ALBERT

Description: We propose a long-term probabilistic multi-hazard assessment for El Misti Volcano, a composite cone located 〈20 km from Arequipa. The second largest Peruvian city is a rapidly expanding economic centre and is classified by UNESCO as World Heritage. We apply the Bayesian Event Tree code for Volcanic Hazard (BET_VH) to produce probabi- listic hazard maps for the predominant volcanic phenomena that may affect c.900,000 people living around the volcano. The methodology accounts for the natural variability displayed by volcanoes in their eruptive behaviour, such as different types/sizes of eruptions and possible vent locations. For this purpose, we treat probabilistically several model runs for some of the main hazardous phenomena (lahars, pyroclastic density currents (PDCs), tephra fall and ballistic ejecta) and data from past eruptions at El Misti (tephra fall, PDCs and lahars) and at other volcanoes (PDCs). The hazard maps, although neglecting possible interactions among phenomena or cascade effects, have been produced with a homogeneous method and refer to a common time window of 1 year. The probability maps reveal that only the north and east suburbs of Arequipa are exposed to all volcanic threats except for ballistic ejecta, which are limited to the uninhabited but touristic summit cone. The probability for pyroclastic density currents reaching recently expanding urban areas and the city along ravines is around 0.05 %/year, similar to the probability obtained for roof-critical tephra load- ing during the rainy season. Lahars represent by far the most probable threat (around 10 %/year) because at least four radial drainage channels can convey them approximately 20 km away from the volcano across the entire city area in heavy rain episodes, even without eruption. The Río Chili Valley repre- sents the major concern to city safety owing to the probable cascading effect of combined threats: PDCs and rockslides, dammed lake break-outs and subsequent lahars or floods. Although this study does not intend to replace the current El Misti hazard map, the quantitative results of this probabilistic multi-hazard assessment can be incorporated into a multi-risk analysis, to support decision makers in any future improvement of the current hazard evaluation, such as further land-use planning and possible emergency management.

Description: Published

Description: 771

Description: 3V. Dinamiche e scenari eruttivi

Description: JCR Journal

Description: restricted

Description: Uncertainties in modelling volcanic hazards are often amplified in geographically large systems and in volcanoes which have a diverse eruption history that comprises variable eruption compositions and styles from different vent locations. The large ~ 700 km2 Okataina Volcanic Centre (OVC) is a large silicic caldera complex in a geodynamic region of New Zealand which has displayed a range of eruption styles and compositions over its current phase of activity (26 ka - present), including one basaltic maar-forming eruption, one basaltic Plinian eruption, and nine rhyolitic Plinian eruptions. All three of these eruption styles have occurred within the past 3.5 ky, and any of these styles could occur in the event of a future eruption. The location of a future eruption is also unknown. Future vents could potentially open in one of three different possible areas of the OVC: the Tarawera linear vent zone (LVZ) (5 eruptions over the past 26 ky), the Haroharo LVZ (5 eruptions over the past 26 ky), or outside of these LVZs (1 eruption over the past 26 ky). A future rhyolitic or basaltic Plinian eruption from the OVC is likely to generate widespread tephra fall in loads that will cause significant disruption and socio- economic impacts throughout the surrounding region. Past OVC tephra studies have focused on evaluating hazard from a rhyolitic Plinian eruption at select vent locations in the OVC's Tarawera LVZ. Here, we expand upon these past studies by evaluating tephra hazard for all possible OVC eruption vent areas and for both rhyolitic and basaltic Plinian eruption styles, and exploring how these parameters influence tephra hazard forecasts. Probabilistic volcanic hazard model BET_VH and advection-diffusion tephra hazard model TEPHRA2 were used to assess the hazard of accumulating ≥ 10 kg m-2 of tephra from both basaltic Plinian and rhyolitic Plinian eruption styles, occurring from within the Tarawera LVZ, the Haroharo LVZ, and other potential vent areas within the caldera. We present the results of these analyses as a first-order tephra hazard assessment for the entire OVC. Our results highlight the importance of considering all the potential vent locations of a volcanic system, in order to capture the full eruption catalogue in analyses (e.g., 11 eruptions over 26 ky for the OVC, versus only 5 eruptions over 26 ky for the Tarawera LVZ), as well as the full potential distribution of tephra hazard. Although the Tarawera LVZ has been prominently discussed in studies of OVC hazard because of is recent activity (1886 and ~1315 AD), we find that, in the event of future eruption, the likelihood of a vent opening within the Haroharo LVZ (last eruption 5.6 ka) is equivalent (〈 1% difference) to that for the Tarawera LVZ (31.8% compared to 32.5%). We also find that an eruption from within the Haroharo LVZ presents a relatively higher hazard to several localities, such as the town of Kawerau, where the average absolute probability of accumulating ≥ 10 kg m-2 of tephra is 1.3 times greater than for an eruption from within the Tarawera LVZ. While the absolute probabilities of accumulating ≥ 10 kg m-2 of tephra in the next one year from a basaltic Plinian eruption are on average 7.2 times lower than for a rhyolitic Plinian eruption throughout the surrounding region, our results suggest that the hazard posed by a basaltic Plinian eruption does contribute to the overall OVC tephra hazard, raising absolute probabilities for the entire OVC by an order of 0.14, which may have implications when considering sensitive decision-making thresholds.

