Michael J Heap
email: heap@unistra.fr

Tobias S Baumann

Marina Rosas-Carbajal

Jean-Christophe Komorowski

H Albert

Albert Gilg

Marlène Villeneuve

Roberto Moretti

Patrick Baud

Lucille Carbillet

Claire Harnett

H Albert Gilg

Thierry Reuschlé

Alteration-Induced Volcano Instability at La Soufrière de Guadeloupe (Eastern Caribbean)

Keywords: hydrothermal alteration, uniaxial compressive strength, Young's modulus, upscaling, flank collapse

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Volcanoes are inherently unstable structures that are built haphazardly, in both space and time, from the products of successive effusive and explosive eruptions and endogenous growth. These materials have highly variable physical and mechanical properties [START_REF] Heap | The mechanical behaviour and failure modes of volcanic rocks: a review[END_REF] and often form oversteepened and unstable slopes. As a result, volcano deformation (such as volcano spreading; [START_REF] Borgia | Spreading volcanoes[END_REF] and mass wasting events (such as debris avalanches resulting from partial flank collapse, lahars, and rockfalls; [START_REF] Roverato | Factors contributing to volcano lateral collapse[END_REF]) are commonplace at many volcanoes worldwide. Catastrophic collapse resulting from volcano spreading (van Wyk de [START_REF] Van Wyk De Vries | Catastrophic collapse at stratovolcanoes induced by gradual volcano spreading[END_REF] and mass wasting events present a significant volcanic hazard, and can also trigger hazardous laterally-directed explosions and devastating-both economically and in terms of loss of life-pyroclastic density currents [START_REF] Cole | Hazards from pyroclastic density currents[END_REF][START_REF] Glicken | Rockslide-debris avalanche of May 18[END_REF]Sparks et al., 2020;[START_REF] Voight | Catastrophic rockslide avalanche of May 18[END_REF]. Indeed, partial flank collapses at about 200 volcanoes have resulted in at least 20,000 fatalities in the last 10,000 years [START_REF] Siebert | Volcanoes of the World[END_REF]. Although about 52% of these flank collapses are associated with magmatic eruptions, about 22% are associated with non-magmatic phreatic and/or hydrothermal eruptions [START_REF] Siebert | Large volcanic debris avalanches: characteristics of source areas, deposits, and associated eruptions[END_REF][START_REF] Siebert | Volcanic hazards from Bezymianny-and Bandai-type eruptions[END_REF][START_REF] Siebert | Volcanoes of the World[END_REF]. As a result, stability assessments at volcanoes are an essential component of volcano monitoring and volcanic hazard mitigation.

Hydrothermal alteration, common to many volcanoes worldwide, is often invoked as a mechanism that contributes significantly to volcano instability. Indeed, studies using geological evidence, geochemical or geophysical data, numerical modeling, laboratory experiments, or a combination of these approaches have highlighted that hydrothermal alteration has or can weaken a volcanic slope sufficiently to promote collapse [START_REF] Ball | An assessment of hydrothermal alteration in the Santiaguito lava dome complex, Guatemala: implications for dome collapse hazards[END_REF][START_REF] Ball | The hydrothermal alteration of cooling lava domes[END_REF][START_REF] Ball | Combining multiphase groundwater flow and slope stability models to assess stratovolcano flank collapse in the Cascade Range[END_REF][START_REF] Cecchi | Flank spreading and collapse of weak-cored volcanoes[END_REF][START_REF] Del Potro | The decrease in the shear strength of volcanic materials with argillic hydrothermal alteration, insights from the summit region of Teide stratovolcano, Tenerife[END_REF][START_REF] Finn | Three-dimensional geophysical mapping of shallow water saturated altered rocks at Mount Baker, Washington: Implications for slope stability[END_REF][START_REF] John | Characteristics, extent and origin of hydrothermal alteration at Mount Rainier Volcano, Cascades Arc, USA: Implications for debris-flow hazards and mineral deposits[END_REF][START_REF] López | Catastrophic volcanic collapse: relation to hydrothermal processes[END_REF][START_REF] Opfergelt | The 1998 debris avalanche at Casita volcano, Nicaragua: Investigation of the role of hydrothermal smectite in promoting slope instability[END_REF][START_REF] Reid | Volcano collapse promoted by hydrothermal alteration and edifice shape[END_REF][START_REF] Rosas-Carbajal | Volcano electrical tomography unveils edifice collapse hazard linked to hydrothermal system structure and dynamics[END_REF][START_REF] Salaün | Hydrothermal alteration in andesitic volcanoes: trace element redistribution in active and ancient hydrothermal systems of Guadeloupe (Lesser Antilles)[END_REF][START_REF] Van Wyk De Vries | Sector collapse forming at Casita volcano, Nicaragua[END_REF][START_REF] Voight | The 1997 Boxing Day Sector Collapse and Debris Avalanche, Soufriere Hills Volcano, Montserrat, W.I[END_REF][START_REF] Watters | Rock mass strength assessment and significance to edifice stability, Mount Rainier and Mount Hood, Cascade Range volcanoes[END_REF]. Fluid and pore fluid pressure re-distributions caused by alteration are also thought to promote instability [START_REF] Ball | Combining multiphase groundwater flow and slope stability models to assess stratovolcano flank collapse in the Cascade Range[END_REF][START_REF] Day | Hydrothermal pore fluid pressure and the stability of porous, permeable volcanoes[END_REF]Heap et al., 2021a;[START_REF] Reid | Massive collapse of volcano edifices triggered by hydrothermal pressurization[END_REF] and erratic explosive behavior [START_REF] De Moor | Insights on hydrothermal-magmatic interactions and eruptive processes at Poás Volcano (Costa Rica) from highfrequency gas monitoring and drone measurements[END_REF][START_REF] Heap | Hydrothermal alteration of andesitic lava domes can lead to explosive volcanic behaviour[END_REF][START_REF] Mick | Hydrothermal alteration and sealing at Turrialba volcano, Costa Rica, as a mechanism for phreatic eruption triggering[END_REF].

Geological evidence is provided by the abundance of hydrothermally altered materials typically found in debris avalanche deposits resulting from partial flank collapse. For example, in their analysis of the 26 December 1997 partial edifice collapse at Soufrière Hills volcano (Montserrat, Eastern Caribbean), [START_REF] Voight | The 1997 Boxing Day Sector Collapse and Debris Avalanche, Soufriere Hills Volcano, Montserrat, W.I[END_REF] found varicolored, hydrothermallyaltered materials within the avalanche deposits, highlighting that alteration could have contributed to the collapse of the southern sector of the volcano. Avalanche deposits and material ejected during phreatic explosions at La Soufrière de Guadeloupe (Eastern Caribbean;

France) were also found to contain various parts of the active and ancient hydrothermal systems of the volcano [START_REF] Salaün | Hydrothermal alteration in andesitic volcanoes: trace element redistribution in active and ancient hydrothermal systems of Guadeloupe (Lesser Antilles)[END_REF]. Debris avalanche deposits at Tutupaca volcano (Peru) also contain hydrothermally altered materials [START_REF] Samaniego | The historical (218±14 aBP) explosive eruption of Tutupaca volcano (Southern Peru)[END_REF]. Extensive fumarole activity and hydrothermal alteration on the unstable flanks of Casita volcano (Nicaragua) was documented by van Wyk de [START_REF] Van Wyk De Vries | Sector collapse forming at Casita volcano, Nicaragua[END_REF], a volcano that experienced a partial flank collapse in 1998 with associated debris avalanches that killed about 2,500 people (Kerle and van Wyk de Vries, 2001;[START_REF] Van Wyk De Vries | Sector collapse forming at Casita volcano, Nicaragua[END_REF]. [START_REF] Opfergelt | The 1998 debris avalanche at Casita volcano, Nicaragua: Investigation of the role of hydrothermal smectite in promoting slope instability[END_REF] further suggested that the presence of abundant smectite (up to 50 wt.%) in the hydrothermally-altered core of Casita volcano contributed to slope instability by acting as a barrier to water infiltration thereby promoting fluid circulation along marked discontinuities, gradually decreasing the shear strength of the host-rock. Finally, more than 55 debris flows have originated from Mt Rainer (USA) in the Holocene, and hydrothermally derived clays have been found in some of the most widespread lahar deposits [START_REF] Reid | Volcano collapse promoted by hydrothermal alteration and edifice shape[END_REF].

Geophysical and geochemical evidence has been provided by a wide range of different techniques. For example, electrical tomography data exposed the extent of the hydrothermally altered zone, the location and geometry of low-strength detachment planes, and potential collapse volumes at La Soufrière de Guadeloupe [START_REF] Rosas-Carbajal | Volcano electrical tomography unveils edifice collapse hazard linked to hydrothermal system structure and dynamics[END_REF]. Hyperspectral remote sensing [START_REF] Kereszturi | Synthesis of hydrothermal alteration, rock mechanics and geophysical mapping to constrain failure and debris avalanche hazards at Mt. Ruapehu (New Zealand)[END_REF] and helicopter electromagnetic and magnetic measurements [START_REF] Finn | Three-dimensional geophysical mapping of shallow water saturated altered rocks at Mount Baker, Washington: Implications for slope stability[END_REF] have been used to map hydrothermal alteration zones to inform slope stability modeling at Mt Ruapehu (New Zealand) and Mount Baker (USA), respectively, highlighting the most likely location of future collapses. Finally, geochemical data and modeling provided by [START_REF] López | Catastrophic volcanic collapse: relation to hydrothermal processes[END_REF] suggested that hydrothermal alteration along faults can create weak sliding planes that can facilitate slope failure.

Numerical and analog modeling has also provided key insights into how hydrothermal alteration can influence volcano stability. For example, using analogue experiments in which sand-plaster mixtures and silicone were used to represent fresh and altered rock, respectively, van Wyk de [START_REF] Van Wyk De Vries | Sector collapse forming at Casita volcano, Nicaragua[END_REF] showed that a weak and ductile (hydrothermally altered) core can reproduce the deformation structures observed at Casita volcano, suggesting that hydrothermal alteration destabilized the volcano. Motivated by intensive alteration on the flanks and summit of Mt Rainer, [START_REF] Reid | Volcano collapse promoted by hydrothermal alteration and edifice shape[END_REF] examined the influence of alteration on volcano stability using a geotechnical method of 3D column limit-equilibrium slope stability analysis. Using a best-estimate distribution of the alteration, and typical values of cohesion, internal friction angle, and rock unit weight for fresh, lightly altered, and highly altered rocks, [START_REF] Reid | Volcano collapse promoted by hydrothermal alteration and edifice shape[END_REF] concluded that steep slopes and large volumes of altered and weak rock can promote potentially large gravitational failures.

