10⁻⁵</td><td></td><td></td><td></td></tr><tr><td>Penny-shaped crack (Fialko et al. 2001)</td><td>Location (east and north, m from origin)</td><td></td><td>Depth (m)</td><td>Volume change (km³)</td><td>Radius (m)</td><td></td><td>RMS (mm)</td></tr><tr><td>Input range</td><td>1 km radius around vent</td><td></td><td>0–5000</td><td>3 × 10⁻⁶–1.5 × 10⁻⁴</td><td>100–2000</td><td></td><td>12.1</td></tr><tr><td>Best fit</td><td>-219 ± 114, 257 ± 140</td><td></td><td>1004 ± 276</td><td>1.5 × 10⁻⁴ ± 4.6 × 10⁻⁵</td><td>523.7 ± 157</td><td></td><td></td></tr></tbody></table>

a westward dip (Fig. S4b) with opening values ranging from 0.05 to 1.7 m. The best-fitting point source (Fig. S5), which gives an RMS misfit of 12.1 mm, is located at a depth of c. 900 m around 200 m NE

of the vent with a volume increase of c. 0.1 × 10⁶ m³ (Fig. 9). Of all the simulations, only 8 of the Mogi models provide fits to the data within 10% of the best solution clustering between c. 850

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /><figcaption>**Fig. 9.** Comparison of best-fit modelled sources for near-field pre-eruptive (effusive) deformation using ALOS-2 radar alongside source model for syn-eruptive (effusive) deformation using Sentinel-1 radar from Joseph *et al.* (2022). Black polygons denote the volcano crater (larger) and 1979 dome (smaller). Model sources are as follows: small blue circle = point source (Mogi 1958), purple rectangle = dyke and large yellow circle = penny-shaped crack (Fialko *et al.* 2001). The dyke modelled in Joseph *et al.* (2022) is denoted as a vertical rectangle half-filled in blue. The 2020 vent is shown as a red triangle.</figcaption></figure>
<table><caption>**Table 4.** Results from Bayesian analysis (GBIS) for syn-eruptive (explosive phase, G4) deformation using different sources</caption><thead><tr><th>Point (Mogi 1958)</th><th>X (m)</th><th>Y (m)</th><th>Depth (m)</th><th>ΔV × 10<sup>6</sup> (m<sup>3</sup>)</th><th>RMS (mm)</th></tr></thead><tbody><tr><td>Optimal</td><td>773</td><td>2178</td><td>6052</td><td>50.2</td><td rowspan="4">26.5</td></tr><tr><td>2.5%</td><td>530</td><td>1824</td><td>5758</td><td>54.4</td></tr><tr><td>97.5%</td><td>981</td><td>2500</td><td>6417</td><td>46.5</td></tr><tr><td>Lower</td><td>-5000</td><td>-5000</td><td>1000</td><td>-100</td></tr><tr><td>Upper</td><td>5000</td><td>5000</td><td>30 000</td><td>100</td><td></td></tr><tr><td>Finite spherical (McTigue 1987)</td><td>X (m)</td><td>Y (m)</td><td>Depth (m)</td><td>Radius (m)</td><td>DP/μ</td><td>RMS (mm)</td></tr><tr><td>Optimal</td><td>776</td><td>2177</td><td>6059</td><td>25</td><td>-974</td><td rowspan="4">26.3</td></tr><tr><td>2.5%</td><td>539</td><td>1828</td><td>5747</td><td>12</td><td>-9219</td></tr><tr><td>97.5%</td><td>982</td><td>2497</td><td>6398</td><td>59</td><td>-78</td></tr><tr><td>Lower</td><td>-5000</td><td>-5000</td><td>100</td><td>5</td><td>-10 000</td></tr><tr><td>Upper</td><td>5000</td><td>5000</td><td>30 000</td><td>5000</td><td>10 000</td><td></td></tr><tr><td>Prolate ellipsoid (Yang et al. 1988)</td><td>X (m)</td><td>Y (m)</td><td>Depth (m)</td><td>Ax (m)</td><td>a/r</td><td>Strike</td><td>Plunge</td><td>DP/μ</td><td>RMS (mm)</td></tr><tr><td>Optimal</td><td>919</td><td>648</td><td>6045</td><td>6096</td><td>0.02</td><td>104</td><td>16</td><td>-0.22</td><td rowspan="4">37.7</td></tr><tr><td>2.5%</td><td>-162</td><td>712</td><td>4684</td><td>221</td><td>0.01</td><td>76</td><td>0.06</td><td>-136</td></tr><tr><td>97.5%</td><td>915</td><td>2416</td><td>7422</td><td>5930</td><td>0.7</td><td>117</td><td>75</td><td>-0.06</td></tr><tr><td>Lower</td><td>-5000</td><td>-5000</td><td>100</td><td>0</td><td>0.01</td><td>0.1</td><td>0</td><td>-10 000</td></tr><tr><td>Upper</td><td>5000</td><td>5000</td><td>30 000</td><td>8000</td><td>1</td><td>359</td><td>90</td><td>10 000</td><td></td></tr><tr><td>Horizontal rectangular sill (Okada 1985)</td><td>Length (m)</td><td>Width (m)</td><td>Depth (m)</td><td>Strike</td><td>X (m)</td><td>Y (m)</td><td>Opening (m)</td><td></td><td>RMS (mm)</td></tr><tr><td>Optimal</td><td>398</td><td>12 278</td><td>3</td><td>18</td><td>3298</td><td>4960</td><td>10</td><td></td><td rowspan="4">63.6</td></tr><tr><td>2.5%</td><td>137</td><td>2916</td><td>3</td><td>17</td><td>3164</td><td>4335</td><td>4.8</td><td></td></tr><tr><td>97.5%</td><td>1088</td><td>14 684</td><td>79</td><td>23</td><td>4955</td><td>4996</td><td>10</td><td></td></tr><tr><td>Lower</td><td>0</td><td>0</td><td>0</td><td>0</td><td>-5000</td><td>-5000</td><td>0</td><td></td></tr><tr><td>Upper</td><td>15 000</td><td>15 000</td><td>30 000</td><td>360</td><td>5000</td><td>5000</td><td>10</td><td></td><td></td></tr></tbody></table>

