Samaniego, Pablo

We assess the volcanic hazard posed by pyroclastic density currents (PDCs) at Tungurahua volcano, Ecuador, using a probabilistic approach based on the analysis of calibrated numerical simulations. We address the expected variability of explosive eruptions at Tungurahua volcano by adopting a scenario-based strategy, where we consider three cases: violent Strombolian to Vulcanian eruption (VEI 2), sub-Plinian eruption (VEI 3), and sub-Plinian to Plinian eruption (VEI 4-5). PDCs are modeled using the branching energy cone model and the branching box model, considering reproducible calibration procedures based on the geological record of Tungurahua volcano. The use of different calibration procedures and reference PDC deposits allows us to define uncertainty ranges for the inundation probability of each scenario. Numerical results indicate that PDCs at Tungurahua volcano propagate preferentially toward W and NW, where a series of catchment ravines can be recognized. Two additional valleys of channelization are observed in the N and NE flanks of the volcano, which may affect the city of Baños. The mean inundation probability calculated for Baños is small (6 ± 3%) for PDCs similar to those emplaced during recent VEI 2 eruptions (

Sangay volcano is considered as one of the most active volcanoes worldwide. Nevertheless, due to its remote location and low-impact eruptions, its eruptive history and hazard scenarios are poorly constrained. In this work, we address this issue by combining an analysis of monitoring data and historical chronicles with expert elicitation. During the last 400 years, we recognize periods of quiescence, weak, and enhanced eruptive activity, lasting from several months to several years, punctuated by eruptive pulses, lasting from a few hours to a few days. Sangay volcano has been mainly active since the seventeenth century, with weak eruptive activity as the most common regime, although there have also been several periods of quiescence. During this period, eruptive pulses with VEI 1–3 occurred mainly during enhanced eruptive activity and produced far-reaching impacts due to ash fallout to the west and long-runout lahars to the south-east. Four eruptive pulse scenarios are considered in the expert elicitation: strong ash venting (SAV, VEI 1–2), violent Strombolian (VS, VEI 2–3), sub-Plinian (SPL, VEI 3–4), and Plinian (PL, VEI 4–5). SAV is identified as the most likely scenario, while PL has the smallest probability of occurrence. The elicitation results show high uncertainty about the probability of occurrence of VS and SPL. Large uncertainties are also observed for eruption duration and bulk fallout volume for all eruptive scenarios, while average column height is better characterized, particularly for SAV and VS. We interpret these results as a consequence of the lack of volcano-physical data, which could be reduced with further field studies. This study shows how historical reconstruction and expert elicitation can help to develop hazard scenarios with uncertainty assessment for poorly known volcanoes, representing a first step towards the elaboration of appropriate hazard maps and subsequent planning.

Tephra fallout hazard assessment is undertaken with probabilistic maps that rely on numerical models. Regarding maps production, the input parameters of the model (including atmospheric conditions), the physical approximations of the numerical simulations, and the probabilities of occurrence of different eruption types in specific time frames are among the most critical sources of uncertainty. We therefore present a tephra fallout hazard assessment study for two active volcanoes (Cotopaxi and Guagua Pichincha) in Ecuador. We utilize PLUME-MoM/HYSPLIT models, and a procedure for uncertainty quantification where: (a) the uncertainty on eruptive source parameters and eruption type occurrence is quantified through expert elicitation; (b) we implement a new procedure for correlations between the different parameters, and (c) we use correction coefficients to take into account the uncertainty of the numerical model. Maps of exceedance probability given a deposit thickness threshold, and thickness maps given a probability of exceedance, are produced (a) for two eruptive scenarios (sub-Plinian and Plinian) and (b) as a combination of these scenarios in case the next eruption will be sub-Plinian or Plinian. These maps are described according to the uncertainty distribution of eruption type occurrence probabilities, considering their 5th percentile, mean, and 95th percentile values. We finally present hazard curves describing exceeding probabilities in 10 sensitive sites within the city of Quito. Additional information includes the areal extent and the population potentially affected by different isolines of tephra accumulation. This work indicates that full uncertainty quantification helps in providing more robust scientific information, improving the hazard assessment reliability.

Pyroclastic 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.

