Exploring the influence of vent location and eruption style on tephra fall hazard from the Okataina Volcanic Centre, New Zealand

Abstract

Uncertainties in modelling volcanic hazards are often amplified in geographically large systems which have a diverse eruption history that comprises variable eruption styles from many different vent locations. The ~700 km2 Okataina Volcanic Centre (OVC) is a caldera complex in 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 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 areas which have been activated in the past 26 ky at the OVC: the Tarawera linear vent zone (LVZ) (five eruptions), the Haroharo LVZ (five eruptions) or outside of these LVZs (one eruption). 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 have severe socio-economic impacts. Past OVC tephra hazard 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 past studies by evaluating tephra hazard for all possible OVC eruption vent areas and for both rhyolitic and basaltic Plinian eruption styles, and explore how these parameters influence tephra hazard forecasts. Probabilistic volcanic hazard model BET_VH and advection–diffusion 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 or other potential vent areas within the caldera. 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 five eruptions over 26 ky for the Tarawera LVZ), as well as the full spatial distribution of tephra hazard. Although the Tarawera LVZ has been prominently discussed in studies of OVC hazard because of its recent activity (1886 and ~1315 ad), we find that in the event of a future eruption, the estimated 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 %). Including both the Haroharo LVZ and the Tarawera LVZ as possible source areas in the hazard analysis allows us to assess the full spatial extent of OVC tephra fall hazard. By considering both basaltic Plinian and rhyolitic Plinian eruption styles, as well as multiple vent location areas, we present a hazard assessment which aims to reduce bias through incorporating a greater range of eruption variables

Beyond eruptive scenarios: assessing tephra fallout hazard from Neapolitan volcanoes

“…5 . For Campi Flegrei, we also explore the effect of the vent position variability, first by showing the comparison of the results obtained combining all possible vent positions given an eruptive size, and then by combining all the possible size classes from any possible vent 16 28 conditional probability to the occurrence of an eruption, i.e. , considering the uncertainty on both the size class and vent position.…”

TephraProb: a Matlab package for probabilistic hazard assessments of tephra fallout

“…grids of fine resolutions). Resulting hazard assessments are conditional on the occurrence of the associated scenarios, and can serve as direct inputs to probabilistic frameworks such as Bayesian event trees to assess the long-term probability of tephra accumulation (Thompson et al 2015) or to fit in multi-hazards assessments (Sandri et al 2014). In order to facilitate the integration of further analyses, each output of TephraProb is saved in a variety of formats (e.g.…”

“…First, the GVP database is not a direct input to the probabilistic modelling in TephraProb and should be used as a support tool to develop eruption scenarios and identify critical ESPs with the full knowledge of the limitations and assumptions behind such databases (Biass and Bonadonna 2013;Dzierma and Wehrmann 2010;Jenkins et al 2012; Mendoza-Rosas and De la Cruz-Reyna 2008; Siebert et al 2010;Simkin and Siebert 1994). Second assessing the long-term probability of a future eruption is not trivial and should be achieved using a rigorous probabilistic framework in order to quantify and propagate various sources of uncertainties on final estimates (Bear-Crozier et al 2016;Connor et al 2003;Jenkins et al 2012;Marzocchi and Bebbington 2012;Sandri et al 2016;Sheldrake 2014;Thompson et al 2015). Probabilities resulting from a Poisson process should not be viewed as more than a first-order estimate, and only when the hypotheses of stable rate and independent events are satisfied (Borradaile 2003).…”

“…In the latter case, the hazard is quantified as a probability to exceed a given tephra accumulation and can be expressed either as conditional to the occurrence of the eruption scenario (e.g. scenariobased hazard assessment; Biass et al 2016;Bonadonna et al 2005;Scaini et al 2014;Volentik and Houghton 2015) or as absolute when the long-term probability of the eruption scenario is also quantified (Bear-Crozier et al 2016;Jenkins et al 2012;Marzocchi and Bebbington 2012;Sandri et al 2014Sandri et al , 2016Thompson et al 2015).…”

Great Balls of Fire: A probabilistic approach to quantify the hazard related to ballistics — A case study at La Fossa volcano, Vulcano Island, Italy

“…Here, we aim at providing a fully probabilistic assessment for VBP impacts as a basis to produce long-term multi-hazard assessments based on Bayesian event trees (e.g. Marzocchi et al, 2008;Selva et al, 2010;Sandri et al, 2014;Sheldrake, 2014;Thompson et al, 2015). The probabilistic approach adopted here is associated with a dependency on both the number of simulated VBPs and on the size of the zones of interest defined to average VBP impacts.…”