Although these aforementioned studies, and others, have improved our understanding of alteration-induced volcano instability, the shape and size of the alteration zones and/or the geomechanical properties used in large-scale stability modeling are often poorly constrained or arbitrarily chosen. Indeed, there is a paucity of mechanical data for variably altered volcanic rocks for use in large-scale numerical models designed to better understand flank stability. As noted by [START_REF] Ball | Combining multiphase groundwater flow and slope stability models to assess stratovolcano flank collapse in the Cascade Range[END_REF], the scarcity of rock physical and mechanical properties for altered volcanic rocks limits the investigation of volcano stability using large-scale models. We present here a multidisciplinary study in which we first provide physical and mechanical properties for variably altered andesites from La Soufrière de Guadeloupe. We then assign laboratory-scale and upscaled (i.e. modified to account for large-scale discontinuities that are not captured in laboratory-scale samples) physical and mechanical properties to zones identified by a recent electrical survey of the dome of La Soufrière de Guadeloupe [START_REF] Rosas-Carbajal | Volcano electrical tomography unveils edifice collapse hazard linked to hydrothermal system structure and dynamics[END_REF] and perform large-scale numerical modeling to showcase the importance of hydrothermal alteration on volcano stability. Finally, we combine recent muon tomography data [START_REF] Rosas-Carbajal | Threedimensional density structure of La Soufrière de Guadeloupe lava dome from simultaneous muon radiographies and gravity data[END_REF] with our laboratory data to create a 3D compressive strength map of the volcano.

Although we use La Soufrière de Guadeloupe as a case study, we consider our salient conclusions relevant for andesitic stratovolcanoes worldwide.

2 La Soufrière de Guadeloupe (Eastern Caribbean, France) La Soufrière de Guadeloupe (Figure 1a), henceforth called La Soufrière, is located on the island of Guadeloupe in the Eastern Caribbean (Figure 1b) and is an archetype of the hazardous andesitic volcanoes that occur in many subduction zones. The current lava dome formed in 1530 CE and, since 1635 CE, there have been six non-magmatic phreatic or hydrothermal eruptions [START_REF] Komorowski | Guadeloupe. In: Volcanic Atlas of the Lesser Antilles[END_REF]. The largest and most-recent eruption occurred in 1976-1977. After months of seismic unrest, the volcano exploded suddenly and without precursory signs in 1976-1977, resulting in a volcanic crisis that led to the evacuation of about 70,000 people and serious socioeconomic consequences for the island [START_REF] Feuillard | The 1975-1977 crisis of La Soufrière de Guadeloupe (FWI): a still-born magmatic eruption[END_REF][START_REF] Hincks | Retrospective analysis of uncertain eruption precursors at La Soufrière volcano, Guadeloupe, 1975-77: volcanic hazard assessment using a Bayesian Belief Network approach[END_REF][START_REF] Komorowski | Guadeloupe. In: Volcanic Atlas of the Lesser Antilles[END_REF][START_REF] Komorowski | Improving crisis decision-making at times of uncertain volcanic unrest (Guadeloupe, 1976)[END_REF].

Volcanic unrest has been increasing since the volcano reawakened in 1992, marked by the restart of fumarolic and seismic activity [START_REF] Komorowski | Guadeloupe. In: Volcanic Atlas of the Lesser Antilles[END_REF] and, in April 2018, the largest felt volcano-tectonic earthquake since the 1976-1977 crisis (ML 4.1 or MW 3.7;[START_REF] Moretti | The 2018 unrest phase at La Soufrière of Guadeloupe (French West Indies) andesitic volcano: Scrutiny of a failed but prodromal phreatic eruption[END_REF]. The 2018 episode of accelerated unrest was interpreted as a failed phreatic eruption and raised serious concerns for the stability of the southwest flank of the dome, which shows the largest displacements (up to 9 mm/year towards the southwest) over the last 20 years [START_REF] Moretti | The 2018 unrest phase at La Soufrière of Guadeloupe (French West Indies) andesitic volcano: Scrutiny of a failed but prodromal phreatic eruption[END_REF]. The main features of the ongoing unrest include the continuous expansion of the outgassing area on top of the lava dome and the appearance of new steam-dominated fumaroles in July 2014 and February 2016 that are characterized by a deep magmatic gas component [START_REF] Brombach | Geochemistry of the thermal springs and fumaroles of Basse-Terre Island, Guadeloupe, Lesser Antilles[END_REF][START_REF] Moretti | The 2018 unrest phase at La Soufrière of Guadeloupe (French West Indies) andesitic volcano: Scrutiny of a failed but prodromal phreatic eruption[END_REF][START_REF] Villemant | Evidence for a new shallow magma intrusion at La Soufrière of Guadeloupe (Lesser Antilles): insights from long-term geochemical monitoring of halogen-rich hydrothermal fluids[END_REF]. Such expansion was accompanied by an increase in the heat output from the entire dome, which evolved from ~1.2 MW in 2010 to ~7.6 MW in 2020 [START_REF] Jessop | A multidecadal view of the heat and mass budget of a volcano in unrest: La Soufrière de Guadeloupe (French West Indies)[END_REF]. This increase in heat output is of great concern for La Soufrière as it has been recently demonstrated that volcanoes with a high heat flux are more likely to experience magmatic or phreatic eruptions [START_REF] Girona | Large-scale thermal unrest of volcanoes for years prior to eruption[END_REF].

High-flow rate thermal acid sulfate-chloride springs, associated with degassing of the underlying magmatic body, are observed on the slopes and at the base of the dome and supply hot acid fluids and CO2, but also SO2, H2S, HCl, and HF, to the above hydrothermal system [START_REF] Villemant | The memory of volcanic waters: shallow magma degassing revealed by halogen monitoring in thermal springs of La Soufrière volcano (Guadeloupe, Lesser Antilles)[END_REF]2014). These acidic fluids are (1) transferred through the dome and feed the steam-rich summit fumaroles and (2) circulate through the dome structure along listric, concave-upward structural discontinuities that drain to the southwest [START_REF] Brombach | Geochemistry of the thermal springs and fumaroles of Basse-Terre Island, Guadeloupe, Lesser Antilles[END_REF][START_REF] Rosas-Carbajal | Volcano electrical tomography unveils edifice collapse hazard linked to hydrothermal system structure and dynamics[END_REF][START_REF] Salaün | Hydrothermal alteration in andesitic volcanoes: trace element redistribution in active and ancient hydrothermal systems of Guadeloupe (Lesser Antilles)[END_REF][START_REF] Villemant | Evidence for a new shallow magma intrusion at La Soufrière of Guadeloupe (Lesser Antilles): insights from long-term geochemical monitoring of halogen-rich hydrothermal fluids[END_REF]. Evidence for the preferential circulation of acidic thermal fluids along these structures is provided by the 3D geometry of thermal springs [START_REF] Moretti | The 2018 unrest phase at La Soufrière of Guadeloupe (French West Indies) andesitic volcano: Scrutiny of a failed but prodromal phreatic eruption[END_REF][START_REF] Villemant | The memory of volcanic waters: shallow magma degassing revealed by halogen monitoring in thermal springs of La Soufrière volcano (Guadeloupe, Lesser Antilles)[END_REF] and geophysical imaging [START_REF] Bouligand | Sub-surface structure of La Soufrière of Guadeloupe lava dome deduced from a ground-based magnetic survey[END_REF][START_REF] Brothelande | Fluid circulation pattern inside La Soufrière volcano (Guadeloupe) inferred from combined electrical resistivity tomography, self-potential, soil temperature and diffuse degassing measurements[END_REF][START_REF] Nicollin | Electrical tomography of La Soufrière of Guadeloupe Volcano: Field experiments, 1D inversion and qualitative interpretation[END_REF][START_REF] Rosas-Carbajal | Volcano electrical tomography unveils edifice collapse hazard linked to hydrothermal system structure and dynamics[END_REF]. The shallow hydrothermal system is considered responsible for the recent observed shallow deformation and seismic activity (at depths at or above sea level, with magnitudes less than one) [START_REF] Moretti | The 2018 unrest phase at La Soufrière of Guadeloupe (French West Indies) andesitic volcano: Scrutiny of a failed but prodromal phreatic eruption[END_REF]. Indeed, the monthly bulletins from the observatory (Observatoire Volcanologique et Sismologique de Guadeloupe, Institut de Physique du Globe de Paris (OVSG-IPGP), 1999-2021; available at http://www.ipgp.fr/fr/ovsg/bulletins-mensuels-de-lovsg) document hundreds of shallow, lowmagnitude (M < 1) earthquakes each month. Deeper and higher magnitude off-axis seismic activity are thought to be related to increases in pore pressure in response to the pulsatory arrival of hot magmatic fluids along the larger faults that cut the dome [START_REF] Moretti | The 2018 unrest phase at La Soufrière of Guadeloupe (French West Indies) andesitic volcano: Scrutiny of a failed but prodromal phreatic eruption[END_REF].

The recent unrest, suggesting that La Soufrière is undergoing a renewed period of structural and chemical weakening, raises concern for the stability of the volcano. Indeed, the most-recent unrest and instability at La Soufrière was associated with hydrothermal activity.