Note: Optimal is from the maximum *a posteriori* probability solution. The percentages 2.5% and 97.5% are the lower and upper boundaries of the 95% credible intervals. Lower and Upper are the bounds of the prior distribution used for the inversion. X and Y are the local coordinates of the source. Depth is positive downward. ΔV is the volume change. Ax is the length of the major semi-axis of the ellipsoid. a/r is the dimensionless aspect ratio between semi-axis (minor/major). Strike is the strike angle of the major semi-axis of the ellipsoid with respect to north in degrees. Plunge is the inclination angle of the major semi axis with respect to the horizontal in degrees. DP/μ is dimensionless excess pressure (pressure change/shear modulus). Opening is the opening of the dislocation plane. RMS (misfit between measured and modelled displacements) is calculated for both horizontal and vertical components; lowest values indicate the best model fit.

and 1700 m depth (Fig. S4a). For the sill, the best-fit model (Fig. S5) has an RMS misfit of 12.1 mm with a volume increase of $0.2 \times 10^6 \text{ m}^3$ at a depth of 1 km with a radius of c. 500 m (Fig. 9). All of the best-fitting models (51 total) lie between c. 450 m and c. 1.6 km focused beneath the vent area (Fig. S4c). InSAR points used in modelling are reported in Table S2a.

