The strength and stiffness of lavas, as well as their propensity to fracture and develop permeable flow networks, are all governed by microstructural heterogeneity. In vesicular andesitic–dacitic lavas from the Nevados de Chillán Volcanic Complex (NChVC) in Chile, we show that uniaxial compressive strength (UCS) and Young’s modulus (E) depend not only on connected porosity but also on the distribution, orientation, size and aspect ratio of pores. Pore fabrics were first quantified via 2D analyses (transparent sections and surface imaging) and 3D micro–computed tomography (micro-CT). Then, using cylinders with pores aligned parallel, perpendicular, and inclined to the loading direction, we performed UCS tests instrumented with acoustic emission (AE) monitoring. Loading perpendicular to the pore major axis presents the lowest strength and stiffness and triggers AE output at low stress, whereas loading parallel to the pore major axis initiates damage at higher stress, attains relatively higher strength, and culminates in macroscopic failure with higher rates of AE concentrated near the peak stress. Mixed AE patterns are observed when loading at an inclined angle to the pore major axis. These observations are consistent with the theory of microcrack nucleation and propagation controlled by stress concentration around elliptical pores. While porosity exerts a first-order control on the overall magnitude of strength and stiffness, we conclude that pore alignment, aspect ratio and directional variability fundamentally modulate failure style, damage evolution, and rock mechanical properties. Operationally, this framework anticipates preferential directions of weakness and damage evolution prior to failure, thereby supporting assessments of the mechanical stability of lava domes, levees, and scoria cones composed of high porosity materials.

As easily accessible natural resources become depleted, it is necessary to extract material from deeper levels and so mines may opt to develop a process of transition from open-pit to underground mining methods. In some cases, however, the process develops in the opposite direction where shallower resources from historic underground districts are mined by surface extraction methods. In both cases, it is necessary to maintain a crown pillar to ensure the stability of the pit and underground infrastructure. The dimensions of these crown pillars are typically designed using a combination of empirical methods and numerical modelling. In both methodologies, rocks are often treated as elastic and isotropic materials, even when they exhibit a clear direction of anisotropy caused by bedding planes, foliation, or closely spaced joints. To explore the role of this anisotropy in the stress state surrounding and within crown pillars, a series of two-dimensional finite element models were built using the code FEniCS. The results of this study show that tectonic loading leads to significantly higher compressive stresses, 2 to 4 times greater than gravitational loads alone. Tensile stress also increases notably, with values reaching almost −11 MPa compared to −1 ​MPa under gravitational loads. Therefore, the degrees of anisotropy and its orientation is likely to play a significant role in stress distribution. Our findings highlight the importance of constraining the in-situ stress, the geology of the host rock and the degree of anisotropy at laboratory scale for adequately addressing the risk of crown pillar failure and mining subsidence.

Determining the contributing factors, distribution, and volume of unstable regions in volcanic flanks is critical for constraining the susceptibility of volcanic debris avalanche generation. These are rapid-onset catastrophic events that can cause substantial damage to infrastructure and loss of life within minutes to hours. Antuco, a basaltic stratovolcano located in the Southern Andean Volcanic Zone of Chile, catastrophically collapsed at ca. 7.1 ka BP, producing a 6.4 km(3) debris avalanche deposit. Rapid post-collapse edifice growth has substantially regenerated the central cone. In this contribution, we seek to describe the lithotechnical units and rock mechanical properties of both pre- and post-collapse units of Antuco. We performed in-situ geotechnical characterization and rebound hardness measurements, complemented by sampling representative blocks of lithological units for petrological, geochemical, and geomechanical characterization. Using a geological profile, we constructed a 3D model of the present-day volcano. We incorporated empirical data into a limit-equilibrium model to simulate unstable regions and potential failure planes within the flank under static and pseudostatic conditions. Results indicate that the upper portion of the western flank is stable under static conditions, but a volume up to 0.59 km(3) could slip with horizontal peak ground accelerations of >0.58 g. In addition, dyke intrusions have the potential to increase flank instability to the point of failure, whereas changes to the water table appears insufficient.

