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Aravena, Álvaro
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- PublicationOpen AccessEvolution of Conduit Geometry and Eruptive Parameters During Effusive EventsThe dynamics of effusive events is controlled by the interplay between conduit geometry and source conditions. Dyke‐like geometries have been traditionally assumed for describing conduits during effusive eruptions, but their depth‐dependent and temporal modifications are largely unknown. We present a novel model which describes the evolution of conduit geometry during effusive eruptions by using a quasi steady state approach based on a 1‐D conduit model and appropriate criteria for describing fluid shear stress and elastic deformation. This approach provides time‐dependent trends for effusion rate, conduit geometry, exit velocity, and gas flow. Fluid shear stress leads to upward widening conduits, whereas elastic deformation becomes relevant only during final phases of effusive eruptions. Simulations can reproduce different trends of effusion rate, showing the effect of magma source conditions and country rock properties on the eruptive dynamics. This model can be potentially applied for data inversion in order to study specific case studies.
311 26 - PublicationOpen AccessScenario-based probabilistic hazard assessment for explosive events at the San Salvador volcanic complex, El Salvador(2023)
; ; ; ; ; ; ; ; ;; ; ; ; ; ;We present a scenario-based, probabilistic hazard assessment for the San Salvador volcanic complex (SSVC), a volcanic field located in the vicinity of San Salvador that includes the El Boquer´on stratovolcano and 25 monogenetic vents. We define a set of likely eruption scenarios for tephra fallout and pyroclastic density currents (PDCs). The eruption scenarios range from violent Strombolian eruptions with a significant uncertainty in source position to sub-Plinian and Plinian activity fed from the central cone. The adopted methodology is mainly based on numerical modeling using Tephra2 (adopting the software TephraProb) to study tephra fallout and the branching box model and the branching energy cone model (adopting the programs BoxMapProb 2.0 and ECMapProb 2.0) to describe inertial and frictional PDCs, respectively. Despite the dominant W-WSW-trending winds, numerical results show that Plinian eruptions at El Boquer´on volcano are able to deposit thick tephra layers in the metropolitan area of San Salvador city, likely reaching mass loads of the order of 100 kg/m2 (conditional probability of 50%). The simulated sub-Plinian events highlight the seasonal influence of wind patterns. In fact, the conditional probability of significant tephra sedimentation in San Salvador city is strongly reduced when eruptions occur during the rainy season. Numerical modeling of violent Strombolian eruptions is performed considering uncertainty in vent position. Results show that the conditional probability of depositing tephra mass loads higher than 10 kg/m2 at a given point reaches a maximum value of ~7% on the NW flank of the volcano, at about 8 km from the central crater. On the other hand, very low conditional probabilities (<1%) are obtained for San Salvador city for any relevant threshold (10 kg/m2 or more) of tephra mass load during violent Strombolian events. Regarding PDCs, results show that those produced during large-scale Plinian eruptions are able to invade significant areas of the volcano surroundings, including San Salvador city. PDCs generated from the partial collapse of a sub-Plinian eruption column exhibit maximum inundation probabilities on the N, W and S flanks of the volcano. Cerro El Picacho exerts a significant shield effect on the propagation of these PDCs, with low inundation probabilities for San Salvador city (<3%). Finally, coupling published vent opening probability maps and numerical modeling of small-scale PDCs yields maximum inundation probabilities on the NW flank of the volcano, reaching maximum conditional probabilities of the order of ~10% and values of about 5% near the village of Nuevo Sitio del Nino.82 13 - PublicationOpen AccessPyroclastic density currents and tephra fallout hazard assessment at Tungurahua volcano, Ecuador: hazard maps with uncertainty quantificationPyroclastic currents (PCs) and tephra fallout are among the major volcanic hazards at explosive volcanoes and have been widely studied over the past decades in order to model the physical processes controlling them. The aim of such efforts is using numerical models for producing probabilistic hazard maps, and complementing such maps with a quantification of the major sources of uncertainty. In this contribution we chose Tungurahua volcano (Ecuador) as a case-study for producing hazard maps for both PCs and tephra fallout for two eruption types (VEI 3 and 4). Concerning PCs we adopt the models ECMapProb 2.0 and BoxMapProb 2.0; the first model is based on the energy cone assumption, while the second is a “box model” integral model. Both follow a tree-branching approach to enhance the channeled features of the flows. We implement structured and reproducible strategies to calibrate input