GI4: syn-eruptive (explosive) deflation

The improved station density existing during the explosive phase allowed more source geometries to be tested. Here, using only GPS data, we apply a Bayesian approach implemented with open-source Geodetic Bayesian Inversion Software (GBIS) (Bagnardi and Hooper 2018), testing point (Mogi
1958), finite spherical (McTigue 1987), prolate ellipsoid (Yang *et al.* 1988) and horizontal rectangular sill (Okada 1985) models to our displacement data (Table 4). The inversion algorithm uses a Markov chain Monte Carlo method, incorporating the Metropolis-Hastings algorithm, to find posterior probability density functions of model parameters taking into account uncertainties in the data. Mogi (1958), McTigue (1987) and Yang *et al.* (1988) model sources all converged on land at various

locations NE of the vent, at depths of 6 km. Using RMS misfit (taking into account both horizontal and vertical displacements), the best fit was obtained using point and finite spherical source models (RMS misfit values 26.5 and 26.3 mm respectively). The prolate ellipsoid source had a higher RMS (37.7 mm), while a horizontal rectangular sill had the largest RMS misfit (63.6 mm), with the centre of the source located just off island to the north. We report on results obtained using

<figure><img src="image_13.png" /><figcaption>**Fig. 10.** Spatial relationship between syn-eruptive deflation (explosive phase 9–22 April 2021) as measured by the cGPS network, pre-explosive volcanic earthquakes (23 March to 6 April 2021) and modelled Mogi sources pre- and syn-eruption. The vent is shown as a green triangle. Observed horizontal displacement vectors, as measured by the cGPS network during 9–22 April 2021, are shown as blue arrows with associated uncertainties (95% confidence level) shown as blue ellipses. Modelled displacements (Mogi) are shown as red arrows. Mogi pre-eruptive (same location as vent) and syn-explosive sources are shown as orange and yellow spheres respectively. Earthquake epicentres (black circles) are taken from Joseph *et al.* (2022).</figcaption></figure>
a simple Mogi point source (lowest number of
changeable parameters), resolving at 6 (±0.2) km
depth with a volume change of 50 (±2) × 10⁶ m³
(Fig. 10).

Although an inversion was not attempted incor-
porating SAOCOM data, we computed a forward
model (InSAR points used reported in Table S2b)
with spherical source (McTigue 1987) parameters
from Table 4 using dMODELS software package
by Battaglia et al. (2013) confirming a close agree-
ment with modelling results obtained using GPS
data only (Fig. 7b, c).

Discussion

Analytical model limitations

We use analytical models to link the surface defor-
mation recorded as ground displacements on St Vin-
cent to La Soufrière’s magma plumbing system, a
feature we cannot directly observe from the surface.
These models cover a range of geometries embedded
in a homogeneous, isotropic and elastic half-space.
The corollary is that this computational simplicity
comes at the expense of their inherent assumptions,
which do not capture the complexities of the natural
situation (Taylor et al. 2021). Geophysical surveys
have shown that isolated vats of liquid-dominated
magma chambers with sharp transitions to surround-
ing country rock do not generally exist, with melts
instead being disseminated via crystal mushes and
closely spaced melt lenses (sills) over a wide depth
range (Cashman et al. 2017). The mechanical behav-
iour of crystalline systems is distinct from melt and
elastic host rock (Liao et al. 2018, 2021; Mullet
and Segall 2022). Nonetheless, modelling magma
reservoirs as dyke (Okada 1985), pressurized point
(Mogi 1958) and finite spherical (McTigue 1987)
sources provide the best fit to our data, and places
some constraints on the depths of magma storage,
associated volume changes and subsurface geome-
try. Prolate spherical (Yang et al. 1988) and horizon-
tal sill (Okada 1985) source models are better able to
simulate magma distribution over an extended depth
range (e.g. Field et al. 2012). However, uncertainties
introduced into ground-based and satellite data due
to variations in station density and spatial coherence
during unrest and eruptive periods, motivate us to
focus here on results obtained using the simplest
models with the lowest number of inversion param-
eters to provide first order insight into source depths
and volumes. We acknowledge that there is usually a
trade-off between source depth and volume when
different source geometries are applied to deforma-
tion data (e.g. Pritchard and Simons 2004) and there-
fore expect the application of more sophisticated
modelling techniques in the future to generate a
larger range of acceptable results.