Fluid circulation in Earth’s crust influences fault slip, mineralization, and volcanic activity. The geometry and efficiency of fluid transport depend on the orientation and magnitude of crustal stresses, but laboratory insights into this relationship remain limited. Most experiments use simplified stress states and one-directional flow measurements, missing the full three-dimensional behavior of permeability. We present results from a novel apparatus that quantifies the directional permeability of cubic rock samples when subjected to three independent principal stresses. We identify two regimes for stress-controlled anisotropic fluid flow in initially isotropic basalt. First, permeability decreases due to anisotropic crack closure, with higher flow reduction in the direction of maximum principal stress. Then, once new crack damage develops, permeability increases sharply along the intermediate principal stress direction. These results provide laboratory evidence that stress anisotropy alone can reorganize fluid pathways, with implications for crustal fluid flow during seismic rupture, volcanic unrest, and ore formation.

Caldera volcano systems in compressional tectonic settings exhibit complex deformation histories shaped by the interaction between magmatic and tectonic processes. This study presents a series of analogue experiments designed to investigate how tectonic shortening induced from regional compression, represented by a fold-and-thrust belt (FTB), interacts with caldera collapse structures. Two experimental scenarios were tested: (1) collapse followed by regional compression and (2) regional compression preceding collapse. Regional shortening after collapse yielded elliptical collapse structures with a thrust-parallel elongation of the caldera outline. The models further indicate that advancing thrusts can locally reactivate suitably orientated sections of collapse-related ring-faults at the thrust front. Conversely, collapse after regional shortening did not involve significant reactivation of pre-existing thrust faults, because their low dip angles render them sub-optimally orientated for reactivation. On the other hand, models show that a thrust-related thickening of the magma reservoir roof can inhibit ring-fault localisation and propagation and can potentially lead to a thrust-perpendicular elongation of the caldera outline. Comparisons with natural calderas in the Central Andes (e.g., Jorquera, Bellavista, Puquios and Diamante calderas) and in Japan (Akaigawa caldera) reveal similar aspect ratios and structural trends, supporting the wider relevance of the experimental findings for understanding interactions between caldera formation and regional tectonics.

Fault zones exert first-order control on crustal fluid migration, and fault intersections further enhance permeability both "structurally," via damage, and "geometrically," by partitioning crustal deformation. This study examines how fault intersections affect strain localization and stress regimes during far-field tectonic transpression through two case studies in the Southern Volcanic Zone, Chile (SVZ). Three-dimensional, temperature-dependent, visco-elasto-plastic models simulate the mechanical response of an upper crust segment containing intersecting, pre-existing, weak fault zones set immediatetly above hot, reactive and partially molten domains.Geometry Type I consists of a margin-parallel vertical fault intersected by a high-angle transverse fault (ATF), while Type II includes two orthogonal, margin-oblique vertical faults; these are inspired by the Puyehue-Cordón Caulle and Nevados de Chillán volcanic complexes (33°S–46°S), respectively. Model results reveal deformation partitioning: NW-striking faults preferentially localize shear strain, whereas NE-trending faults concentrate volumetric strain. In both geometries, fault intersections display local transitions from compressional to strike-slip or transtensional stress regimes within a roughly 2 km radius and down to depths of approximately 2 km. Type I intersections produce nearly twice the dilation of Type II, emphasizing how intersection orientations influence long-term rock damage.These modeled stress and strain patterns offer a mechanical framework for understanding the spatial distribution of volcanic and hydrothermal features, including the location of the Puyehue stratovolcano and the contrasting alignments of dikes and monogenetic cones at Nevados de Chillán. Further tests incorporating (i) plastic dilatancy and (ii) poro-elasticity illustrate their importance in controlling fluid flow toward the surface. These results provide foundation for future, more evolved self-consistent and coupled fluid-solid rheologies to help understand: (i) why upward magmatic and geothermal fluid migration in the upper crust may not be vertical under transpressional conditions, particularly near fault intersections, and (ii) the link between deformation and geoflluids flow over timescales intermediate between volcanic and geological scales.