Description: Published

Description: 38

Description: 3V. Dinamiche e scenari eruttivi

Description: JCR Journal

Description: restricted

Description: By using BET_VH, we propose a quantitative probabilistic hazard assessment for base surge impact in Auckland, New Zealand. Base surges resulting from phreatomagmatic eruptions are among the most dangerous phenomena likely to be associated with the initial phase of a future eruption in the Auckland Volcanic Field. The assessment is done both in the long-term and in a specific short-term case study, i.e. the simulated pre-eruptive unrest episode during Exercise Ruaumoko, a national civil defence exercise. The most important factors to account for are the uncertainties in the vent location (expected for a volcanic field) and in the run-out distance of base surges. Here, we propose a statistical model of base surge run-out distance based on deposits from past eruptions in Auckland and in analogous volcanoes. We then combine our hazard assessment with an analysis of the costs and benefits of evacuating people (on a 1km x 1km cell grid). In addition to stressing the practical importance of a cost-benefit analysis in creating a bridge between volcanologists and decision makers, our study highlights some important points. First, in the Exercise Ruaumoko application, the evacuation call seems to be required as soon as the unrest phase is clear; additionally, the evacuation area is much larger than what is recommended in the current Contingency Plan. Secondly, the evacuation area changes in size with time, due to a reduction in the uncertainty in the vent location and increase in the probability of eruption. It is the tradeoff between these two factors that dictates which cells must be evacuated, and when, thus determining the ultimate size and shape of the area to be evacuated.

Description: Published

Description: 705-723

Description: 4.3. TTC - Scenari di pericolosità vulcanica

Description: JCR Journal

Description: reserved

Description: The complexity of the processes responsible for volcanic eruptions makes a theoretical approach to forecasting the evolution of volcanic unrest rather difficult. A feasible strategy for this purpose appears to be the identification of possible repetitive schemes (patterns) in the pre-eruptive unrest of volcanoes. Nevertheless, the limited availability and the heterogeneity of pre-eruptive data, and the objective difficulty in quantitatively recognizing complex pre-eruptive patterns, make this task very difficult. In this work we address this issue by using a pattern recognition approach applied to the seismicity recorded during 217 volcanic episodes of unrest around the world. In particular, we use two non-parametric algorithms that have proven to give satisfactory results in dealing with a small amount of data, even if not normally distributed and/or characterized by discrete or categorical values. The results show evidence of a longer period of instability in the unrest preceding an eruption, compared to isolated unrest. This might indicate, even if not necessarily, a difference in the energy of processes responsible for the two types of unrest. However, if the unrest is followed by an eruption, it seems that the seismic energy released during the unrest (parameterized by the duration of the swarm and the maximum magnitude recorded) is not indicative of the magnitude of the impending eruption. We also found that, in general, unrest followed by the largest explosive eruptions have a longer repose time than those related to moderate eruptions. This evidence supports the fact that the occurrence of a large eruption needs a sufficient amount of time after the last event in order to re-charge the feeding system and to achieve a closed-conduit regime so that a sufficiently large amount of gas can be accumulated.

Description: Published

Description: 263–275

Description: JCR Journal

Description: restricted

Description: The Campi Flegrei caldera is a restless structure affected by general subsidence and ongoing resurgence of its central part. The persistent activity of the system and the explosive character of the volcanism lead to a very high volcanic hazard that, combined with intense urbanization, corresponds to a very high volcanic risk. One of the largest sources of uncertainty in volcanic hazard/risk assessment for Campi Flegrei is the spatial location of the future volcanic activity. This paper presents and discusses a long- term probability hazard map for vent opening in case of renewal of volcanism at the Campi Flegrei caldera, which shows the spatial conditional probability for the next vent opening, given that an eruption occurs. The map has been constructed by building a Bayesian inference scheme merging prior information and past data. The method allows both aleatory and epistemic uncertainties to be evaluated. The probability map of vent opening shows that two areas of relatively high probability are present within the active portion of the caldera, with a probability approximately doubled with respect to the rest of the caldera. The map has an immediate use in evaluating the areas of the caldera prone to the highest volcanic hazard. Furthermore, it represents an important ingredient in addressing the more general problem of quantitative volcanic hazards assessment at the Campi Flegrei caldera.

Description: Published

Description: 497-510

Description: 4.3. TTC - Scenari di pericolosità vulcanica

Description: JCR Journal

Description: reserved

Description: Pyroclastic density currents (PDCs) are gravitydriven hot mixtures of gas and volcanic particles which can propagate at high speed and cover distances up to several tens of kilometers around a given volcano. Therefore, they pose a severe hazard to the surroundings of explosive volcanoes able to produce such phenomena. Despite this threat, probabilistic volcanic hazard assessment (PVHA) of PDCs is still in an early stage of development. PVHA is rooted in the quantification of the large uncertainties (aleatory and epistemic) which characterize volcanic hazard analyses. This quantification typically requires a big dataset of hazard footprints obtained from numerical simulations of the physical process. For PDCs, numerical models range from very sophisticated (not useful for PVHA because of their very long runtimes) to very simple models (criticized because of their highly simplified physics). We present here a systematic and robust validation testing of a simple PDC model, the energy cone (EC), to unravel whether it can be applied to PVHA of PDCs. Using past PDC deposits at Somma-Vesuvius and Campi Flegrei (Italy), we assess the ability of EC to capture the values and variability in some relevant variables for hazard assessment, i.e., area of PDC invasion and maximum runout. In terms of area of invasion, the highest Jaccard coefficients range from 0.33 to 0.86 which indicates an equal or better performance compared to other volcanic mass-flow models. The p values for the observed maximum runouts vary from 0.003 to 0.44. Finally, the frequencies of PDC arrival computed from the EC are similar to those determined from the spatial distribution of past PDC deposits, with high PDC-arrival frequencies over an ∼8-km radius from the crater area at Somma-Vesuvius and around the Astroni crater at Campi Flegrei. The insights derived from our validation tests seem to indicate that the EC is a suitable candidate to compute PVHA of PDCs.

Description: Published

Description: 79

Description: 3V. Dinamiche e scenari eruttivi

Description: JCR Journal

Description: restricted