Probabilistic volcanic hazard analysis is becoming an increasingly popular component of volcanic risk reduction strategies worldwide. While probabilistic hazard analyses offer many advantages for decision-making, displaying the statistical results of these analyses on a map presents new hazard communication challenges. Probabilistic information is complex, difficult to interpret, and associated with uncertainties. Conveying such complicated data on a static map image without careful consideration of user perspectives or context, may result in contrasting interpretations, misunderstandings, or aversion to using the map. Here, we present the results of interviews and surveys conducted with organisational stakeholders and scientists in New Zealand which explored how probabilistic volcanic hazard map properties influence map interpretation, understanding, and preference. Our results suggest that data classification, colour scheme, content, and key expression play important roles in how users engage with and interpret probabilistic volcanic hazard maps. Data classification was found to influence the participants' perceived uncertainty and data reading accuracy, with isarithmic style maps reducing uncertainty and increasing accuracy best. Colour scheme had a strong influence on the type of hazard messages interpreted, with a red-yellow scheme conveying the message of a hazard distribution (high to low), and a red-yellow-blue scheme conveying the message of hazard state (present or absent) and/or risk. Multiple types of map content were found to be useful, and hazard curves were viewed as valuable supplements. The concept of "confidence" was more easily interpreted than upper and lower percentiles when expressing uncertainty on the hazard curves. Numerical and verbal expression in the key also had an influence on interpretation, with a combination of both a percent (e.g., 25%) and a natural frequency (e.g., 1 in 4) "probability" being the most inclusive and widely-understood expression. The importance of these map property choices was underscored by a high portion of participants preferring to receive maps in unalterable formats, such as PDF. This study illustrates how engaging with users in a bottom-up approach can complement and enhance top-down approaches to volcanic hazard mapping through a collaborative and integrative design process which may help to prevent miscommunications in a future crisis when maps are likely to be drafted and disseminated rapidly.

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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

Hazard analysis at caldera volcanoes is challenging due to the wide range of eruptive and environmental conditions that can plausibly occur during renewed activity. Taupo volcano, New Zealand, is a frequently active and productive rhyolitic caldera volcano that has hosted the world's youngest known supereruption and numerous smaller explosive events. To assess ashfall hazard from future eruptions, we have simulated atmospheric ash dispersal using the Ash3d model. We consider five eruption scenarios spanning magma volumes of 0.1–500 km3 and investigate the main factors governing ash dispersal in modern atmospheric conditions. Our results are examined in the context of regional synoptic weather patterns (Kidson types) that provide a framework for assessing the variability of ashfall distribution in different wind fields. For the smallest eruptions (~0.1‐km3 magma), ashfall thicknesses >1 cm are largely confined to the central North Island, with dispersal controlled by day‐to‐day weather and the dominance of westerly winds. With increasing eruptive volume (1–5‐km3 magma), ashfall thicknesses >1 cm would likely reach major population centers throughout the North Island. Dispersal is less dependent on weather patterns as the formation of a radially expanding umbrella cloud forces ash upwind or crosswind, although strong stratospheric winds significantly restrict umbrella spreading. For large eruptions (50–500‐km3 magma), powerful expansion of the umbrella cloud results in widespread ashfall at damaging thicknesses (>10 cm) across most of the North Island and top of the South Island. Synoptic climatology may prove a useful additional technique for long‐term hazard planning at caldera volcanoes.