Rock masses containing pre-existing damage are more susceptible to time-dependent deformation and failure under long-term loading. To better understand the nonlinear time-dependent deformation characteristics of fractured rock with pre-existing (initial) damage, a damage evolution model that considers the combined effects of initial macro- and mesoscopic damage is proposed. Based on this, a novel nonlinear creep damage model for fractured rock has been developed. This model accounts for the influence of initial damage on creep deformation and effectively captures the decelerating–accelerating characteristics of brittle creep in fractured rock. Subsequently, the effects of initial macroscopic damage on the creep damage behavior of sandstone were analyzed through uniaxial compression and stress-stepping creep loading tests on sandstone specimens with macroscopic fractures at various inclinations. The relationship between initial macroscopic damage and the long-term strength was also investigated. Finally, the proposed model was evaluated against laboratory creep data of fractured rock with different initial macro- and mesoscopic damage levels, demonstrating its good applicability. Overall, this study establishes a combined analysis framework for creep damage that incorporates both initial macro- and mesoscopic damage, and provides a new approach for long-term performance evaluation of geotechnical engineering in complex environments.

Active slip partitioning between the subduction megathrust and the upper plate is investigated in the oblique-convergence setting of the Nazca-South American plate boundary between 33° and 47° S. This segment has two major along-strike bends: the Maipo Orocline (~34° S) and the Arauco Peninsula (~38° S), whereas south of 38° S, lies the intra-arc Liquiñe-Ofqui Fault System (LOFS). Here we examine long- and short-term upper-plate deformation by combining a harmonized catalog of about 2,300 fault-slip measurements in the forearc and arc regions, from which we derive P-T axes using kinematic inversions, along with an integrated seismological database for upper-plate events (1976-2025), including global and local networks. These data are categorized by forearc, arc, and back-arc regions. We identify four distinct tectonic segments based on the spatial distribution of P and T axes in the long-and short-term: (1) 33°-34° S, showing both ~E-W and ~N-S subhorizontal shortening in the forearc and arc areas, suggesting active radial shortening; (2) 34°-37° S, dominated by mostly blind, seismogenic, margin-parallel dextral faults along with NW- and NE-trending structures running at a high angle with respect to the plate margin (called transverse faults here); (3) 37°- 41° S, where margin-orthogonal subhorizontal shortening in the submerged forearc coexists with nearly margin-parallel shortening in the emerged forearc, with contemporaneous dextral slip along the LOFS and ~E-W shortening accommodated by transverse NE-trending dextral and NW-trending sinistral seismically active faults; and (4) 41°-47° S, a region governed mainly by the geometry and kinematics of the LOFS strike-slip duplex. Kinematic indicators on mesoscopic faults (mostly slickenfibers) and a lack of pseudotachylytes suggest a considerable aseismic component to upper-plate fault slip, implying that morphotectonic slip rates may overestimate seismic hazard. Future detailed geodetic data may help better constrain the relative contributions of aseismic and seismic slip on the upper-plate faults in this Andean segment.

The expansion toward deep mining poses new geomechanical challenges. To understand the deformation and seismic behavior of rocks under conditions at mining relevant depths and under elevated stress prior to failure, holistic characterization of stress-induced anisotropy should be performed. To do so, we designed a suite of triaxial tests under confining pressures of 25 MPa, at ambient temperature conditions, on 100 mm × 40 mm cylinders of five representative lithologies of the El Teniente underground mine: a sericitic breccia, a veined andesite, a tonalite, diorite, and finally a dacite. We contemporaneously recorded the output of acoustic emissions (AE: a laboratory analog to earthquake activity) and performed active seismic surveys to understand the evolution of compressional wave velocities in six different directions. Both the mechanical and seismic responses differed between the lithologies. The tonalite exhibited the highest strength, while the breccia was the most compliant of the samples. The tonalite, dacite, and veined andesite samples all exhibit extremely brittle behavior with very little acoustic-emission output prior to failure. However, while much of the deformation prior to failure, in these rocks, was apparently accommodated with little AE output and predominantly elastically, we noted directionally dependent changes in P-wave velocity (Vp) at up to 73

