Hazard assessment at the Quaternary La Garrotxa Volcanic Field (NE Iberia)
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While volcanic events are commonly characterized by multiple eruptive stages, most probabilistic tephra hazard analyses only simulate the major (paroxysmal) stage. In this study, we reconsider this simplified treatment by comparing hazard outcomes from simulated single‐ and multistage eruption sequences, using the Okataina Volcanic Center (OVC) in New Zealand as a case study. Our study draws upon geological evidence particular to the OVC as well as generalized patterns of eruptive behavior from other analogous volcanic centers. Exceedance probabilities of simulated tephra thickness, the cumulative duration of explosive behavior, and the duration of the entire eruptive sequence were all compared. Multistage simulations show an increased hazard with the greatest differences lying close to the vent for long duration and high thickness thresholds and at intermediate distances between the vent and the maximum extent of the deposit for lower thickness and duration thresholds. Multiple explosive stages increase the likelihood of an event lasting longer than 1 month by up to sevenfold and, for given low‐probability events, accumulated tephra thicknesses in some locations may increase by 1 order of magnitude and impact up to 22% more of New Zealand's North Island. Given our understanding of the eruptive history of the Okataina Volcanic Center, multistage simulations provide a better understanding of the potential hazard from this source.
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Abstract. Nowadays, modeling of tephra fallout hazard is coupled with probabilistic analysis that takes into account the natural variability of the volcanic phenomena in terms of eruption probability, eruption sizes, vent position, and meteorological conditions. In this framework, we present a prototypal methodology to carry out the long-term tephra fallout hazard assessment in southern Italy from the active Neapolitan volcanoes: Somma–Vesuvius, Campi Flegrei, and Ischia. The FALL3D model (v.8.0) has been used to run thousands of numerical simulations (1500 per eruption size class), considering the ECMWF ERA5 meteorological dataset over the last 30 years. The output in terms of tephra ground load has been processed within a new workflow for large-scale, high-resolution volcanic hazard assessment, relying on a Bayesian procedure, in order to provide the mean annual frequency with which the tephra load at the ground exceeds given critical thresholds at a target site within a 50-year exposure time. Our results are expressed in terms of absolute mean hazard maps considering different levels of aggregation, from the impact of each volcanic source and eruption size class to the quantification of the total hazard. This work provides, for the first time, a multi-volcano probabilistic hazard assessment posed by tephra fallout, comparable with those used for seismic phenomena and other natural disasters. This methodology can be applied to any other volcanic areas or over different exposure times, allowing researchers to account for the eruptive history of the target volcanoes that, when available, could include the occurrence of less frequent large eruptions, representing critical elements for risk evaluations.
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We subdivided volcaniclastic layers drilled during Leg 157 around Gran Canaria at distances up to 70 km from the shore of the island at Hole 953C, 955A, and 956B deposited between 14 and ~11.5 Ma into >100 volcaniclastic units at each site.Most volcaniclastic layers are <20 cm thick, but complex turbidite units up to 1.5 m thick make up 10% to 20% of all volcaniclastic units in Holes 953C and 956B.We distinguish several types of clasts: felsic vitroclasts, (1) bubble-wall/junction shards, (2) brown nonvesicular felsic shards, (3) welded tuff clasts, (4) pumice shards, and (5) sideromelane shards.Mineral phases comprise anorthoclase and lesser amounts of plagioclase, calcic and sodic amphibole (kaersutite, richterite, and edenite), clinopyroxene (titanaugite to aegirine), hypersthene, minor enstatite, phlogopite, Fe/Ti oxides, sphene, chevkinite, apatite, and zircon.Xenocrysts are dominantly titanaugite derived from the subaerial and submarine shield basalts.Lithoclasts are mainly tachylitic and crystalline basalt, the latter most common in Hole 953C, and fragments of felsic lava and ignimbrite.Bioclasts consist of open planktonic foraminifers and nannofossil ooze in the highly vitric layers, while filled planktonic foraminifers, benthic foraminifers, and a variety of shallow water calcareous and siliceous fossils and littoral skeletal debris are common in the basal coarser grained parts of turbidites.Volcaniclastic sedimentation during the time interval 14-9 Ma was governed dominantly by direct and indirect volcanic processes rather than by climate and erosion.Most volcaniclastic units thought to represent ignimbrite eruptions consist of a coarse basal part in which pumice and large brown nonvesicular and welded tuff shards and crystals dominate, and an upper part that commonly consists of thin turbidites highly enriched in bubble-wall shards.The prominent coarser grained and vitroclast-rich volcaniclastic layers were probably emplaced dominantly by turbidity currents immediately following entry of ash flows into the sea.The brown, blocky and splintery, dense, completely welded, dominantly angular to subrounded, partially to completely welded tuff shards are thought to have formed by quench fragmentation (thermal shock) as the hot pyroclastic flows entered the sea, fragmentation of cooling ignimbrite sheets forming cliffs along the shore, and water vapor explosions in shallow water.Well-sorted beds dominated by bubble-wall/junction shards may have formed by significant sorting processes during turbidite transport into the deep (300-4000 m) marine basins north and south of Gran Canaria.Some may also have been generated largely by grinding of pumice rafts and fallout and/or by interface-shearing of