For example, the materials ejected during the explosive hydrothermal eruption of [1976][1977] were largely hydrothermally altered [START_REF] Feuillard | The 1975-1977 crisis of La Soufrière de Guadeloupe (FWI): a still-born magmatic eruption[END_REF] and geological studies have also shown that La Soufrière has an exceptional record of partial edifice collapse (at least eight collapses in the past 9,150 years) that have produced extensive debris avalanche deposits dominantly composed of hydrothermally altered materials [START_REF] Boudon | Magma and hydrothermally driven sector collapses: The 3100 and 11,500 y. BP eruptions of la Grande Decouverte (la Soufriere) volcano, Guadeloupe, French West Indies[END_REF][START_REF] Komorowski | Guadeloupe. In: Volcanic Atlas of the Lesser Antilles[END_REF][START_REF] Friant | Potential flank-collapse of Soufriere Volcano, Guadeloupe, lesser Antilles? Numerical simulation and hazards[END_REF][START_REF] Peruzzetto | Modeling of partial dome collapse of La Soufrière of Guadeloupe volcano: implications for hazard assessment and monitoring[END_REF][START_REF] Rosas-Carbajal | Volcano electrical tomography unveils edifice collapse hazard linked to hydrothermal system structure and dynamics[END_REF][START_REF] Salaün | Hydrothermal alteration in andesitic volcanoes: trace element redistribution in active and ancient hydrothermal systems of Guadeloupe (Lesser Antilles)[END_REF]. The simulations of Le [START_REF] Friant | Potential flank-collapse of Soufriere Volcano, Guadeloupe, lesser Antilles? Numerical simulation and hazards[END_REF] and [START_REF] Peruzzetto | Modeling of partial dome collapse of La Soufrière of Guadeloupe volcano: implications for hazard assessment and monitoring[END_REF] suggest that the northern and eastern parts of the town of Saint Claude could be impacted by small-volume partial dome collapses and that, in the worst-case scenario of a major dome collapse, a large part of Basse-Terre could be affected. As a result, not only is La Soufrière an ideal natural laboratory to study the influence of hydrothermal alteration on volcano stability, but such a study is also timely because of the ongoing, and increasing, unrest at the volcano. A total of 17 blocks were collected from La Soufrière during a field campaign in 2019 (sampling locations shown as red stars in Figure 1b). Our aim was to sample a range of materials from different locations on the volcano that best represent the variability in porosity and hydrothermal alteration of the rocks forming the edifice. A large proportion of the blocks (eight out of 17) were collected from the collapse scar of the 19 November 2009 landslide, triggered by extreme rainfall [START_REF] Moretti | The 2018 unrest phase at La Soufrière of Guadeloupe (French West Indies) andesitic volcano: Scrutiny of a failed but prodromal phreatic eruption[END_REF][START_REF] Peruzzetto | Modeling of partial dome collapse of La Soufrière of Guadeloupe volcano: implications for hazard assessment and monitoring[END_REF], which was seen as an opportunity to sample blocks that are more representative of the interior of the volcano.

Eight blocks were taken from the collapse scar of the 2009 landslide (H2A, H2B, H3, H4A, H5A, H6, H25, and H29), one block was taken from the collapse scar of the landslide triggered by the 21 November 2004 Les Saintes magnitude Mw 6.3 regional earthquake [START_REF] Feuillet | The Mw = 6.3, November 21, 2004, Les Saintes earthquake (Guadeloupe): Tectonic setting, slip model and static stress changes[END_REF]; Figure 1b) (WP1285), two blocks were taken from the West wall of the fault "Faille 30 août" (H14 and H15), and one block, a volcanic bomb from the 1976-1977 eruption, was taken from the roof of a small disused thermal bathhouse to the South of the dome (WP1317). The remaining samples were taken from the dome: one block was taken from the "Lacroix Supérieur" outgassing fracture (H18), and four blocks were taken from the lava spines of the 1530 CE dome: two blocks from "Cratère Sud Central" (H19 and H20) and two blocks from an adjacent site (H21 and H22). Site descriptions and Global Positioning System (GPS) coordinates are available for each block in the Supplementary Information.

Cylindrical samples were prepared from each of the blocks to a diameter of 20 mm and then cut and precision-ground to a nominal length of 40 mm. The samples were washed and then dried in a vacuum-oven at 40 °C for at least 48 h. The connected porosity of each sample was calculated using the bulk sample volume and the skeletal (solid) sample volume measured by a helium pycnometer. Dry and wet bulk sample densities were calculated using the bulk sample volume and the dry and wet mass, respectively. Polished thin sections were prepared from offcuts of the samples for microstructural analysis, performed using a Tescan Vega 2 XMU scanning electron microscope (SEM). The mineral content of the 17 blocks was analyzed and quantified using X-ray powder diffraction (XRPD) on powdered offcuts of the samples, assisted by Raman spectroscopy on the prepared thin sections (see Supplementary Information for more details).

Dry uniaxial compressive strength was measured on oven-dry samples from all 17 blocks in a uniaxial load frame. Samples were deformed under ambient laboratory pressure and temperature at a constant axial strain rate of 10 -5 s -1 until macroscopic failure. The static Young's modulus was determined from the elastic portion of the uniaxial stress-strain curves (Heap et al., 2020a). Triaxial compression experiments were performed on blocks selected to best represent the variability in observed alteration (samples H2B, H3, and H18). These samples were vacuum-saturated in deionized water, inserted into a rubber jacket, placed inside a pressure vessel, and then taken to the target confining (from 10.5 to 25 MPa) and pore fluid pressures (10 MPa for all experiments) using servo-controlled pumps. Samples were deformed under ambient laboratory temperature at a constant axial strain rate of 10 -5 s -1 until either macroscopic failure (for brittle experiments) or until 3% axial strain (for ductile experiments).

The triaxial apparatus was also used to measure the water-saturated uniaxial compressive strength of samples H2B, H3, and H18. For both uniaxial and triaxial experiments, axial displacement and axial load were measured using a linear variable differential transducer and a load cell, respectively. Axial displacement (minus the displacement accumulated within the load chain) and axial load were converted to axial strain and axial stress using the sample dimensions. Here, we define the effective pressure, 𝑃 "## , as the confining pressure, 𝑃 $ , minus the pore fluid pressure, 𝑃 % . We adopt the convention that compressive stresses and strains are positive.

Microstructure, mineralogy, and alteration

The mineral contents for each of the 17 blocks are given in Table 1, and Figure 2 shows backscattered SEM images of three blocks selected to represent the range of alteration intensity (defined by their percentage of secondary minerals) observed in the rocks collected for this study (relatively unaltered -H18; moderately altered -H3; and highly altered -H2B). SEM images of all the blocks are available in the Supplementary Information. All of the andesite blocks are characterized by a porphyritic texture comprising phenocrysts of dominantly plagioclase and pyroxene (and high-density oxides) within a crystallized groundmass (Figure 2). All of the samples also contain secondary minerals, including silica polymorphs (quartz, cristobalite, tridymite, and opal-A), hematite, pyrite, alunite or natroalunite, gypsum, kaolinite, and talc (Table 1). Plagioclase phenocrysts are often pervasively altered and partially replaced by kaolinite (e.g., Figure 2c) or opal-A. Secondary minerals (natroalunite, alunite, cristobalite, tridymite, pyrite, and kaolinite) are also found precipitated within pores and microcracks (see Supplementary Information for additional SEM images).

Tridymite* - - - - - - - 0.7 - - - - - - - 13.2 13.2 Hematite* - - - - - - 3.4 - 2.8 2.4 - - - - 3.1 - - Pyrite* 3.5 - 3.8 2.3 - - - - - - - 0.4 3.1 0.6 - - - Alunite* - - - - - - - - - - - - - - - - 2.4 Na-Alunite* 1.4 1.6 2.8 1.3 5.4 5.1 5.1 15.0 - 14.2 0.5 0.5 - 9.8 - - - Gypsum* - - - 0.7 - - - - - - 0.8 1.2 - - - - - Kaolinite* 6 59.7 17.4 43.3 23.5 36.0 < 1 34.3 - 2.0 2.0 2.0 < 1 25.3 - - - Talc* - - - - - - - - - - - - 2.9 - - - - Opal-A* - - - - - - - - - 33.0 30.0 25.0 - - 10.0 - - Table 1.
Mineral contents, measured by X-ray powder diffraction and refined using Raman spectroscopy and optical microscopy, of the 17 blocks collected for this study (sampling locations shown in Figure 1b). Values in wt.%. Asterisk denotes a secondary/alteration mineral.

Mechanical data

Representative uniaxial stress-strain curves for selected samples are shown in Figure 3a. Stress is first a non-linearly increasing function of strain, often attributed to the closure of pre-existing microcracks. This stage is followed by a quasi-linear elastic stage. In the next stage, stress is a non-linearly decreasing function of strain, a result of the initiation, propagation, and coalescence of microcracks. Finally, a stress drop signals the formation of a macroscopic shear fracture. Uniaxial compressive strength and Young's modulus are plotted as a function of connected porosity in Figures 3b and3c, respectively (data available in Table 2). We first note that the connected porosity of our samples varies from 0.12 to 0.43. The uniaxial compressive strength and Young's modulus of our sample suite varies from ~3.5 to ~150 MPa and from ~1 to ~43 GPa, respectively (Figure 3). Our data show that uniaxial compressive strength and Young's modulus both decrease as a function of increasing porosity (Figure 3). Sample locations are provided in Figure 1b (site descriptions and GPS coordinates are available in the Supplementary Information).

Samples from three blocks were selected for triaxial compression experiments: a relatively unaltered block (H18), a moderately altered block (H3), and a very altered block (H2B). The microstructure of these samples is provided in Figure 2 and their mineral componentry in Table 1. The percentages of secondary minerals (determined using the XRPD data; Table 1) for these rocks are 15.2, 35.2, and 74.6%, respectively. Triaxial experiments were performed on H18 samples at effective pressures of 0, 2.5, 5, 10, and 15 MPa (Figure 4a), on H3 samples at effective pressures of 0, 2.5, 5, and 7.5 MPa (Figure 4b), and on H2B samples at effective pressures of 0, 0.5, 1, and 2 MPa (Figure 4c) (data available in Table 3). Different effective pressures were chosen for the different samples in an attempt to ensure the brittle behavior required to calculate the cohesion and angle of internal friction. Blocks H18 (Figure 4a) and H3 (Figure 4b) were brittle over the chosen range of effective pressures. The compressive strength of blocks H18 and H3 increased as a function of increasing effective pressure: H18 increased from 65.4 to 143.8 MPa and H3 increased from 31.5 to 54.8 MPa as effective pressure was increased from 0 to 15 and 7.5 MPa, respectively (Figure 4). The stressstrain curves for block H2B are very different to those for blocks H18 and H3 (Figure 4). Subtle strain softening associated with brittle behavior was observed for the experiments performed at effective pressures of 0 and 0.5 MPa, but no strain softening was seen at 1 and 2 MPa, suggestive of ductile behavior (Figure 4c).