In geothermal systems the thermo-physical properties of the rocks change as they interact with fluids passing through the volcanic system and during discrete events such as earthquakes and magma intrusion. To characterize a geothermal system and the flow of fluid through a sub-volcanic complex, targeted rock physical tests are needed for the rocks of the area conducted at natural P-T conditions. Here, we present rock property characterization of the main geological units of the active Nevados de Chillan Geothermal System, located in the Southern Volcanic Zone (SVZ), an area with some of the largest geothermal potential in the Andes. Six representative blocks of the geothermal host reservoir and overlying strata were collected from the volcanic basement. The main geomechanical units of this system are (from oldest to youngest): 1) andesites, tuffs and breccia of the Cura-Mallin Formation (Miocene country rocks); 2) granodiorites and diorites of the Santa Gertrudis Bullileo Batholith (15.7 Ma and 5.9 Ma, respectively); and 3) hornfels from the contact between the granitoids and country rocks. Cylindrical core samples (26 mm diameter x 65 mm length) from each block were used to quantify density, porosity, and ultrasonic wave velocities at different confining pressures. All tests were carried out at the Rock Deformation Laboratory, University of Manchester. Polished thin sections were prepared from blocks of the same orientation as the cored directions and analyzed using petrographic.Granodiorite has the lowest porosity at between 2%) to the granodiorite and 4.8 to 5.7 km/s P-wave velocities and 2.7 to 3.3 km/s S-wave velocities. The andesitic lavas have porosities ranging 3-7%, while the tuffs and breccias have porosities of 12-30%. Elastic waves velocities in the andesitic lavas are around 2 km/s faster than the pyroclastic rocks.Tests with cycles of increasing and decreasing hydrostatic pressure (up to approximately 150 MPa) show that granodiorite and diorite exhibit sharp increases in P-wave velocity (Vp). This is attributable to the stiffening of the rock from the progressive closure of pre-existing cracks. Above 40 MPa, the rate of increase in Vp with pressure reduces markedly, implying that the remaining porosity is less compliant. This is consistent with the maximum burial depth of the rocks suggesting that those cracks formed because of bringing the rocks to the surface.Finally, in terms of microstructural observations, the granodiorites, diorites and hornfels have large intragranular and intergranular fractures with very high aspect ratio, which are commonly oriented and therefore impart anisotropy. In contrast, the andesitic, tuffs and breccias porosity is higher than the crystalline rocks and is mainly composed of intergranular pores with low aspect ratios and relatively isotropic.

It has been suggested that fracture and fault intersections promote enhanced transport of fluids in the brittle crust by forming zones of increased permeability. However, the underlying mechanisms that control the emplacement of magma at fault intersections remain poorly understood. To better understand the relation between magma emplacement, volcano development and fault zone intersections, we examine the Nevados de Chillan Volcanic Complex (NChVC, 36.8 degrees S) in the Southern Andean Volcanic Zone. The complex is thought to be located atop the intersection between two sets of NE-right lateral strike-slip faults and a seismically active regional scale NW- oriented inherited structure, also interpreted as a regional fault zone. We collected data on the orientation and frequency of tens of dykes and thousands of fractures, at the volcano scale, from representative outcrops using three-dimensional digital image correlation techniques, with images taken from Unmanned Aerial Vehicles (UAVs). We use these data to generate a conceptual model of the response of the different fracture sets to regional loads and the potential consequence in terms of magma emplacement. In our conceptual model, N-S to NW-SE striking fractures become reactivated by fault intersection-related local stress field rotations. This, in turn, favors NW-SE aligned magma emplacement, and the evolution of NW-SE aligned volcanoes. Our findings provide a mechanical explanation for rotated magma emplacement pathways, which do not necessarily require a transient stress state imposed by unlocking the megathrust.