coignimbrite ash clouds traveling over the water surface.Generally fresh sideromelane shards that occur dispersed in many felsic volcaniclastic layers and in one hyaloclastite layer are mostly nonvesicular and blocky.They indicate submarine basaltic eruptions at water depths of several hundred meters on the slope of Gran Canaria synchronously with felsic ash flow eruptions on land.Most sideromelane shards are slightly evolved (4-6 wt% MgO), but shards in some layers are mafic (6-8 wt% MgO).Most shards have alkali basaltic compositions.The dense, iron-rich, moderately evolved basaltic magmas are thought to be the direct parent magmas for the trachytic to rhyolitic magmas of the Mogán Group.They were probably unable to erupt beneath the thick, low-density lid of the felsic magma reservoir below the large caldera but were erupted through lateral dikes onto the flanks of the submarine cone.Tholeiitic shards occur low in the stratigraphic section where peraluminous K-poor magmas were erupted, a correlation that supports the parental relationship.Heterogeneity in glass and crystal populations in the absence of other evidence for an epiclastic origin, probably largely reflect systematic primary compositional heterogeneity of most of the ignimbrites, which become more mafic toward the top.This gross compositional zonation is destroyed at the land/sea interface, where the ignimbrites are likely to have resulted in a chaotic buildup of large, quickly cooled, and fragmented mounds of hot ignimbrite.Post-emplacement, erosional mixing is probably reflected in volcaniclastic layers that are well bedded, contain a large amount of shallow water skeletal debris and rounded basaltic lithoclast, and show a wide spectrum of glass and mineral compositions.Basaltic lithoclasts are much more common in volcaniclastic layers at Hole 953C, probably because the northeastern shield basalts were highly dissected in this older part of the composite shield volcano prior to the beginning of ignimbrite volcanism at 14 Ma.As a result, many ignimbrites may have been channeled into the sea via deep canyons.In contrast, erosion was minimal during Mogán time in the southern half of the island, which was gently sloping and practically undissected, leading to concentric sedimentation on the volcanic apron.In general, the submarine, syn-ignimbrite turbidites have preserved a number of characteristics from the pristine stage of ash flow emplacement-especially shape and vesicularity of primary particles and the transient glassy state-that are lacking in the subaerial ignimbrites that cooled and devitrified at high temperatures.,
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Abstract. Nowadays, tephra fallout hazard is based on coupling the physical modeling of the tephra dispersion processes with a probabilistic analysis that takes into account the natural variability of the volcanic phenomena in terms of eruption probability, eruption sizes, vent position and meteorological conditions. In this framework, we present a prototypal methodology to carry out a multi-volcano long-term tephra fallout hazard assessment in Southern Italy from the active Neapolitan volcanoes: Somma-Vesuvius, Campi Flegrei, and Ischia. FALL3D model (v.8.0) has been used to run thousands of numerical simulations (1,500 per eruption size class), considering the ECMWF ERA5 meteorological dataset over the last 30 years. The output in terms of tephra ground load has been processed within a new workflow for large-scale, high-resolution volcanic hazard assessment, in order to quantify the mean annual frequency with which the tephra load at the ground exceeds given critical thresholds at a target site within a 50-years exposure time, and the relative epistemic uncertainty. This work provides, for the first time, a multi-volcano probabilistic hazard analysis for tephra fallout, fully comparable with those used for seismic phenomena and other natural disasters in which multiple sources are integrated together, and it accounts for potential changes in regimes of each single considered volcano. This allows us to discuss also how the full information can be traced back to provide specific information about the prevalence of different volcanoes and eruptive style in the different target areas, based on hazard disaggregation. The methodology is applicable to any other volcanic areas or over different exposure times.
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The goal of probabilistic volcanic hazard assessment is to translate complex volcanological data and numerical models into practical hazard estimates for communities potentially affected by volcanic eruptions. Probabilistic volcanic hazard assessment quantifies volcanic hazards and illustrates uncertainties about the magnitude and consequences of volcanic activity. Planning based on probabilistic volcanic hazard assessment has the potential of mitigating the effects of volcanic eruptions when they occur. This paper presents an approach developed to estimate volcanic hazards related to tephra fallout and illustrates this approach with a tephra fallout hazard assessment for the city of León, Nicaragua, and the surrounding area. Tephra fallout from eruptions of Cerro Negro volcano has caused damage to property and adverse health effects and has disrupted life in this area. By summarizing the geologic and historical records of past eruptions of Cerro Negro on a probability tree, it is shown that the inhabitants of León can expect >1 cm of tephra accumulation from approximately 30% of eruptions, and >4 cm of tephra accumulation from approximately 9% of eruptions of Cerro Negro volcano. This historical record is augmented with simulations of tephra dispersion that estimate the likelihood of tephra accumulation given a range of eruption magnitudes and that map the expected distribution of tephra over a broader region. An upper limit value of 0.5 m is calculated using the tephra dispersion model. Without a fundamental change in the eruptive behavior of Cerro Negro, tephra accumulation in León is not expected to exceed this value.
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