*Relationship between modelled source volume change and erupted volume*

Comparisons of geodynamically modelled subsurface
volume changes with eruptive volumes commonly
show that the former are much smaller than the latter
(McCormick Kilbride *et al.* 2016). This difference is
controlled by competing influences of magma com-
pressibility and magma reservoir properties includ-
ing geometry and host rheology (Rivalta and Segall
2008; McCormick Kilbride *et al.* 2016; Wasser
*et al.* 2021; Yip *et al.* 2022). Magma compressibility
is fundamentally linked to gas exsolution within
magma reservoirs. Volatiles dissolved in magma
exsolve during decompression, cooling and crystalli-
zation, diminishing the pressure changes recorded by
surface ground displacements. Relatedly, volatile-
saturated magmas are ten times more compressible
than magmas with no exsolved volatiles (Edmonds
and Woods 2018). As alluded to in the previous sec-
tion, although analytical models such as Mogi are
widely used to estimate source parameters, they are
highly idealized in simplifying the geometry and
physical properties of the crustal magmatic system
(Pascal *et al.* 2014; Taylor *et al.* 2021). As a result,
modelling of a magma body within a perfectly elastic
crustal rheology rather than a heterogenous magma-
mush network can lead to underestimations in the
volume change of the magma reservoir and magma
compressibility (Mullet and Segall 2022).

For the explosive phase (GI4), our modelled Mogi
source volume change is 50 × 10⁶ m³ (±2). This is
near estimations by Cole *et al.* (2023) of erupted
magma volume using mapped tephra deposits (42
× 10⁶ m³ (±20% Dense Rock Equivalent). Sparks
*et al.* (2023) calculated the exploded volume up to
14 April 2021 using estimates of the duration of indi-
vidual explosions from seismic observations and the
magma flux from satellite observations of column
height. Their results yield a comparatively higher
volume of 83 × 10⁶ m³, the majority of which emit-
ted in the first three days, a time frame matching the
bulk of measured syn-eruptive deflation (Figs 4 &
5). Interestingly the uncertainties surrounding this
calculation (5th percentile 41 × 10⁶ m³ and 95th per-
centile 142 × 10⁶ m³) overlap with our volume.
Depending on which estimate of erupted volume is
used for comparison, our modelled reservoir volume
change could be smaller or larger leading to contrast-
ing inferences about magma compressibility.

Reconstructing the pre-eruptive exsolved volatile
content of Soufrière magmas could place constraints
on its compressibility prior to the explosive phase.
However, a number of factors limit attempts at
these estimations. For explosive eruptions, it can
be assumed that total gas emissions are sourced
entirely from the pre-eruptive exsolved volatile
phase due to high magma ascent rates restricting
volatile exsolution between the chamber and the sur-
face (Yip *et al.* 2022). Although total gas emissions
were measured by satellites during La Soufrière’s
explosive phase (Joseph *et al.* 2022; Taylor *et al.*
2022), its effusive–explosive sequence complicates
this association by introducing the probability for
additional degassing within the conduit facilitated
by slower extrusion rates. Unravelling these pro-
cesses is further hampered by irregular gas measure-
ments during the effusive phase, scrubbing by the
hydrothermal system masking the actual volatile sig-
nature (Joseph *et al.* 2022) and lacking petrological
determinations of volatile concentrations in melt
inclusions or matrix glasses (Wasser *et al.* 2021).

Alternatively, we can use existing conceptual
models to infer magma reservoir conditions pre-
explosions. Stinton et al. (2023) posit that a shallow
degassed magma plug left over from previous erup-
tions fed the effusive phase. This implies that low
concentrations of pre-explosive exsolved volatiles
were present in the magma reservoir with associated
low magma compressibility. An incompressible
magma scenario can also be reconciled with the con-
tinued inflation observed following the onset of
dome extrusion. Segall (2016) shows that viscoelas-
tic relaxation can manifest in inflation, rather than
expected deflation post-eruption, for sufficiently
incompressible magmas without the requirement
for magma recharge.