Numerical models can be utilized to understand and anticipate the future behavior of a geothermal reservoir, and hence aid in the development of efficient reservoir engineering strategies. However, as each system has a unique geological context, individual characterization is required. In this research, the Nevados de Chillan Geothermal System (NChGS) in the Southern Volcanic Zone of the Andes is considered. The NChGS is controlled by the geology of the active Nevados de Chillan Volcanic Complex (NChVC) including their basement units (Miocene lavas and volcaniclastic layers from Cura-Mall & iacute;n Formation and the Miocene, Santa Gertrudis granitoids) as well as the key structural control from crustal scale faults, all of which combine to influence the reservoir characteristics. The presence of faults acts to generate a high secondary permeability which favors the circulation of hydrothermal fluids. Based on previous studies in the NChGS, we designed a thermo-hydraulic model in COMSOL Multiphysics (R) combining equations of heat transfer and Darcy's law in order to determine the distribution of isotherms and surface heat flux. The boundary conditions of the model were informed by a conceptual model of depth 3 km and width of 6.6 km which considers a highly fractured granitic reservoir, a clay cap behavior of Miocene lavas and volcaniclastic units, and transitional zones between a regional zone and the reservoir. A lowangle reverse fault affecting the clay cap unit was also incorporated into the models. Results indicate convective behavior in the reservoir zone and a surface heat flux of 0.102 W/m2 with a local peak up to 0.740 W/m2 in the area affected by the low-angle reverse fault zone. The models suggest hydrothermal fluid residence times of around 9-15 thousand years are required to reach a steady-state thermal configuration, which is consistent with the deglaciation age proposed for the NChVC latitude of the complex (c. 10-15 ka). Permeability in the fractured reservoir is one of the most complex parameters to estimate and the most sensitive and hence requires further constraint. Finally, using the volumetric method and the results obtained in this research, we estimate a geothermal potential of 39 +/- 1 MWe for the NChGS.

Shallow-crustal magma transport occurs mainly via dykes and inclined sheets, which may or may not reach the surface to erupt. Originating from various magma sources, dyke propagation is primarily controlled by magma overpressure, magma rheology, local stress fields and the mechanical properties of the host rock, shaping the complex spatio-temporal evolution of transcrustal plumbing systems. Here, we use finite element method numerical models to investigate how shallow-crust heterogeneities influence dyke pathways at Santorini volcano (Greece). In our models, subvertical dykes predominantly ascend from the roofs of sill-like magma chambers, whereas inclined sheets emerge from lateral chamber ends and occasionally reach the surface beyond the caldera. Our results show that layered systems with contrasting mechanical properties and vertically stacked magma storage promote stress rotations that favour dyke and sheet arrest. The initial site of dyke injection strongly controls whether magma propagates vertically or along an inclined trajectory, emphasizing the role of chamber depth, crustal heterogeneity, and regional stress in magma pathways and recharge locations. This study enhances our understanding of potential shallow and deep magma pathways providing new insights into future unrest episodes at Santorini volcano.

In room-and-pillar mining, pillar stability depends on both the mechanical properties of the rock mass and the excavation geometry. While empirical methods typically assume homogeneous and isotropic conditions, many stratiform ore deposits are intersected by igneous intrusions, introducing heterogeneity. This study presents a comprehensive parametric analysis, implemented in COMSOL Multiphysics, to quantify the effect of dyke geometry, stiffness contrast, and contact strength on the stress distribution within rock pillars. Using the Arqueros Mine in Chile as a case study, we demonstrate that lithological contacts act as stress raisers and can locally increase the maximum compressive stress by up to 58

The study of dyke swarms is crucial for understanding the processes of magma intrusion and the tectonic environments that influence volcanic activity. Dyke swarms are essential records of tectonic and magmatic events, offering insights into magma ascent, chamber dynamics, and stress conditions during emplacement. In this context, the dyke swarm of the central part of the Saghro Massif (Eastern Anti-Atlas) is investigated for the first time, using structural aspects of the dykes and paleo stress inversion to constrain magma overpressure, the depth of magma reservoir, the state of stress during emplacement, and the tectonic regime responsible for emplacement. We selected seventy-eight mafic and intermediate dykes in the swarm of the Central Part of the Saghro Massif (CPSM), characterized by different orientations N-S to NNE-SSW, NE-SW, E-W and NW-SE. In addition, the size distribution of thicknesses and lengths follows a power-law and a log-normal distribution respectively. We calculate magmatic overpressure using selected dyke aspect ratios to estimate the depth of the magma reservoir. This study suggests that the CPSM dykes form mainly due to the injection of magma from a deep magma reservoir at depths >31 km, hence close to the Moho crust-mantle boundary (31-33 km). Paleo stress reconstruction shows that the CPSM dyke swarm was emplaced when the minimum principal compressive stress (63) was oriented WNW-ESE, and the maximum principal compressive stress (61) was vertical. The reconstruction also shows that the emplacement of the CPSM swarm occurred during a transtensional tectonic regime, associated with the WACadomian orogeny.