parameters on the data of past eruptions, by considering well-documented benchmark PC-forming events. We compare the hazard maps derived from the application of different calibration metrics and models. Concerning tephra fallout, we perform Monte Carlo simulations coupling the plume model PLUME-MoM and the tephra dispersal model HYSPLIT. First, we quantify the average under/overestimation of the thickness outputs in a particular scenario. Then we sample the uncertainty distributions of the input parameters in various eruptive scenarios (total fallout mass, eruption duration, average plume height). We compare the results based on different sampling strategies, in which we sample two of the inputs and infer the third from them. This enables the replication of input correlation structures. Finally, we describe the hazard maps of the two phenomena separately and then we discuss them in terms of the implications of their combination. Our results provide the quantitative basis for a multi-hazard assessment that may enable better operative decisions to face future eruptive crises.
69 23 - PublicationOpen AccessTree‐Branching‐Based Enhancement of Kinetic Energy Models for Reproducing Channelization Processes of Pyroclastic Density Currents(2020)
; ; ; ; ; ; ; ; ; ; ; Kinetic energy models, also called kinetic models, are simple tools able to provide a fast estimate of the inundation area of pyroclastic density currents (PDCs). They are based on the calculation of the PDC front kinetic energy as a function of the distance from a source point. On a three‐dimensional topography, the PDC runout distance is estimated by comparing the flow kinetic energy with the potential energy associated with the topographic obstacles encountered by the PDC. Since kinetic models do not consider the occurrence of channelization processes, the modeled inundation areas can be significantly different from those observed in real deposits. To address this point, we present a new strategy that allows improving kinetic models by considering flow channelization processes, and consists in the inclusion of secondary source points in the expected channelization zones, adopting a tree branch‐like structure. This strategy is based on the redistribution of a key physical variable, such as the flow energy or mass depending on the considered kinetic model, and requires the adoption of appropriate equations for setting the characteristics of the secondary sources. Two models were modified by applying this strategy: the energy cone and the box model. We tested these branching models by comparing their results with those derived from their traditional formulations and from a two‐dimensional depth‐averaged model, considering two specific volcanoes (Chaitén and Citlaltépetl). Thereby, we show the capability of this strategy of improving the accuracy of kinetic models and considering flow channelization processes without including additional, unconstrained input parameters.976 5 - PublicationRestrictedNumerical modeling of magma ascent dynamicsIn the world, volcanic systems exhibit a wide range of eruption styles threatening the lives of millions of people. Relatively slow effusive eruptions generate lava flows (low viscosity magma) and lava domes (high viscosity magma) and tend to evolve over days to decades. Alternatively, explosive eruptions can inject very large volumes of fragmented magma and volcanic gas high into the atmosphere over shorter periods (minutes to weeks to months). Mitigation of the associated risk to populations, the built environment, and the cultural heritage relies upon our ability to accurately assess volcanic hazards, and this, in turn, depends on our understanding of the processes that control the style and scale of volcanic eruptions. To this end, technological developments over the last couple of decades have greatly improved our ability to characterize magmatic systems and detect precursors at high spatial and temporal resolution through the use of analytical and observational volcanology, including monitoring-derived data, and volcano geophysics. Numerical modeling of magma ascent can serve to link all of these data and processes to build effective near-real-time strategies. The complexity of the volcanic system, derived from the multiphase, multicomponent character of the magmatic mixtures and from their interaction dynamics with the surrounding host rocks, is however manifested in the complexity of its mathematical representation, and numerical models able to describe several interdependent processes, eventually at disequilibrium conditions, are required to capture the nature of volcanic systems with fidelity. In this chapter, we present the main equations governing magma ascent, highlighting the multiphase and disequilibrium nature of volcanic flows, and the presence of complex feedback mechanisms between gas exsolution, outgassing, and crystallization that are able to influence the most important characteristics of the resulting volcanic events. Then, a suite of numerical simulations is described to show the effect of some parameters and processes in controlling eruption style and scale, and thus the potential eruption hazard.