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 perspectives

The use of numerical models aimed at producing probabilistic maps is becoming more and more a common practice for tephra fallout hazard assessment. However, it is important to complement such maps with a quantification of the major sources of aleatoric/epistemic uncertainties, to help stakeholders and decisionmakers in taking informed decisions. In this contribution, we present an example of uncertainty quantification applied to a tephra fallout hazard assessment. The study is related to two volcanoes (Cotopaxi and Guagua Pichincha) threatening the capital city of Ecuador, Quito. Uncertainty was quantified with respect to three aspects: 1) the numerical model itself; 2) the probability of occurrences of different eruptive styles; 3) the range of variation of three eruptive input parameters (total fallout mass, eruption duration, average plume height). For point 1), the model used (which couples the plume model PLUMEMoM and the tephra dispersal model HYSPLIT) was tested in reproducing recent eruptions from South American volcanoes. This step allowed quantifying the difference between real (observed) and modelled values of several parameters, including mass loading, from which we derived coefficients of average model overestimation and underestimation. Concerning points 2) and 3), we performed an expert judgement (elicitation) session involving 20 experts of different countries and areas of expertise. This allowed deriving detailed uncertainty ranges that we used to i) sample the eruptive input parameters at each iteration during hazard map production; ii) linearly combine maps of different eruptive magnitude/style according to their relative probability of occurrence. The final products of this study are hazard maps of different formats and hazard curves for 10 sensitive sites in the city of Quito. Each of these maps/curves is presented as a set of three maps/curves (“lower”, “mean” and “upper”) which quantify the major sources of uncertainty.

OBJECTIF DU PROJET Création de cartes d’aléa probabilistes des retombées de cendre pour les volcans Cotopaxi et Guagua Pichincha et réalisation d'une étude d’aléa plus détaillé pour la ville de Quito NOUVEAUTÉS Utilisation d'un modèle numérique (PLUME-MoM / HYSPLIT) jamais utilisé pour produire des cartes d’aléa probabilistes des retombées de cendre Quantification explicite des principales sources d'incertitudes 1.Du modèle numérique 2.De la probabilité d'avoir différents styles éruptifs 3.De la plage de variation des paramètres éruptifs

Uncertainty quantification of the model – definition of mean under/overestimation coefficientes of the model Uncertainty quantification for the probability of occurrence of different eruption types for the range of eruptive source parameters – expert elicitation session Hazard maps produced for sub-plinian and plinian eruptions considered separately and together Cotopaxi (4 eruption types) Guagua Pichincha (2 eruption types) Two map types: for a given tephra accumulation threshold and different probabilities for a given probabilité donnée et différents seuils d'accumulation de téphra Three maps (« lower », « natural » et « upper ») that quantify the different sources of uncertainty Quito : hazard curves defined for 10 sensitive sites

Trace volatile elements like He are key for understanding the mantle source signature of magmas and to better constrain the relative roles of subduction and crustal processes to the variability of along-arc chemical and isotopic signatures of magmatic fluids. Here we report on noble gas abundances and isotopic data of Fluid Inclusions (FIs) in eruptive products and/or fumarolic gases from the Colombia-Ecuador segment of Andean Northern Volcanic Zone (NVZ). FIs in olivine phenocrysts from Ecuador (El Reventador, Cotopaxi and Tungurahua) yield air-normalized corrected 3He/4He ratios of 7.0–7.4 RA, within the MORB range (8 ± 1 RA). With exception of the Cotopaxi lavas (opx <

Future occurrence of explosive eruptive activity at Cotopaxi and Guagua Pichincha volcanoes, Ecuador, is assessed probabilistically, utilizing expert elicitation. Eight eruption types were considered for each volcano. Type event probabilities were evaluated for the next eruption at each volcano and for at least one of each type within the next 100 years. For each type, we elicited relevant eruption source parameters (duration, average plume height, and total tephra mass). We investigated the robustness of these elicited evaluations by deriving probability uncertainties using three expert scoring methods. For Cotopaxi, we considered both rhyolitic and andesitic magmas. Elicitation findings indicate that the most probable next eruption type is an andesitic hydrovolcanic/ash-emission (~ 26–44% median probability), which has also the highest median probability of recurring over the next 100 years. However, for the next eruption at Cotopaxi, the average joint probabilities for sub-Plinian or Plinian type eruption is of order 30–40%—a significant chance of a violent explosive event. It is inferred that any Cotopaxi rhyolitic eruption could involve a longer duration and greater erupted mass than an andesitic event, likely producing a prolonged emergency. For Guagua Pichincha, future eruption types are expected to be andesitic/dacitic, and a vulcanian event is judged most probable for the next eruption (median probability ~40–55%); this type is expected to be most frequent over the next 100 years, too. However, there is a substantial probability (possibly >40% in average) that the next eruption could be sub-Plinian or Plinian, with all that implies for hazard levels.