We have developed a new True Triaxial Apparatus (TTA) for rock deformation consisting of six servo-controlled loading rams that transmit maximum stresses of 220 MPa in the two horizontal axes and 400 MPa in the vertical axis to 50 mm side cubic rock samples. The sample and loading platens are introduced in a steel vessel where rock specimens can be subjected to up to 60 MPa of confining pressure, and pore fluids line connected to two pump intensifiers allow for highly accurate permeability measurements along the three loading axes. We present a suite of Finite Element Method (FEM) models implemented to determine the conditions and loading configuration that minimise the loading boundary effects during true triaxial loading. These observations are generic and we expect they will contribute to the development of true triaxial loading systems generally. Finally, we validate our experimental configuration by presenting results on permeability measurements along the three axes on cubic samples of three types of well studied rocks: Darley Dale sandstone, Crab Orchard sandstone, and Etna basalt.

The spatial distribution of surface geothermal manifestations and hydrothermal alteration reflects the complex interplay of multiple factors controlling fluid circulation. This study aims to understand the different controls on the distribution of surface hydrothermal alteration, using the Nevados de Chill & aacute;n Geothermal System (NChGS) as a case study. Lineament maps were created at 1:2500 and 1:5000 scales, and orientations of faults, fractures, and veins were measured. The pH measured ranges from 2.6 to 5.4. Surface temperature was measured and analyzed via Inverse Distance Weighted (IDW) interpolation in ArcGIS (R). Surface hydrothermal alteration was mapped using drones, and minerals were identified through X-Ray Diffraction of whole rock and clay analysis, including chalcedony, opal, native sulfur, illite, kaolinite, iron oxides, sulfates, and hydrated sulfates. Illite crystallinity measured using the Full Width at Half Maximum index (FWHM) ranged from 0.05 to 1.77, while kaolinite crystallinity measuring using Aparicio-Gal & aacute;n-Ferrell index (AGFI), ranged from 0.73 to 1.15. The results show that the distribution of high temperatures (reaching 95 degrees C) and intense advanced argillic hydrothermal alteration is heterogeneous, controlled by lithological contact and the interaction between similar to E-W-trending lineaments associated with subvertical faults and NNW-striking surficial low-angle faults. Circulation of acid-sulfate waters rich in Fe, Al, and Cu occur primarily along fault/fracture networks, promoting the formation of high-crystallinity illite and kaolinite at the surface. This study proposes a link between surface geothermal manifestations with fault-fracture network and lithological controls within a fractured geothermal system in the Southern Andes.

Detailed structural analysis from representative outcrops is necessary to characterize geothermal reservoir dynamics. Here, we estimate fracture density and intensity, as well as the dimensional properties of individual fault and fracture sets in basement rocks of the Nevados de Chillan Geothermal System. We identified several important structural features that could be responsible for controlling local fluid flow; the high-angle sinistral Las Trancas Fault as well as a series of low-angle reverse faults within the Las Termas-Olla de Mote Fault system. Most fractures identified strike either NE-SW, NNE-SSW, and NNW-SSE. Analysis of fault-slip data, supported by seismicity, indicates the presence of a main transtensional regime with subhorizontal NE-trending sigma 1. Structures sub-parallel to the present-day local maximum horizontal stress show significant dilation tendencies, whilst NW- SE fractures are less prone to dilation. NE and E-W high angle faults could be primary conduits facilitating the upward migration of hot fluids from reservoirs within crystalline and fractured rocks. The fracture length distribution was analysed using power law, negative exponential, and log-normal distribution. The power law with a scaling exponent of about-3 provides the best fit to the data. This study advances our understanding of the structural control of the geothermal reservoir and its associated fracture-controlled fluid circulation and thereby improves the prospectivity in the region by quantifying the optimum fracture sets for fluid flow.