The cohesion and angle of internal friction calculated using these triaxial data (excluding the two ductile H2B experiments), necessary for the large-scale numerical modeling, were 14.8 MPa and 46.7° for H18, 7.1 MPa and 37.7° for H3, and 0.4 MPa and 42.5° for H2B, respectively (Figure 4). The method for determining these values is explained in the Supplementary Information. Sample locations are provided in Figure 1b (site descriptions and GPS coordinates are available in the Supplementary Information) and microstructural images are provided in Figure 2.

Discussion

Hydrothermal alteration and mineral precipitation

Volcanic andesitic systems discharging acid-chloride-sulfate hydrothermal fluids [START_REF] Browne | Hydrothermal alteration in active geothermal fields[END_REF][START_REF] Ellis | Chemistry and geothermal systems[END_REF][START_REF] Taran | Acid sulfate-chloride volcanic waters; Formation and potential for monitoring of volcanic activity[END_REF] are typically characterized by either propylitic alteration, resulting from the rapid ascension of acidic fluids and the lack of time available for neutralization [START_REF] Taran | Acid sulfate-chloride volcanic waters; Formation and potential for monitoring of volcanic activity[END_REF], or intermediate to advanced argillic alteration in zones where fluids can circulate. The samples measured for this study were sourced from (1) zones proximal to summit fumarolic vents that tap the central zone characterized by the rapid ascent of steam separated from a boiling hydrothermal aquifer [START_REF] Brombach | Geochemistry of the thermal springs and fumaroles of Basse-Terre Island, Guadeloupe, Lesser Antilles[END_REF][START_REF] Moretti | The 2018 unrest phase at La Soufrière of Guadeloupe (French West Indies) andesitic volcano: Scrutiny of a failed but prodromal phreatic eruption[END_REF]) (e.g., H18-22; Table 1) and ( 2) from zones far from currently active fumarolic zones, where the shallow groundwaters are heated via conduction and mixed with Na-Cl liquids from the hydrothermal aquifer [START_REF] Brombach | Geochemistry of the thermal springs and fumaroles of Basse-Terre Island, Guadeloupe, Lesser Antilles[END_REF][START_REF] Villemant | Evidence for a new shallow magma intrusion at La Soufrière of Guadeloupe (Lesser Antilles): insights from long-term geochemical monitoring of halogen-rich hydrothermal fluids[END_REF]) (e.g., those sampled from the collapse scar of the 2009 landslide:

H2A, H2B, H3, H4A, H5A, H6, H25, and H29; Table 1) (Figure 1b).

The secondary mineral assemblage in our samples (Table 1) is essentially the same as that found in the debris avalanche deposits to the southwest of La Soufrière [START_REF] Salaün | Hydrothermal alteration in andesitic volcanoes: trace element redistribution in active and ancient hydrothermal systems of Guadeloupe (Lesser Antilles)[END_REF].

However, the debris-avalanche deposits investigated by [START_REF] Salaün | Hydrothermal alteration in andesitic volcanoes: trace element redistribution in active and ancient hydrothermal systems of Guadeloupe (Lesser Antilles)[END_REF] contain abundant smectite, whereas the samples collected for this study contain kaolinite and no smectite (Table 1). The presence of kaolinite, a key mineral in the argillic alteration facies, typically indicates fluid-rock interaction at relatively low fluid temperatures (< 150-200 °C) and a pH of ~4.5-6 (Fulignati, 2020;[START_REF] Inoue | Formation of clay minerals in hydrothermal environments[END_REF], but can also occur in the advanced argillic alteration facies (pH < 3; temperatures up to about 300 °C). The altered samples analyzed in this study therefore represent a more advanced stage of hydrolysis reactions than the smectite-bearing products from the Holocene debris-avalanche deposits described in [START_REF] Salaün | Hydrothermal alteration in andesitic volcanoes: trace element redistribution in active and ancient hydrothermal systems of Guadeloupe (Lesser Antilles)[END_REF], indicating that the rocks analyzed here were altered by hydrothermal fluids with a higher H + activity that led to complete alkali leaching.

The most abundant secondary minerals are cristobalite, amorphous silica, natroalunite, and kaolinite (Table 1). Cristobalite is a silica polymorph stable at low-pressure and hightemperature in its cubic b-form [START_REF] Schipper | Volcanic SiO2-cristobalite: A natural product of chemical vapor deposition[END_REF]. Because it is kinetically favored, metastable cristobalite is commonly found in preference to quartz in lava domes, where it precipitates from a vapor-phase enriched in magmatic components or results from groundmass devitrification [START_REF] Horwell | The nature and formation of cristobalite at the Soufrière Hills volcano, Montserrat: implications for the petrology and stability of silicic lava domes[END_REF][START_REF] Martel | Experimental Constraints on the Crystallization of Silica Phases in Silicic Magmas[END_REF][START_REF] Schipper | Volcanic SiO2-cristobalite: A natural product of chemical vapor deposition[END_REF]. Our samples contain tetragonal a-cristobalite, suggesting a final temperature lower than 240 °C, the nominal temperature for the transition from bto a-cristobalite [START_REF] Damby | The α-β phase transition in volcanic cristobalite[END_REF]. Schipper et al.

(2020) identified a lower and broader range for the b-a transition in volcanic cristobalites (~150-200 °C), which overlaps with the temperature interval inferred by the presence of kaolinite. The cristobalite in the studied samples precipitated onto pore walls (see Figure 2 and the Supplementary Information) and so we consider that it predominately formed by vapor deposition. Amorphous silica (opal-A) partially to completely replaced the plagioclase phenocrysts (see Figure 2 and the Supplementary Information), as often observed in acid-sulfate alteration zones [START_REF] Mccollom | Chemical and mineralogical trends during acid-sulfate alteration of pyroclastic basalt at Cerro Negro volcano and implications for early Mars[END_REF].

Alunite is a characteristic family of sulfate minerals produced by hypogene magmatichydrothermal and steam-heated alteration or supergene processes [START_REF] Rye | The stable isotope geochemistry of acid sulfate alteration[END_REF]. As such it is commonly found in the alteration products of rhyolitic to andesitic rock compositions, including surficial solfataric or acid hot spring environments. Alunite and natroalunite are the two most common minerals of the alunite group [START_REF] Stoffregen | Alunite-jarosite crystallography, thermodynamics, and geochronology[END_REF]. Only the volcanic bomb from the 1976-1977 eruption (sample WP1317) contains alunite, whereas natroalunite is commonly observed (Table 1). Natroalunite is the high temperature end-member of the alunite group and typically forms at 200-350 °C, but is also stable at 380-450 °C [START_REF] Henley | Magmatic-vapor expansion and the formation of high-sulfidation gold deposits: Chemical controls on alteration and mineralization[END_REF][START_REF] Meyer | Wall rock alteration in[END_REF]. Natroalunite precipitates from an acidic (pH < 4) fluid with a high sulfate content and high [Na]/[K] ratio [START_REF] Hemley | Some stability relations of alunite[END_REF][START_REF] Stoffregen | An experimental study of Na-K exchange between alunite and aqueous sulfate solutions[END_REF].

Natroalunite precipitation requires fluids with [Na]/[K] about 100 (at pH = 3; Deyell and Dipple, 2005), much higher than measured in circulating fluids and in whole rock and groundmass compositions at La Soufrière [START_REF] Boudon | A new scenario for the last magmatic eruption of La Soufrière of Guadeloupe (Lesser Antilles) in 1530 AD Evidence from stratigraphy radiocarbon dating and magmatic evolution of erupted products[END_REF][START_REF] Metcalfe | Magmatic Processes at La Soufrière de Guadeloupe: Insights From Crystal Studies and Diffusion Timescales for Eruption Onset[END_REF][START_REF] Villemant | The memory of volcanic waters: shallow magma degassing revealed by halogen monitoring in thermal springs of La Soufrière volcano (Guadeloupe, Lesser Antilles)[END_REF], suggesting that equilibrium was not obtained or that the fluids responsible are different from those discharged at the surface [START_REF] Deyell | Equilibrium mineral-fluid calculations and their application to the solid solution between alunite and natroalunite in the El Indio-Pascua belt of Chile and Argentina[END_REF]. If the fluids are different, acidic Na-Cl-SO4 fluids would be required, which form H2S and H2SO4 following the interaction between Na-Cl waters and elemental sulfur at depth [START_REF] Taran | Acid sulfate-chloride volcanic waters; Formation and potential for monitoring of volcanic activity[END_REF].

This hypothesis is compatible with the observation that highly acidic chlorine-sulfate rich fluids (pH < 1) have been rising and circulating within the dome as of early 1998, forming acid ponds within summit craters [START_REF] Komorowski | Guadeloupe. In: Volcanic Atlas of the Lesser Antilles[END_REF][START_REF] Rosas-Carbajal | Volcano electrical tomography unveils edifice collapse hazard linked to hydrothermal system structure and dynamics[END_REF][START_REF] Villemant | Evidence for a new shallow magma intrusion at La Soufrière of Guadeloupe (Lesser Antilles): insights from long-term geochemical monitoring of halogen-rich hydrothermal fluids[END_REF]. Alternative hypotheses include the interaction of Na-Cl neutral water (water/rock volume ratio >> 1) with rocks already showing advanced argillic alteration, and that the rocks collected represent old and buried solfataric fields [START_REF] Marini | The bimodal pH distribution of volcanic lake waters[END_REF] resulting from previous volcanic cycles.