*Interpreting pre- and syn-eruptive deformation events within a structural framework*

Notwithstanding its historical eruptive frequency,
prior to the 2020–21 eruption, knowledge about
the dynamics of La Soufrière’s sub-volcanic system
had been mainly constrained using petrological
observations. It is thought to comprise a vertically
extensive crystal mush network (melt contained in
a framework of crystals) facilitating the differentia-
tion of hydrous, mantle-derived high magnesium
basalts over a range of depths spanning near the
Moho to mid and upper crustal regions (Tollan
et al. 2011; Melekhova et al. 2015). Preferred reser-
voirs for ascending magmas in various stages of evo-
lution have been proposed by Fedele et al. (2021) to
be c. 13 km, c. 7 km and c. 3.5 km. The products of
eruptions over the last 1000 years have been predom-
inantly homogeneous basaltic andesites (Cole et al.
2019) indicating a current steady-state regime; how-
ever, the drivers behind changes in eruptive style
remain unclear. We attach geophysical observations
to a developing petrological framework and infer the
temporal evolution of the plumbing system during
this eruption (Fig. 11).

**Stage 1.** Far-field precursory inflation was
detected approximately 6 months in advance of this
eruption, modelled at a source depth ranging 16–

20 km (including modelling uncertainty). This likely
signalled the accumulation of basaltic andesite mag-
mas and/or gas from depth. We cannot speculate as
to the cause of this initiation. Petrological experi-
ments place the generation of basaltic andesites
beneath St Vincent in the lower crust (0.7–1 GPa)
(Melekhova et al. 2015). Our modelled depth
range is at the top of this pressure range.

**Stage 2.** From the timing of ALOS-2 and SAO-COM acquisitions, shallow precursory deformation occurred between October 2020 and January 2021. Based on our modelling, this signal can be explained either by (i) a shallow accumulation of magma at 800–1000 m depth or (ii) an eastward-propagating dyke to a top depth of c. 650 m. A pre-eruptive build of magma is supported by the Stinton et al. (2023) model whereby shallow degassed magma remnant from previous eruptions was remobilized and pushed out by gas-rich magma during the effusive phase. If this material was indeed the source of the 2021 dome extrusion, bulk rock chemical analyses support this assertion in being more silicic than 2021 explosive and 1979 material (Weber et al. 2023). Our alternative model, which has the best misfit to the data, suggests that there was a failed intrusion beneath the 1979 dome, likely representing a pre-existing pathway for magma extrusion. Arrested dykes are commonly observed (e.g. Gudmundsson 2011; Drymoni et al. 2020), with models suggesting that edifice heterogeneity as well as the local stress field are possible factors preventing an intrusion from reaching the surface (Dahm 2000; Gudmundsson 2011). Given the location of our best-fit dyke, we suggest that the pre-existing dome structure acted as a structural barrier causing the dyke to stall and fail to reach the surface. Priming of the shallow magma system by ascending basaltic andesite magmas and associated exsolved volatiles generated in Stage 1, followed by emplacement of the failed dyke are consistent with above-background seismicity recorded in November 2020 (Joseph et al. 2022).

**Stage 3.** The breaching of the dome through the surface in December 2020, west of the 1979 dome, was accompanied by shallow deformation modelled as a vertical dyke. This shift in the source location implies that magma found a more favourable path to the surface, perhaps unobstructed by the pre-existing 1979 dome. Once the effusive eruption began, the volcano continued to inflate at the scale of the edifice, seemingly uninterrupted whilst the dome extruded. This could be related to a continued influx of basaltic andesite magmas from depth with the extrusion of material at the surface being insufficiently vigorous to compensate for this overall pressure build-up. Alternatively, low compressibility of resident magma facilitated viscous creep of the surrounding host rock without new magma recharge from depth (Segall 2016).
<figure><img src="image_14.png" /><figcaption>**Fig. 11.** (a) Schematic summary illustrating the vertically extensive and interconnected plumbing system architecture beneath La Soufrière as determined from geodetic observations. Basaltic andesite magmas, generated from mantle-derived high magnesium basalts, stall at preferred depths of 16–20 and c. 6 km (dark red spheres). Arrows show the inferred migration of magmas from different regions within the crust. At the surface, within the volcano crater, 2020 (left) and 1979 (right) domes are shown. The shallowest pressure source (<1 km) detected using InSAR likely represents the accumulation of magma from depth or remobilization of magma left over from previous eruptions prior to the 27 December 2020 effusive eruption. This is shown in orange to distinguish this region as partially to mostly crystalline (mushy) at these depths before the start of unrest. The best-fit source geometry is an eastward-propagating dyke likely representing a failed intrusion beneath the 1979 dome. (b) Pre- and syn-eruptive deformation stages detected using GPS and InSAR techniques, as described in the text. We distinguish the I2 phase detected via InSAR into a & b to highlight the detection pre- and syn-eruption deformation sources.</figcaption></figure>

