Hayes, J.L.; Williams, J.H.; Magill, C.R.; O’Leary, T.M.; Bebbington, M.; Wilson, T.M.; Leonard, G.S.; Fitzgerald, R.H.; Craig, H.; Lindsay. J. 2026 National Volcano Model: Probabilistic volcanic ash building damage. Lower Hutt, NZ: Earth Sciences New Zealand. GNS Science report 2026/21. 95 p.; doi: 10.21420/NSEH-1881
Abstract
Volcanic ashfall is the most widespread volcanic hazard affecting Aotearoa New Zealand and has the potential to cause substantial damage to buildings over large geographic areas. Despite significant advances in probabilistic volcanic hazard modelling, nationally consistent assessments of building damage and associated financial loss have been limited by the absence of appropriate vulnerability models and integrated risk frameworks. This report presents the development and application of the ashfall building damage and loss component of the National Volcano Model (NVM), providing the first national probabilistic assessment of volcanic ash risk to New Zealand buildings. The NVM integrates probabilistic eruption occurrence and magnitude, ashfall hazard modelling using Tephra2, a national building exposure dataset and newly developed New Zealand-specific vulnerability models derived through structured expert elicitation. Expert judgement was calibrated using Cooke's Classical Model to quantify and aggregate uncertainty in building response to volcanic loading. Fragility functions were developed for representative New Zealand building typologies and implemented within a probabilistic loss modelling framework to estimate damage, repair costs, average annual loss and loss exceedance probabilities. The expert elicitation also developed fragility functions for building damage from volcanic mass-flow hazards. Although these are not applied in the ashfall-focused analysis presented here, their inclusion extends the NVM vulnerability framework to support future multi-hazard volcanic risk assessments. Loss modelling demonstrates substantial spatial variability in volcanic ashfall risk across New Zealand, reflecting differences in volcanic hazard, exposure and building vulnerability. Results identify the relative contributions of individual volcanoes, regions and building components to national risk, while illustrating the sensitivity of loss estimates to alternative vulnerability assumptions. The probabilistic framework enables consistent comparison of risk across volcanic sources and geographic regions, and provides advancements toward a fully operationalised NVM with quantitative metrics that will be suitable for emergency management, resilience planning, infrastructure investment and financial risk assessment. This study represents a significant advancement in national volcanic risk assessment by integrating hazard, exposure and vulnerability within a consistent probabilistic framework in a New Zealand context. While future work will further refine hazard characterisation, vulnerability models and multi-hazard capability, the framework provides a robust foundation for the continued development of the NVM and improved evidence-based volcanic disaster risk reduction in New Zealand (auths)