We conclude that the observed secondary mineral assemblages (Table 1) are the result of intense fluid-rock interactions. Advanced argillic alteration of the shallow portion of the edifice was promoted by the efficient circulation of cooled (below 350 °C, and down to 150°C-200 °C), acidic (pH < 4) hydrothermal fluids, possibly mixed with meteoric waters. The alteration assemblages observed at La Soufrière are similar to those commonly identified at domes and craters of active volcanoes worldwide [START_REF] Heap | Hydrothermal alteration of andesitic lava domes can lead to explosive volcanic behaviour[END_REF][START_REF] Zimbelman | Origin of secondary sulfate minerals on active andesitic stratovolcanoes[END_REF][START_REF] Yilmaz | Rapid alteration of fractured volcanic conduits beneath Mt Unzen[END_REF], although the rocks from Mt Unzen volcano (Japan) were overprinted by propylitic alteration [START_REF] Yilmaz | Rapid alteration of fractured volcanic conduits beneath Mt Unzen[END_REF], not observed here. Multiple reaction paths and variable alteration intensity reflect a variable degree of reaction progress. For a given alteration rate, reaction progress will increase as exposure to altering fluids and pathway length increase, or as water flow and water/rock volume ratio decrease. Variations in these environmental parameters simply reflect the heterogeneity and structural complexity of the dome and thus the complex architecture of the hydrothermal system, as revealed by geological and geophysical data [START_REF] Brothelande | Fluid circulation pattern inside La Soufrière volcano (Guadeloupe) inferred from combined electrical resistivity tomography, self-potential, soil temperature and diffuse degassing measurements[END_REF][START_REF] Jessop | A multidecadal view of the heat and mass budget of a volcano in unrest: La Soufrière de Guadeloupe (French West Indies)[END_REF][START_REF] Moretti | The 2018 unrest phase at La Soufrière of Guadeloupe (French West Indies) andesitic volcano: Scrutiny of a failed but prodromal phreatic eruption[END_REF][START_REF] Nicollin | Electrical tomography of La Soufrière of Guadeloupe Volcano: Field experiments, 1D inversion and qualitative interpretation[END_REF][START_REF] Rosas-Carbajal | Volcano electrical tomography unveils edifice collapse hazard linked to hydrothermal system structure and dynamics[END_REF][START_REF] Tamburello | Spatio-temporal relationships between fumarolic activity, hydrothermal fluid circulation and geophysical signals at an arc volcano in degassing unrest: La Soufrière of Guadeloupe (French West Indies)[END_REF]. In particular, the complex 3D geometry

The influence of porosity and alteration on uniaxial compressive strength

New mechanical data show that the uniaxial compressive strength and Young's modulus of variably-altered andesites from La Soufrière both decrease as a function of increasing porosity (Figure 3), in accordance with previous studies on volcanic rocks (Heap et al., 2014a(Heap et al., , 2020a;;[START_REF] Heap | The mechanical behaviour and failure modes of volcanic rocks: a review[END_REF][START_REF] Mordensky | Physical and mechanical property relationships of a shallow intrusion and volcanic host rock, Pinnacle Ridge, Mt. Ruapehu, New Zealand[END_REF][START_REF] Schaefer | Geomechanical rock properties of a basaltic volcano[END_REF][START_REF] Zhu | Micromechanics of brittle faulting and cataclastic flow in Mount Etna basalt[END_REF]. We compare our new data with those previously published for andesites in Figure 5a and 5b, which show that the data for the rocks from La Soufrière are in broad agreement with those previously published. This comparison shows that the andesites forming La Soufrière are generally more porous, and therefore weaker (Figure 5a) and softer (Figure 5b), than andesites from other stratovolcanoes (the block with the lowest porosity, H22, has a porosity of 0.12). Indeed, muon tomography data have shown that the material forming the dome at La Soufrière is typically low-density (typically ≤ 1500 kg.m -3 ; [START_REF] Lesparre | Density muon radiography of La Soufriere of Guadeloupe volcano: comparison with geological, electrical resistivity and gravity data[END_REF][START_REF] Rosas-Carbajal | Threedimensional density structure of La Soufrière de Guadeloupe lava dome from simultaneous muon radiographies and gravity data[END_REF] and, therefore, likely comprises high-porosity rocks. However, we note that the relative abundance of high-porosity rocks with a low strength and a low Young's modulus from La Soufrière may be a consequence of sampling only 17 blocks. In other words, low-porosity rocks may exist at La Soufrière and, if many more blocks were sampled, maybe the data would encompass a similar range to the compiled dataset for other volcanoes. volcanoes worldwide (data from: [START_REF] Karaman | Evaluation of the influence of porosity on the engineering properties of volcanic rocks from the Eastern Black Sea Region: NE Turkey[END_REF][START_REF] Heap | Microstructural controls on the physical and mechanical properties of edifice-forming andesites at Volcán de Colima, Mexico[END_REF][START_REF] Siratovich | Physical property relationships of the Rotokawa Andesite, a significant geothermal reservoir rock in the Taupo Volcanic Zone, New Zealand[END_REF][START_REF] Wyering | Mechanical and physical properties of hydrothermally altered rocks, Taupo Volcanic Zone, New Zealand[END_REF]Heap et al., 2015a;[START_REF] Mordensky | Physical and mechanical property relationships of a shallow intrusion and volcanic host rock, Pinnacle Ridge, Mt. Ruapehu, New Zealand[END_REF][START_REF] Harnett | Evolution of mechanical properties of lava dome rocks across the 1995-2010 eruption of Soufrière Hills volcano, Montserrat[END_REF][START_REF] Kennedy | Pressure controlled permeability in a conduit filled with fractured hydrothermal breccia reconstructed from ballistics from Whakaari (White Island), New Zealand[END_REF][START_REF] Heap | The mechanical behaviour and failure modes of volcanic rocks: a review[END_REF]. Uniaxial compressive strength (c) and Of interest to the goal of this study is whether hydrothermal alteration has influenced the compressive strength and Young's modulus of the materials collected. Our triaxial data highlight that alteration (percent of secondary minerals) reduces rock cohesion, but does not systematically influence the angle of internal friction (Figure 4). Experimental studies have

shown that alteration can influence the strength of volcanic rock [START_REF] Del Potro | The decrease in the shear strength of volcanic materials with argillic hydrothermal alteration, insights from the summit region of Teide stratovolcano, Tenerife[END_REF][START_REF] Farquharson | Acid-induced dissolution of andesite: evolution of permeability and strength[END_REF][START_REF] Frolova | Effects of hydrothermal alterations on physical and mechanical properties of rocks in the Kuril-Kamchatka island arc[END_REF][START_REF] Heap | Quantifying the role of hydrothermal alteration in creating geothermal and epithermal mineral resources: The Ohakuri ignimbrite (Taupō Volcanic Zone, New Zealand)[END_REF][START_REF] Heap | The tensile strength of volcanic rocks: Experiments and models[END_REF][START_REF] Mordensky | Physical and mechanical property relationships of a shallow intrusion and volcanic host rock, Pinnacle Ridge, Mt. Ruapehu, New Zealand[END_REF], 2019;[START_REF] Wyering | Mechanical and physical properties of hydrothermally altered rocks, Taupo Volcanic Zone, New Zealand[END_REF]. The type of alteration, porosity increasing dissolution and alteration to clay [START_REF] Del Potro | The decrease in the shear strength of volcanic materials with argillic hydrothermal alteration, insights from the summit region of Teide stratovolcano, Tenerife[END_REF][START_REF] Farquharson | Acid-induced dissolution of andesite: evolution of permeability and strength[END_REF][START_REF] Opfergelt | The 1998 debris avalanche at Casita volcano, Nicaragua: Investigation of the role of hydrothermal smectite in promoting slope instability[END_REF][START_REF] Watters | Effect of argillic alteration on rock mass stability[END_REF] or porosity decreasing mineral precipitation [START_REF] Heap | Quantifying the role of hydrothermal alteration in creating geothermal and epithermal mineral resources: The Ohakuri ignimbrite (Taupō Volcanic Zone, New Zealand)[END_REF](Heap et al., , 2021)), dictates whether the alteration decreases or increases the strength and Young's modulus of the rock. Uniaxial compressive strength and Young's modulus are reduced from ~60-140 to < 10 MPa and from 20-45 to < 5 GPa as alteration increases from ~16-18 to ~75% (Figure 5c and5d).

The low uniaxial compressive strengths and Young's moduli of the two relatively unaltered blocks that form the most obvious outliers (H14 and H20) can be explained by meso-scale fractures found within sample H14 (sample photographs are provided in the Supplementary Information) and the high porosity of sample H20, respectively. Although the data in Figure 5c and 5d suggest that an increase in alteration results in a decrease in strength and Young's modulus, it is difficult to untangle the influence of porosity. In other words, are the weak samples weak because of alteration or are the more porous samples simply more altered because of their higher surface area available for rock-fluid interactions? Indeed, our data do show that, in general, alteration increases as a function of increasing porosity. Although laboratorycontrolled hydrothermal alteration experiments [START_REF] Farquharson | Acid-induced dissolution of andesite: evolution of permeability and strength[END_REF] are required to definitively answer this question, we contend here that the observed alteration served to reduce the strength and Young's modulus of these andesites from La Soufrière, regardless of their porosity, due to the presence of abundant clay minerals, phases considered to reduce the strength of volcanic rocks [START_REF] Del Potro | The decrease in the shear strength of volcanic materials with argillic hydrothermal alteration, insights from the summit region of Teide stratovolcano, Tenerife[END_REF][START_REF] Nicolas | Influence of hydrothermal alteration on the elastic behaviour and failure of heat-treated andesite from Guadeloupe[END_REF][START_REF] Opfergelt | The 1998 debris avalanche at Casita volcano, Nicaragua: Investigation of the role of hydrothermal smectite in promoting slope instability[END_REF][START_REF] Watters | Effect of argillic alteration on rock mass stability[END_REF].

5.3 Influence of alteration on the stability of La Soufrière de Guadeloupe: large-scale numerical modeling

We used the open-source hydro-thermo-mechanical modeling code LaMEM (Lithosphere and Mantle Evolution Model; https://bitbucket.org/bkaus/lamem/src/master/; [START_REF] Kaus | Forward and inverse modelling of lithospheric deformation on geological timescales[END_REF], which models the non-linear, visco-elastoplastic deformation of rocks, to investigate the influence of upscaling and alteration on the stability of La Soufrière. This model has recently been used to model large-scale volcano deformation (Heap et al., 2021a).

Mathematically, the model solves a coupled system of conservation equations, the conservation equations of momentum and mass. The various deformation mechanisms are connected in an additive constitutive relationship, where the total strain rate is the sum of the individual strain rates of elastic, viscous, and plastic deformation. LaMEM uses the Drucker-Prager failure criterion to determine the magnitude of the plastic strain rate. Numerically, LaMEM uses a staggered-grid finite difference method for discretization and a marker-and-cell method to assign and track rock properties within a Eulerian advection framework [START_REF] Harlow | Numerical calculation of time-dependent viscous incompressible flow of fluid with free surface[END_REF]. This approach enables the model to attain the large deformations required in, for example, salt tectonics, continental collision, and magmatic systems [START_REF] Baumann | Constraining effective rheology through parallel joint geodynamic inversion[END_REF][START_REF] Pusok | Development of topography in 3-D continental-collision models[END_REF][START_REF] Reuber | Unraveling the physics of the Yellowstone magmatic system using geodynamic simulations[END_REF]. The ability to model large deformations is seen here as an advantage of LaMEM over commonly-used commercial packages such as FLAC (by Itasca Consulting Group), RS2 (by Rocscience), and PLAXIS 2D (by Virtuosity). We refer the reader to [START_REF] Kaus | Forward and inverse modelling of lithospheric deformation on geological timescales[END_REF] and the Supplementary Information for a more detailed description of LaMEM.