**Stage 4.** Prior to the explosive phase, an increased NE motion was observed on SVGF (c. 31 March 2021). However, this lone signal cannot

be taken as an independent indicator of an augmented volcanic inflation rate. Nonetheless, this timing falls within the range of a number of key features
observed from other monitoring streams signalling a
change in the dynamics of the magmatic system.
Late March heralded the start of volcano-tectonic
earthquake swarms (23–24 March), followed by
their transition to greater depths from around 5
April (Fig. 10) (Joseph *et al.* 2022). Early April
also saw an increase in dome volume and extrusion
rate (Joseph *et al.* 2022). We hypothesize that in
advance of the explosive phase, these changes
could have been facilitated by an increase in mag-
matic overpressure favoured by a crater configura-
tion where the dome effectively capped a
significant portion of the crater floor, decreasing
the efficiency of outgassing. Similar mechanisms
related to edifice and conduit permeability have
been proposed for changing eruptive styles at sub-
marine Kick-'em-Jenny volcano (Dondin *et al.*
2017). Alternatively or simultaneously, this period
may represent the moment in which volatile-rich
magmas reached the surface after all remnant
degassed magma from previous eruptions was
pushed out (Stinton *et al.* 2023).

**Stage 5.** Explosions began on 9 April 2021 accompanied by a deflation of the volcano. Our modelling places this pressure source at 6 km. This is marginally shallower than thermobarometric estimates of the magma reservoir feeding the current eruption from petrology (8–13 km depth) (Weber *et al.* 2023). Pre-explosive volcano-tectonic earthquakes span above and below our source depth (Fig. 10) likely emulating stress changes accompanying increasing overpressure and destabilization in the trans-crustal melt–mush system (e.g. Christopher *et al.* 2015) as the activity evolved from effusive to explosive. Seismic epicentres are offset from the vent mirroring the modelled explosive deflation source location (Fig. 10). Although detailed investigation into the mechanism behind pre-explosive seismicity at La Soufrière is beyond the scope of this paper, we postulate stress transfer to nearby fault structures (inferred on past geological maps e.g. Robertson 2005) relative to the conduit due to dyke inflation and/or elevated pore pressures in surrounding water-saturated aquifers known to be characteristic of La Soufrière’s edifice (Lahr *et al.* 1994; White and Power 2001; Roman *et al.* 2008; Fournier *et al.* 2011).

SVGF’s atypical behaviour

La Soufrière’s northernmost cGPS station, SVGF (Fig. 1), was distinctive in increasing its horizontal motion just before the explosive phase and expanding away from the volcano after the 2020–21 eruption had ended (Fig. 5b). Here, we speculate as to the reasons for this curious behaviour. Out of all the stations deployed during the effusive phase, SVGF was the last to be completed (Fig. 2) and the

only monument-style installation (Fig. 3c) (SVGS is excluded from this since it went offline shortly after it was installed). It is therefore probable that data recorded by SVGF reflect the coinciding effects of concrete settling and magma withdrawal.

SVGF’s time-series post-eruptive trajectory may also underscore some of La Soufrière’s structural heterogeneities. Its edifice consists of an older strato-cone or Somma, within which a younger pyroclastic cone emerges that has been the source of historical eruptions (Robertson 2005). Remnants of this Somma are defined by steep arcuate ridges north of the volcano comprising basaltic lava flows. Geographically, SVGF was the only station present during this eruption to reside upon this prehistoric edifice. Following the removal of pressure brought about by the explosive phase, it can be inferred that this distinct country rock surrounding the magma reservoir to the north influenced the partitioning of strain as the edifice rebounded inelastically during recovery (e.g. Head et al. 2019).