61 11 - PublicationOpen AccessThematic vent opening probability maps and hazard assessment of small-scale pyroclastic density currents in the San Salvador volcanic complex (El Salvador) and Nejapa-Chiltepe volcanic complex (Nicaragua)(2021)
; ; ; ; ; ; ; ; ; ;; ; ; ; ;The San Salvador volcanic complex (El Salvador) and Nejapa-Chiltepe volcanic complex (Nicaragua) have been characterized by a significant variability in eruption style and vent location. Densely inhabited cities are built on them and their surroundings, including the metropolitan areas of San Salvador (∼2.4 million people) and Managua (∼1.4 million people), respectively. In this study we present novel vent opening probability maps for these volcanic complexes, which are based on a multi-model approach that relies on kernel density estimators. In particular, we present thematic vent opening maps, i.e., we consider different hazardous phenomena separately, including lava emission, small-scale pyroclastic density currents, ejection of ballistic projectiles, and low-intensity pyroclastic fallout. Our volcanological dataset includes: (1) the location of past vents, (2) the mapping of the main fault structures, and (3) the eruption styles of past events, obtained from critical analysis of the literature and/or inferred from volcanic deposits and morphological features observed remotely and in the field. To illustrate the effects of considering the expected eruption style in the construction of vent opening maps, we focus on the analysis of small-scale pyroclastic density currents derived from phreatomagmatic activity or from low-intensity magmatic volcanism. For the numerical simulation of these phenomena we adopted the recently developed branching energy cone model by using the program ECMapProb. Our results show that the implementation of thematic vent opening maps can produce significantly different hazard levels from those estimated with traditional, non-thematic maps.310 9 - PublicationRestrictedStability of volcanic conduits during explosive eruptionsGeological evidences of volcanicconduitwidening arecommonin mostpyroclastic deposits (e.g. presence oflith- ic fragments from different depths), suggesting a continuous modi fi cation of the conduit geometry during volca- nic eruptions. However, the controlling factors of the mechanisms driving conduit enlargement (e.g. erosion, local collapse) are still partially unclear, as well as the in fl uence of conduit geometry on the eruptive dynamics. Although numerical models have been systematically employed to study volcanic conduits, their mechanical sta- bility and the eruptive dynamics related to non-cylindrical conduits have been poorly addressed. We present here a 1D steady-state model which includes the main processes experimented by ascending magmas (i.e. crystallization, rheological changes, fragmentation, drag forces, outgassing and degassing), and the application of two mechanical stability criteria (Mohr – Coulomb and Mogi – Coulomb), in order to study the collapse conditions of volcanic conduits during a representative explosive rhyolitic eruption. It emerges that me- chanical stability of volcanic conduits is mainly controlled by its radial dimension, and a minimum radius for reaching stable conditions can be computed, as a function of water content and inlet overpressure. Additionally, for a set of input parameters thought typical of explosive rhyolitic volcanism, we estimated a minimum magma fl ux for developing a mechanically stable conduit (~7 ∙ 10 7 − 3 ∙ 10 8 kg/s). Results are consistent with the unsteady character usually observed in sub-Plinian eruptions, opposite to mainly stationary Plinian eruptions, commonly characterized by higher magma discharge rates. We suggest that cylindrical conduits represent a mechanically stable con fi guration only for large radii. Because the instability conditions are not uniform along the conduit, the widening processes probably lead to conduit geometries with depth-varying width. Consequently, as our model is able to consider volcanic conduits with depth-dependent radius, two plausible and previously untested geometries have been studied, evidencing major and complex modi fi cations in some eruptive parameters (par- ticularly, exit pressure and mass discharge rate), and suggesting that the geometry acquired by the conduit as it is widened influences the eruptive dynamics