Models were run on simplified 2D North-South cross-sections of the volcano (643 km Easting), built using the geomIO tool [START_REF] Bauville | geomIO: An Open-Source MATLAB Toolbox to Create the Initial Configuration of 2-D/3-D Thermo-Mechanical Simulations From 2-D Vector Drawings[END_REF]. The model domain was first separated into deformable and non-deformable domains, guided by geological observations and the recent electrical survey of [START_REF] Rosas-Carbajal | Volcano electrical tomography unveils edifice collapse hazard linked to hydrothermal system structure and dynamics[END_REF]. The models have a dimension of 2000 × 800 m, a model resolution of 4 × 3 m, free-slip boundary conditions at the sides, and a free surface at the top. All our models assumed an entirely liquid saturated volcano. The average topographic height was used as the reference surface for determining the hydrostatic pressure, 𝑝 * , using 𝑝 # = 𝑝 * + 𝜆 / (𝑝 1/2* -𝑝 * ), where 𝑝 # and 𝑝 1/2* are the pore fluid and lithostatic pressure, respectively, and 𝜆 / is the pore fluid pressure ratio. Following Heap et al. (2021a), we assumed a value of 0.2 for 𝜆 / (Table 4). To account for the uncertainty in 𝜆 / , additional models using 𝜆 / = 0.4 are provided in the Supplementary Information. We highlight that, although the assumption of complete volcano saturation is an oversimplification [START_REF] Ball | The hydrothermal alteration of cooling lava domes[END_REF][START_REF] Ball | Combining multiphase groundwater flow and slope stability models to assess stratovolcano flank collapse in the Cascade Range[END_REF][START_REF] Hurwitz | Groundwater flow, heat transport, and water table position within volcanic edifices: Implications for volcanic processes in the Cascade Range[END_REF], the main goal of our modeling was to understand, all else being equal, whether hydrothermal alteration is a contributing factor to the deformation observed at La Soufrière. Further, the distribution of liquid-saturated and dry zones within the volcano is currently poorly constrained. However, using the average topographic height as the reference surface resulted in a "dry" volcano summit with a pore fluid pressure of zero (see Supplementary Information for further details).

By modifying the segmentation of the deformable domain, we prepared three crosssections: (1) a homogeneous volcano, (2) a volcano containing alteration zones identified by the electrical conductivity data of [START_REF] Rosas-Carbajal | Volcano electrical tomography unveils edifice collapse hazard linked to hydrothermal system structure and dynamics[END_REF], and (3) a volcano in which we artificially enlarged the highly altered zone identified by [START_REF] Rosas-Carbajal | Volcano electrical tomography unveils edifice collapse hazard linked to hydrothermal system structure and dynamics[END_REF]. Zones within the volcano cross-sections were designated as either slightly altered (V3), moderately altered (V2), or highly altered (V1) (Figures 6a-d). Zones V3, V2, and V1 were assigned the mechanical properties of samples H18, H6, and H2B, respectively, the three blocks chosen to represent relatively unaltered, moderately altered, and highly altered rock (Figure 2; Table 1).

The laboratory data used in the modeling were: water-saturated bulk density, Young' modulus, cohesion, and the angle of internal friction. Poisson's ratio, not measured here, was assumed to be 0.2 (as recommended by Heap et al., 2020a). The model parameters for V3, V2, and V1 are provided in Table 4. As the models are dynamic, we must decide when the models reach a quasi-steady state and can be compared (see [START_REF] Baumann | Geodynamic inversion to constrain the non-linear rheology of the lithosphere[END_REF]. Here, we stop the models after an initial deformation phase that is dominated by an elastic response. For each of the three modeled scenarios, we ran one model in which we used the laboratory data and one model in which we used upscaled values of the laboratory data (Table 4). The laboratory-scale Young's modulus and Poisson's ratio were upscaled using empirical relationships for Young's modulus [START_REF] Hoek | Empirical estimation of rock mass modulus[END_REF]) and Poisson's ratio [START_REF] Vásárhelyi | A possible method for estimating the Poisson's rate values of the rock masses[END_REF] that account for rock-mass structure using the Geological Strength Index (GSI), as described in Heap et al. (2020a). Following Heap et al. (2020a), GSI was assumed to be 55 and the damage parameter in the Hoek-Diederichs equation was considered zero. The cohesion and angle of internal friction were upscaled by transforming the generalized form of the Hoek-Brown failure criterion to the Mohr-Coulomb failure criterion (as in Hoek et al., 2002). We used the mean uniaxial compressive strength of samples H18, H6, and H2B for layers V3, V2, and V1, respectively. The empirical fitting parameter 𝑚 / required for the generalized Hoek-Brown failure criterion was determined from the triaxial data (Figure 4) using RocData (Rocscience; https://www.rocscience.com). To do so, we again assumed a GSI of 55 and a damage parameter of zero. Since a depth is required to upscale the cohesion and angle of internal friction, we selected a depth half of that between the summit of the volcano and the top of the undeformable zone in the model (275 m). The upscaled model parameters for V3, V2, and V1 are provided in Table 4. A more detailed description of the upscaling methods can be found in the Supplementary Information. The model results, the velocity magnitude at the surface and the 2 nd invariant of the deviatoric strain rate tensor, which we refer to as the strain rate, within the deformable domain, are presented in Figure 6. The strain rate allows us to understand whether the deformation is widespread or localized, and the velocity at the surface can be compared to geophysical data.

V1 (highly altered) V2 (moderately altered) V3 (relatively unaltered) Laboratory-scale input parameters

Figures 6e-h and 6i-l show model runs using the laboratory-scale and upscaled values, respectively. We first highlight that our results emphasize the importance of using upscaled values for large-scale modeling. Not only are the strain rates and velocities much lower when using the laboratory-scale values, but the deformation also manifests slightly differently (Figure 6).

To discuss the influence of alteration, we will restrict our discussion to the upscaled models (Figures 6i-l). We first note that strain rates and velocities are low in the homogenously altered volcanoes, that deformation is largely restricted to the surface and the base of southern slope, and that there is little difference in strain rates and velocities when the alteration is increased from slightly to moderately altered (Figures 6i and6j). When the highly-altered zone guided by the recent electrical survey of Rosas-Carbajal et al. ( 2016) is included, the strain rates and velocities increase significantly (Figure 6k). Surface velocities increase from < 0.01 to ~0.05 mm/yr and a large listric, concave-upward sliding surface emerges, which stretches from the top of the dome to near the base of the southern flank (Figure 6k). A localized slip surface at the base of the deformable domain and two smaller slip surfaces also emerge near the base of the southern flank (Figure 6k). Therefore, the inclusion of the highly altered zone, the consequence of pervasive alteration resulting from the circulation of acid hydrothermal fluids [START_REF] Rosas-Carbajal | Volcano electrical tomography unveils edifice collapse hazard linked to hydrothermal system structure and dynamics[END_REF], has significantly increased volcano deformation rates and therefore decreased the stability of the volcano. As outlined above, recent manifestations of increased hydrothermal activity, such as fumaroles and hot springs [START_REF] Moretti | The 2018 unrest phase at La Soufrière of Guadeloupe (French West Indies) andesitic volcano: Scrutiny of a failed but prodromal phreatic eruption[END_REF][START_REF] Jessop | A multidecadal view of the heat and mass budget of a volcano in unrest: La Soufrière de Guadeloupe (French West Indies)[END_REF], also provide evidence that the measured ground deformation is, in part, due to the alteration of the rock forming the dome. Continuous and campaign Global Navigation Satellite System (GNSS) measurements over ~20 years show that the dome is sliding at a rate of 0.3-0.7 cm/yr [START_REF] Moretti | The 2018 unrest phase at La Soufrière of Guadeloupe (French West Indies) andesitic volcano: Scrutiny of a failed but prodromal phreatic eruption[END_REF]. Therefore, although our modeling captures the dynamics of the deformation of the southern flank, our simplified 2D model setup (Figure 6) underestimates the deformation measured at the volcano. This underestimation is likely the result of the presence of large-scale faults, discontinuities, extremely weak clay-rich layers, and zones of high pore pressure, not included in our modeling. Indeed, the strain rate in the southern flank increases when we increase the pore fluid pressure ratio, 𝜆 / , from 0.2 to 0.4 (see additional models presented in the Supplementary Information). Future models will aim to incorporate these features.

In a final step, and to further explore how alteration can influence volcano stability, we modeled a scenario in which the highly altered zone identified by [START_REF] Rosas-Carbajal | Volcano electrical tomography unveils edifice collapse hazard linked to hydrothermal system structure and dynamics[END_REF] was artificially enlarged to incorporate the entire southern flank (Figure 6d). Increasing the size of the alteration zone increases the surface velocities from ~0.05 to ~0.1 mm/yr and significantly increases the strain rate within the flank (Figure 6l). The slip surface that at the base of the deformable domain is now larger and has a higher strain rate, and the entire southern flank is now characterized by higher strain rates (Figure 6l). In addition, we also observe that expanding the alteration zone likely increases the potential collapse volume (Figure 6l). Based on these models, increasing the size of the highly altered zone (i.e. to a size larger than the present-day altered zone) will significantly increase the risk posed by the volcano. Deformation snapshots from additional models, presented in the Supplementary Information, show the progression of deformation for the present-day and increased-alteration scenarios from 5 to 100 kyrs (using upscaled physical and mechanical properties). These additional models also show that increasing the extent of the alteration increases the rate of volcano spreading.

Three-dimensional strength model for La Soufrière de Guadeloupe

The results of the previous section highlight the importance of combining geophysical surveys, laboratory data, and large-scale modeling to evaluate the influence of alteration on volcanic flank instability. Our 2D model geometry is based on qualitative interpretations from an electrical conductivity model to delimit different regions based on their degree of alteration.