Utility of GPS and InSAR for volcano
monitoring in the Caribbean: lessons from La
Soufrière

The 2020–21 eruption propelled the integration of InSAR with traditional GPS monitoring at La Soufrière allowing for the effectiveness of each for detecting deformation to be equally assessed. During volcanic crises, monitoring scientists are largely interested in forecasting the start to and progression of eruptive activity for purposes of risk management. GPS was demonstrably the more successful technique for detecting precursory unrest in this scenario. Despite its sparsity, the lengthy time-series provided by the far-field 2-station cGPS network revealed deformation about 6 months before the effusive phase (G1), albeit not in real time. One of the major attractions of InSAR over GPS is the theoretically larger geographical extent the former captures, especially when compared to ground-based station networks. Yet, even with high return rates, the shorter wavelength of Sentinel-1 resulted in the coherent part of interferograms being restricted to the less vegetated crater area of the volcano. The longer wavelength instruments mounted on ALOS-2 and SAOCOM are capable of penetrating vegetation and generating interferograms that are coherent over a greater portion of the island. However, the very long repeat cycle of ALOS-2 and SAOCOM during the years prior to the eruption meant that the pre-eruptive pairs produced interferograms which were only coherent in the region of the volcanic crater and a little beyond it. Therefore InSAR was restricted to detecting near-field deformation before (and mostly throughout) the eruption, with only
ALOS-2 and SAOCOM radar uncovering pre-
eruptive deformation using pre- and post-eruptive
(effusive) pairs. Whilst providing additional, useful
insight after the fact on shallow precursory deforma-
tion, the infrequency of ALOS-2 and SAOCOM
acquisitions hypothetically thwarted the provision
of this information during the unrest period.

After repeat times were increased for ALOS-2 in 2021, it was possible to generate interferograms that were coherent across much of the island. However, no obvious deformation related to continued dome growth was measured with this or SAOCOM radar. Although 2021 brought with it the densification of the GPS network, logistical challenges prevented the continuous operation of the summit station during the entire eruptive period, essentially restricting the network's detection capability to the far field. Deformation during the effusive phase was detected only by GPS. The closest station to the volcano (SVGF) detected a change in deformation rate days before the explosive phase. If this was indeed a definitive signal, the pressure source responsible for this change might have been characterized had the summit station (SVGS) been operational and/or satellite repeat times for longer wavelength radar (ALOS-2 and SAOCOM) been similar to this deformation timescale.

Once the explosions began, the extremely heavy
ash fall and destruction of the crater area made it
impossible to obtain coherent interferograms from
post-explosive phase ALOS-2 (Fig. S6) and
Sentinel-1 images. SAOCOM radar conversely was
able to detect the syn-eruptive (explosive) deflation
also measured by the cGPS network (Fig. 7). GPS
instrumentation was also affected by ashfall with
recording interruptions and the addition of noise to
the data (Fig. 5).

Overall, our retrospective analysis of GPS and InSAR data reveals the wealth of information that could have been utilized before and during this erup-
tion had they both been optimized/available in real time. GPS provided a more consistent data stream and is capable of detecting deformation sources cov-
ering a large depth range, but this is solely dependent on the density and functioning of the network. InSAR's applicability depended on the radar used, capable of detecting shallow (≤6 km) deformation sources when a cocktail of different satellites (prefer-
ably long wavelength) are used with frequent over-
passes. For St Vincent, ideal InSAR monitoring would combine L-band (for improved coherence), high-spatial resolution (for detection of shallow pro-
cesses), short revisit interval (to increase temporal resolution and mitigate atmospheric noise), and an open-access/near-real-time data dissemination strat-
egy (to facilitate near-real-time exploitation in a crisis monitoring setting). At present, UWI-SRC's access to the required radar and associated processing

requirements is contingent upon collaboration. The
forthcoming 2023 satellite mission NASA-ISRO
SAR (NISAR) will offer an important opportunity
for exploiting the application of InSAR to Lesser
Antilles volcanoes with its L-band wavelength,
12-day repeat cycle and free data access policy.
When both techniques are used together, resolution
shortfalls in each can be compensated for, as demon-
strated by this study.