Steep slopes, excessive volcanic edifice volume, weathering and/or alteration generating weak substratum, and magmatic-volcanic or seismic activity are known factors that all control volcano flank instability. These factors are critical in generating volcanic debris avalanches, which are rapid-onset catastrophic events that can cause substantial damage to infrastructure and loss of life within minutes to hours. Despite this, volcano hazard maps of Andean volcanoes do not specifically incorporate analysis of zones or areas prone to flank collapse. In a new project (Fondecyt 11241126), we aim to determine the distribution and geometry of potential sites of flank instability, and their controlling factors, that may produce potential future volcanic debris avalanches at the Chillán Viejo (3,195 m asl) and Antuco (2,979 m asl) volcanoes. In 1883, the upper south-southeast flank of Chillan Viejo volcano collapsed after an eruptive cycle, producing a 600 m-length scar. Comparatively, at ca. 7 ka BP, the western flank of Antuco collapsed catastrophically, producing a 6.4 km3 debris avalanche deposit [1]. Rapid edifice growth and regeneration at both volcanoes demands the assessment of future flank collapse scenarios. In the field, we sampled representative lithological units of both fresh and hydrothermally altered rocks for petrological and geochemical characterisation. Simultaneously, we performed rebound tests in substratum units, lava flows, and intrusive bodies using N-type Schmidt test hammers. These measurements were complimented by the construction of three-dimensional outcrop models from Unmanned Aerial Vehicle surveys for structural analysis. The samples collected were analysed in the laboratory in order to constrain the physical and mechanical rock properties of both intact and hydrothermally altered blocks. Preliminary results are compared with textural and compositional features of the effusive products to better understand the mechanical behaviour of both volcanic edifices, identify potential sites of future collapse, and ascertain potential collapse drivers.This is a contribution to Fondecyt iniciación 11241126 and the European Research Council (ERC) SYNERGY grant 101118491 "ROTTnROCK". References:[1] Romero, J. E., Moreno, H., Polacci, M., Burton, M., & Guzmán, D. (2022). Mid-Holocene lateral collapse of Antuco volcano (Chile): debris avalanche deposit features, emplacement dynamics, and impacts. Landslides, 19(6): 1321-1338.

Collapse calderas result from subsidence of a magma reservoir roof during large-volume eruptions. Whilst calderas form in various tectonic settings, it is unclear how regional ("far-field") forces influence caldera fault nucleation, orientation and architecture. Furthermore, although the presence of a pore fluid is known to reduce the effective stress, it is typically neglected in past caldera collapse models. Utilizing two-dimensional Distinct Element Method (DEM) models, we explore the influences of regional stress and pore fluid pressure on the evolutions of stress, strain and faulting during caldera subsidence. We simulate a shallow magma volume as an inviscid inclusion within a homogeneous crust and decrease the inclusion's pressure to model magma withdrawal. Results reveal that the critical underpressure needed to trigger collapse is reduced in extensional regimes, particularly in fluid-saturated conditions, due to lowered frictional resistance on faults. We observe three progressive deformation stages: initial fracturing at the reservoir roof, collapse onset, and complete roof failure. The geometry of faults depends on the tectonic setting, with extensional conditions favoring steeper fault dips and compressional settings promoting shallower, outward-dipping reverse faults. Models simulating a fluid-saturated crust exhibit similar effects to those models that simulate lower strength materials. This study highlights the need to account for regional stress states and crustal properties in volcanic hazard assessment, especially in caldera systems with complex hydrothermal or tectonic influences. Our findings are compared with recent collapse episodes, underscoring the utility of DEM modeling in understanding crustal responses to magma depletion.