A quantitative approach would require a petrophysical relation to convert the conductivity values to rock strength and cohesion, but such relations are highly non-unique as electrical conductivity depends on many factors including clay content, fluid temperature, and composition. Instead, rock strength is controlled to a first order by porosity, as shown in Figure 3b, and thus to the bulk rock density.

Here, we combine a 3D model of the bulk density of the present-day lava dome [START_REF] Rosas-Carbajal | Threedimensional density structure of La Soufrière de Guadeloupe lava dome from simultaneous muon radiographies and gravity data[END_REF] with an empirical power-law function relating wet bulk density and uniaxial compressive strength, 𝜎 % , where 𝜎 % = 1.326 ;<= 𝜌 ? @.AB (𝜎 % in Pa and 𝜌 ? in kg/m 3 ) to create a 3D strength map of the volcano (Figure 7). The empirical function was specifically fitted to our new data for La Soufrière (Table 2; Figure 7a). Because the distribution of liquidsaturated and dry zones within the volcano is currently poorly constrained, we chose to use the bulk density of our samples saturated with liquid water, a scenario we consider better represents the natural state of the volcano than a completely dry volcano. The 3D bulk density model of [START_REF] Rosas-Carbajal | Threedimensional density structure of La Soufrière de Guadeloupe lava dome from simultaneous muon radiographies and gravity data[END_REF] was obtained by jointly inverting gravity data and muon data acquired using three simultaneous muon detectors that scanned the lava dome from different points of view. Muon tomography is a novel technique, which uses the attenuation experienced by cosmic muons as they pass through the volcano to estimate its density [START_REF] Tanaka | Threedimensional computational axial tomography scan of a volcano with cosmic ray muon radiography[END_REF].

Whereas a single detector can provide 2D profiles of the average density of the volcano in the different directions sampled, multiple simultaneous detectors from different points of view can be used to create a 3D model of the volcano [START_REF] Bonechi | Atmospheric muons as an imaging tool[END_REF]. Soufrière showing the distribution of uniaxial compressive strength. Color scale is the Roma (seismic tomography) color scale of Fabio Crameri [START_REF] Crameri | The misuse of colour in science communication[END_REF]. UCS -uniaxial compressive strength.

Figure 7 highlights that large portions of the dome are characterized by very low values of strength (< 5 MPa). In particular, the southern flank of the volcano hosts a significant weak zone (Figures 7b and7c). This low-strength zone is coincident with the location of the main hydrothermal reservoir at La Soufrière, as exposed by electrical conductivity measurements [START_REF] Lesparre | Density muon radiography of La Soufriere of Guadeloupe volcano: comparison with geological, electrical resistivity and gravity data[END_REF][START_REF] Rosas-Carbajal | Volcano electrical tomography unveils edifice collapse hazard linked to hydrothermal system structure and dynamics[END_REF]. Based on (1) the influence of hydrothermally altered rock on edifice stability and spreading rates at La Soufrière revealed by the numerical modeling in the previous section, and (2) the large volume of low-strength rock in the La Soufrière dome highlighted by our 3D strength model, we conclude that the stability of La Soufrière is compromised by its high degree of rock alteration, and that the rate of volcano spreading and the probability of an eventual partial edifice collapse may increase solely due to the ongoing alteration caused by circulating hydrothermal fluids, and may not necessarily require intensified hydrothermal and/or magmatic activity.

Based on our 3D strength model, we can provide estimates for the volumes of rock within the lava dome below a certain strength threshold. For example, we estimate that rock with a strength lower than 1 and 10 MPa comprises minimum dome volumes of 2.7 ´ 10 6 and 36

´ 10 6 m 3 , respectively, corresponding to at least 4% and 55% of the dome above 1200 m. In other words, more than half of the La Soufrière dome could be considered to have a low strength. We highlight that these volume and percentage estimates should be considered conservative due to the positioning of the three telescopes at La Soufrière, which provides better coverage of the southern flank of the volcano [START_REF] Rosas-Carbajal | Threedimensional density structure of La Soufrière de Guadeloupe lava dome from simultaneous muon radiographies and gravity data[END_REF]. It is likely, therefore, that the volumes and percentages of weak rock within the dome at La Soufrière are higher than estimated here.

Although the strength map shown in Figure 7 is currently simplistic (for example, it is well known that strength increases with depth in the brittle field; [START_REF] Heap | Fracture and compaction of andesite in a volcanic edifice[END_REF]Figure 4), this 3D model permits the quick identification of zones within the volcano that could promote instability and more experimental data, when available, will allow us to account for depth and other factors that can influence rock mechanical behavior, such as temperature and saturation state [START_REF] Heap | The mechanical behaviour and failure modes of volcanic rocks: a review[END_REF].

Conclusions

Hydrothermal alteration is often invoked as a mechanism that greatly contributes to volcano instability [START_REF] Reid | Volcano collapse promoted by hydrothermal alteration and edifice shape[END_REF][START_REF] Van Wyk De Vries | Sector collapse forming at Casita volcano, Nicaragua[END_REF]. However, owing to the paucity of experimental data and the difficulty in accurately imaging subsurface alteration zones, the rock properties and the shape and size of the alteration zones required for large-scale modeling are often poorly constrained or even assumed. Here we use a combination of laboratory and geophysical data to numerically model the influence of hydrothermal alteration on volcano stability, using La Soufrière de Guadeloupe as a case study. Our numerical modeling suggests that hydrothermal alteration can significantly increase the deformation rate within the flanks of the volcano, and also increase the potential collapse volume.

The volcano stability modeling and 3D strength map presented herein, firsts for La Soufriere, help to validate the hypothesis forwarded by the numerous geophysical studies performed at the volcano: that the identified hydrothermal reservoir characterized by altered materials promotes volcano instability that could result in collapse [START_REF] Bouligand | Sub-surface structure of La Soufrière of Guadeloupe lava dome deduced from a ground-based magnetic survey[END_REF][START_REF] Brothelande | Fluid circulation pattern inside La Soufrière volcano (Guadeloupe) inferred from combined electrical resistivity tomography, self-potential, soil temperature and diffuse degassing measurements[END_REF][START_REF] Lesparre | Density muon radiography of La Soufriere of Guadeloupe volcano: comparison with geological, electrical resistivity and gravity data[END_REF][START_REF] Nicollin | Electrical tomography of La Soufrière of Guadeloupe Volcano: Field experiments, 1D inversion and qualitative interpretation[END_REF][START_REF] Rosas-Carbajal | Volcano electrical tomography unveils edifice collapse hazard linked to hydrothermal system structure and dynamics[END_REF], 2017). As discussed above, eventual partial edifice collapse at La Soufrière may not necessarily require intensified hydrothermal and/or magmatic activity. Not only would the debris avalanche resulting from partial or major dome collapse threaten the inhabited areas surrounding the volcano [START_REF] Friant | Potential flank-collapse of Soufriere Volcano, Guadeloupe, lesser Antilles? Numerical simulation and hazards[END_REF][START_REF] Peruzzetto | Modeling of partial dome collapse of La Soufrière of Guadeloupe volcano: implications for hazard assessment and monitoring[END_REF], but collapse could also depressurize the hydrothermal system leading to steam-driven eruptions, dangerous laterally-directed explosions, and associated high energy pyroclastic density currents, similar to the activity during the 1976-1977 volcanic crisis [START_REF] Feuillard | The 1975-1977 crisis of La Soufrière de Guadeloupe (FWI): a still-born magmatic eruption[END_REF][START_REF] Komorowski | Guadeloupe. In: Volcanic Atlas of the Lesser Antilles[END_REF]. We also highlight that the fragmentation threshold (the overpressure required for complete fragmentation upon depressurization), the proportion of fines produced as a result of fragmentation, and the ejection speed are also influenced by hydrothermal alteration [START_REF] Mayer | Experimental constraints on phreatic eruption processes at Whakaari (White Island volcano)[END_REF](Mayer et al., , 2016;;2017;[START_REF] Montanaro | Experimental investigations on the explosivity of steam-driven eruptions: A case study of Solfatara volcano (Campi Flegrei)[END_REF], reinforcing the need for a multidisciplinary approach to unravel how hydrothermal alteration affects volcanic hazards (see also [START_REF] De Moor | Insights on hydrothermal-magmatic interactions and eruptive processes at Poás Volcano (Costa Rica) from highfrequency gas monitoring and drone measurements[END_REF][START_REF] Mick | Hydrothermal alteration and sealing at Turrialba volcano, Costa Rica, as a mechanism for phreatic eruption triggering[END_REF].