Conclusions

We have used geodetic observations obtained from GPS and InSAR techniques gathered retrospectively and during the 2020–21 effusive–explosive eruption of La Soufrière volcano to approximate its magma plumbing system architecture down to the lower crust. The far-field cGPS network geometry (closest operational station 5 km from the vent) only allowed for a depth sensitivity to deformation events ≥6 km. Despite the various radar employed in this study (Sentinel-1, ALOS-2 and SAOCOM), InSAR was most useful for detecting shallower deformation events, otherwise imperceptible by the existing cGPS network. This was mainly due to limitations introduced by satellites with the highest penetrating wavelengths having infrequent overpasses. Together both techniques proved to be complementary resources for distinguishing the magmatic processes influencing this eruptive episode. Five deformation periods are identified: (1) deep and (2) shallow pre-ruptive inflation, (3) syn-eruptive (effusive) inflation, (4) syn-eruptive (explosive) deflation and (5) post-eruptive recovery. Our modelled source depth estimates support the growing concept of a vertically extensive magmatic system beneath La Soufrière with preferred magma storage regions at c. 16–20 km, c. 6 km and <1 km (Fig. 11). This broadly matches independent estimates from petrological studies on this volcano. However, although providing first-order constraints on magma reservoir depths, our analytical modelling approach does not fully characterize the geometry of the plumbing system. Such detail will require additional constraints provided by further petrological and geophysical studies, which will in turn allow geodetic modelling parameters to be better resolved. There is room for continued development of geodetic monitoring in St Vincent, including sustaining a consistent data stream from the current 8-station cGPS network and new station installations. This study highlights the benefits of integrating InSAR into routine monitoring at UWI-SRC for supplementing the GPS network especially in the near field where logistical challenges have historically impeded station upkeep.

**Acknowledgements** The authors are grateful to the
Soufrière Monitoring Unit for field assistance during GPS
occupations, and the Japanese space agency (JAXA) and European Space Agency for tasking systematic ALOS-2 and Sentinel-1 acquisitions over La Soufrière during the eruptive period. We would like to thank S. Sparks, J. Blundy and W. Aspinall for valuable suggestions and discussion. Two anonymous reviewers provided helpful and constructive comments that improved the clarity of the paper.

**Competing interests** The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

**Author contributions** MC-H: conceptualization (lead), formal analysis (equal), investigation (equal), methodology (equal), software (equal), visualization (equal), writing – original draft (lead), writing – review & editing (lead); KP: conceptualization (supporting), formal analysis (equal), investigation (equal), methodology (equal), software (equal), writing – original draft (supporting), writing – review & editing (supporting); PE: conceptualization (supporting), formal analysis (equal), investigation (equal), methodology (equal), software (equal), visualization (equal), writing – original draft (supporting), writing – review & editing (supporting); RG: conceptualization (supporting), formal analysis (equal), investigation (equal), methodology (equal), software (equal), visualization (equal), writing – original draft (supporting), writing – review & editing (supporting); IH: conceptualization (supporting), formal analysis (equal), investigation (equal), methodology (equal), software (equal); RC-A: conceptualization (supporting), formal analysis (supporting), writing – review & editing (supporting); GAR: conceptualization (supporting), writing – review & editing (supporting); JLL: conceptualization (supporting), writing – review & editing (supporting); LL: conceptualization (supporting); MJ: conceptualization (supporting).

## Funding information

MC-H acknowledges funding from a Royal Society Newton International Fellowship (NIF\R1\211276).

**Data availability** The data that support the findings of this study are available from The University of the West Indies Seismic Research Centre, Japanese Space Agency (JAXA) and Argentina Space Agency (CONAE) but restrictions apply to the availability of these data, which were used under licence for the current study, and so are not publicly available. Data are, however, available from the authors upon reasonable request and with permission of the above agencies.

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