185 180 - PublicationOpen AccessA novel strategy to enhance kinetic energy models by considering channelization processes of PDCs(MISCELLANEA INGV 52, ISSN 2039-6651, 2020)
; ; ; ; ; ; ; ;; ; ; Kinetic energy models are tools able to provide an estimate to the inundation area of pyroclastic density currents (PDCs). We present here a novel strategy that allows improving these models in order to consider the occurrence of channelization processes of pyroclastic material. In this strategy, the inundation area associated with a basal collapse process, represented by a root conoid that interacts with the topography, is complemented with the inundation zones derived from the inclusion of secondary source points located in the expected zones of pyroclast channelization (represented by branch conoids). For that, we adopt a tree branch-like structure and appropriate assumptions for setting the position and the initial characteristics of the secondary source points. Two widely used kinetic models are modified by applying this strategy: the energy cone and the box model, giving place to two open-source and freely downloadable codes (ECMapProb and BoxMapProb). We tested these branching formulations by comparing their results with those derived from the traditional formulations of these kinetic energy models, with other numerical solvers, and with the invasion area of real PDC deposits. We show the capability of the presented strategy of improving the accuracy of kinetic models without adding new, unconstrained input parameters or significantly increasing the computational cost, allowing the assessment of volcanic hazards using a probabilistic approach. The application of this strategy represents a time-effective alternative to the use of more sophisticated models for describing PDC transport and deposition.54 18 - PublicationOpen AccessTephra fallout and pyroclastic density currents hazard maps for Ecuadorian volcanoes: examples from Cotopaxi, Guagua Pichincha, Tungurahua and Sangay volcanoes(2022)
; ; ; ; ; ; ; ; ; ; ; ; ; ; ;; ; ; ; ;; ; ;Introduction Brief overview on studied volcanoes Numerical modelling Tephra fallout (TF) Pyroclastic Density Currents (PDC) Uncertainty quantification procedure Hazard maps Cotopaxi/Guagua Pichincha (TF) Tungurahua (TF/PDC) Sangay (expert elicitation for maps production) Conclusions and perspectives81 14 - PublicationOpen AccessCalibration strategies of PDC kinetic energy models and their application to the construction of hazard maps(2022)
; ; ; ; ; ; ;; ; ; ; The availability of computer tools able to describe the behavior of pyroclastic density currents (PDCs) with uncertainty quantification is of primary importance for the assessment of volcanic hazard. A common strategy to assess the intrinsic variability of these phenomena is based on the analysis of large sets of numerical simulations with variable input parameters. The use of models fast enough to allow for a large number of simulations, such as the so-called kinetic energy models, is thus advantageous. Due to the sensitivity of kinetic energy models to poorly constrained input parameters, the definition of their variation ranges is a critical step in the construction of hazard maps and a numerical calibration becomes necessary. We present a set of reproducible and structured calibration procedures of numerical models based either on a reference deposit or on the distribution of runout distance or inundation area of documented PDCs. In the first case, various metrics can be adopted to compare the model results with the reference PDC deposit (root mean square distance, Hausdorff distance, and Jaccard index), facilitating the development of scenario-based hazard assessments. Calibrations based on the distribution of runout distance or inundation area allow the construction of probabilistic hazard maps that are not conditioned on the occurrence of a specific scenario, but rather reflect the variability of the documented PDCs during the time window considered. Importantly, our calibration strategies allow one to set the input parameters considering their potential statistical dependence. These procedures have been implemented on the user-friendly versions of two kinetic energy models: ECMapProb 2.0 and BoxMapProb 2.0, whose functionalities are presented for the first time in this paper. The different calibration strategies and the functionalities of the two programs are illustrated by considering three case studies: El Misti (Peru), Merapi (Indonesia), and Campi Flegrei (Italy).533 97