Our modeling and 3D strength map therefore highlight the importance of monitoring the extent and evolution of hydrothermal alteration, and the formation of low-strength altered layers, at La Soufrière and at other active volcanoes worldwide. Such monitoring can be achieved, for example, using geophysical methods such as electrical tomography (Ahmed et al., 2018;[START_REF] Byrdina | Geophysical image of the hydrothermal system of Merapi volcano[END_REF][START_REF] Ghorbani | Complex conductivity of volcanic rocks and the geophysical mapping of alteration in volcanoes[END_REF][START_REF] Rosas-Carbajal | Volcano electrical tomography unveils edifice collapse hazard linked to hydrothermal system structure and dynamics[END_REF] and muon tomography [START_REF] Lesparre | Density muon radiography of La Soufriere of Guadeloupe volcano: comparison with geological, electrical resistivity and gravity data[END_REF][START_REF] Rosas-Carbajal | Threedimensional density structure of La Soufrière de Guadeloupe lava dome from simultaneous muon radiographies and gravity data[END_REF], near-surface seismic imaging [START_REF] Amoroso | Integrated tomographic methods for seismic imaging and monitoring of volcanic caldera structures and geothermal areas[END_REF], thermal and gas monitoring [START_REF] De Moor | Insights on hydrothermal-magmatic interactions and eruptive processes at Poás Volcano (Costa Rica) from highfrequency gas monitoring and drone measurements[END_REF][START_REF] Edmonds | SO2 emissions from Soufrière Hills Volcano and their relationship to conduit permeability, hydrothermal interaction and degassing regime[END_REF][START_REF] Jessop | A multidecadal view of the heat and mass budget of a volcano in unrest: La Soufrière de Guadeloupe (French West Indies)[END_REF][START_REF] Moretti | The 2018 unrest phase at La Soufrière of Guadeloupe (French West Indies) andesitic volcano: Scrutiny of a failed but prodromal phreatic eruption[END_REF][START_REF] Tamburello | Spatio-temporal relationships between fumarolic activity, hydrothermal fluid circulation and geophysical signals at an arc volcano in degassing unrest: La Soufrière of Guadeloupe (French West Indies)[END_REF], geological mapping (van Wyk de [START_REF] Van Wyk De Vries | Sector collapse forming at Casita volcano, Nicaragua[END_REF], deformation monitoring [START_REF] Moretti | The 2018 unrest phase at La Soufrière of Guadeloupe (French West Indies) andesitic volcano: Scrutiny of a failed but prodromal phreatic eruption[END_REF], magnetic methods (Finn et al., 2007), and remote and/or ground-based optical and spectroscopic methods [START_REF] Crowley | Mapping hydrothermally altered rocks on Mount Rainier, Washington, with airborne visible/infrared imaging spectrometer (AVIRIS) data[END_REF][START_REF] Darmawan | Morphological and structural changes at the Merapi lava dome monitored in 2012-15 using unmanned aerial vehicles (UAVs)[END_REF][START_REF] John | Characteristics, extent and origin of hydrothermal alteration at Mount Rainier Volcano, Cascades Arc, USA: Implications for debris-flow hazards and mineral deposits[END_REF][START_REF] Kereszturi | Hydrothermal Alteration on Composite Volcanoes: Mineralogy, Hyperspectral Imaging, and Aeromagnetic Study of Mt Ruapehu, New Zealand[END_REF][START_REF] Mueller | Surveying fumarole sites and hydrothermal alteration by unoccupied aircraft systems (UAS) at the La Fossa cone, Vulcano Island (Italy)[END_REF]. Further, although we document a reduction in strength as a function of alteration (Figure 5c), alteration can also increase strength and potentially promote volcanic instability by creating zones of high pore fluid pressure (Heap et al., 2021a;[START_REF] Reid | Massive collapse of volcano edifices triggered by hydrothermal pressurization[END_REF]. As noted in Heap et al. (2021a), it is important not only to map the extent and evolution of hydrothermal alteration, but also the locations in the volcano prone to porosity increasing and porosity decreasing alteration.

We have also shown that microtextural and mineralogical analysis of host-rock material erupted during phreatic explosive eruptions, as well as material from landslides and debrisavalanche deposits, can provide highly valuable insights into the nature, extent, and dynamics of alteration processes and fluid circulation in the pre-collapse edifice (see also [START_REF] Boudon | The hydrothermal system at Soufriere Hills volcano, Montserrat (West Indies): Characterization and role in the ongoing eruption[END_REF][START_REF] Komorowski | Insights into processes and deposits of hazardous vulcanian explosions at Soufrière Hills Volcano during 2008 and 2009 (Montserrat, West Indies)[END_REF][START_REF] Salaün | Hydrothermal alteration in andesitic volcanoes: trace element redistribution in active and ancient hydrothermal systems of Guadeloupe (Lesser Antilles)[END_REF]. Hence, we highlight that it is also important to quantify the mineral content of new material exposed by landslides and collapses, as well as their deposits, and to continuously monitor fluid compositions and water table fluctuations [START_REF] Hurwitz | Groundwater flow, heat transport, and water table position within volcanic edifices: Implications for volcanic processes in the Cascade Range[END_REF] with respect to the geometry of weakened low-strength structures that are characteristic of the internal framework volcanic edifices hosting hydrothermal systems. A better understanding of the spatio-temporal evolution of hydrothermal alteration, and how it can influence volcano stability using large-scale modeling, will help improve the mitigation of volcanic hazards at active volcanoes worldwide.

Figure 1 .

 1 Figure 1. (a) Photograph of La Soufrière de Guadeloupe (Eastern Caribbean, France). This

Figure 2 .

 2 Figure 2. Backscattered scanning electron microscope images of three andesites from La

Figure 3 .

 3 Figure 3. (a) Representative stress-strain curves for selected andesite samples from La

Figure 4 .

 4 Figure 4. Stress-strain curves for andesites from La Soufrière de Guadeloupe (Eastern

  of large faults and fractures in the dome allow for the efficient drainage of fluids and the compartmentalization of supergene and hypogene alteration. In such an environment:[START_REF] Heap | The tensile strength of volcanic rocks: Experiments and models[END_REF] intermediate to advanced argillic alteration will develop preferentially along discontinuities favoring fluid circulation, leading to the formation of low-strength layers dominated by clay and sulfate minerals; (2) partial edifice collapse or erosion favored in these weakened zones will expose pyrite-rich zones of the hydrothermal system below the water table and promote the deepening of supergene oxidation and the alteration aureole (as seen in the rain-triggered landslide in 2009;[START_REF] Rosas-Carbajal | Volcano electrical tomography unveils edifice collapse hazard linked to hydrothermal system structure and dynamics[END_REF]; and (3) H2S will be easily released by deeper, boiling fluids or by the disproportionation of magmatic SO2 to H2S and aqueous sulfate during the condensation of magmatic vapor plume at intermediate depths.

Figure 5 .

 5 Figure 5. (a) Uniaxial compressive strength and (b) Young's modulus as a function of

  Young's modulus (d) as a function of alteration (the percentage of secondary minerals). Symbol color indicates the connected porosity (red colors indicate low porosity and yellow colors indicate high porosity). Outliers-blocks H14 and H20-are labelled on panels (c) and (d).

  Figure 5c and 5d show plots of uniaxial compressive strength and Young's modulus, respectively, as a function of the percentage of secondary minerals for the rocks from La Soufrière. The symbols in Figure 5c and 5d are color-coded to show their porosity: red colors indicate low porosity and yellow colors indicate high porosity. Although there are two notable outliers, Figure 5c and 5d, respectively, show that uniaxial compressive strength and Young's modulus decrease as a function of increasing hydrothermal alteration.

Figure 6 .

 6 Figure 6. Results of the large-scale 2D numerical modeling designed to better understand the

Figure 7 .

 7 Figure 7. (a) Uniaxial compressive strength as a function of wet bulk sample density for

Table 2 .

 2 Table2). Connected porosity, uniaxial compressive strength, Young's modulus, and the percentage of secondary minerals (i.e. alteration) for the samples prepared for this study.

	Sample	Connected	Uniaxial	Young's modulus	Percentage of
		porosity	compressive	(GPa)	secondary
			strength (MPa)		minerals
	H2A_2	0.18	63.7	31.9	23.2
	H2A_5	0.19	80.5	25.5	23.2
	H2A_6	0.19	65.4	25.8	23.2
	H2A_11	0.19	59.3	24.4	23.2
	H2A_12	0.18	70.4	28.0	23.2
	H2B_3	0.42	6.6	1.8	74.6
	H2B_10	0.42	6.4	1.9	74.6
	H2B_11	0.41	8.7	2.3	74.6
	H2B_12	0.41	8.1	2.3	74.6
	H2B_15	0.43	4.6	1.6	74.6
	H3_3	0.16	70.3	27.3	35.2
	H3_7	0.16	69.9	24.9	35.2
	H3_8	0.16	64.8	27.5	35.2
	H3_11	0.16	59.7	24.0	35.2
	H3_13	0.16	71.2	26.2	35.2
	H4A_2	0.23	40.3	10.4	60.0
	H4A_4	0.23	42.0	10.6	60.0
	H4A_6	0.23	43.9	8.1	60.0
	H4A_8	0.23	37.2	9.3	60.0
	H4A_9	0.22	40.4	9.9	60.0
	H5A_2	0.16	88.3	26.4	42.4
	H5A_3	0.16	86.6	22.8	42.4
	H5A_5	0.16	83.9	22.5	42.4
	H5A_8	0.18	68.8	21.2	42.4
	H5A_10	0.17	74.5	23.0	42.4
	H6_6	0.18	55.1	13.5	52.7
	H6_9	0.18	68.0	18.0	52.7
	H6_9	0.18	53.0	12.5	52.7
	H6_12	0.18	64.2	17.6	52.7
	H6_13	0.18	63.0	18.3	52.7
	H14_2	0.18	65.4	26.1	23.7

Table 3 .

 3 Experimental summary for the triaxial deformation experiments (and wet uniaxial

experiments) performed on relatively unaltered (H18), moderately altered (H3), and very altered (H2B) andesites from La Soufrière de Guadeloupe (Eastern Caribbean, France).

Table 4 .

 4 Laboratory-scale and upscaled input parameters for the numerical modeling performed using hydro-thermo-mechanical modeling code LaMEM (Lithosphere and Mantle Evolution Model).

	Bulk density(kg/m³)	1900	2400	2500
	Young's modulus (GPa)	0.4	9.6	20.3
	Poisson's ratio	0.2	0.2	0.2
	Cohesion (MPa)	0.4	7.1	14.8
	Internal friction angle (°)	42.5	37.7	46.7
	Pore fluid pressure ratio	0.2	0.2	0.2
		Upscaled input parameters
	Bulk density(kg/m³)	1900	2400	2500
	Young's modulus (GPa)	0.2	3.9	8.3
	Poisson's ratio	0.3	0.3	0.3

Acknowledgements

This work was supported by the TelluS Program of INSU-CNRS ("Assessing the role of hydrothermal alteration on volcanic hazards") and ANR grant MYGALE ("Modelling the phYsical and chemical Gradients of hydrothermal ALteration for warning systems of flank collapse at Explosive volcanoes"), awarded to the first author. M. Heap also acknowledges support from the Institut Universitaire de France (IUF). We thank Tomaso Esposti Ongaro, Gilles Morvan, Christophe Nevado, Marie Cherrier, Océane Rocher, and Lisa Tomasetto. We thank the IPGP for general funding for the Observatoires Volcanologiques et Sismologiques (OVS), INSU-CNRS for the funding provided by the Service National d'Observation en Volcanologie (SNOV), and the Ministère pour la Transition Ecologique (MTE) for financial support for the monitoring of the instable flank of La Soufrière de Guadeloupe. This study contributes to the IdEx Université de Paris ANR-18-IDEX-0001. Olivier Roche, Jessica Ball, and Gabor Kereszturi are thanked for providing comments that helped improve this manuscript.

All the mechanical data collected for this study can be downloaded here: https://doi.org/10.6084/m9.figshare.14701137.v1.