Simon Barker, University of Auckland (EQC funded project 16/724)
Hazard analysis at caldera volcanoes is challenging because of the wide range of potential eruptive sizes and conditions that can plausibly occur in any single event. Ashfall hazards can impact huge areas, with even small amounts of volcanic ash causing widespread damage, disruption and economic costs.
Taupo caldera in the central North Island is one of the most frequently active and productive caldera volcanoes on Earth and host to the youngest known supereruption ~25,400 years ago. Over the past 12,000 years there have been at least 25 more eruptions from Taupo that span 3-4 orders of magnitude in size and recent studies suggest that there is still a large magma system that could generate another eruption on timescales of human concern.
In this EQC-funded project we have combined our current understanding of Taupo's eruptive record with one of the most advanced ash dispersal models; Ash3d, to build a better understanding of which areas of New Zealand would experience ashfall across a range of eruption sizes and wind conditions.
For the smallest eruptions considered (~0.1 km3 magma), ashfall to >1 cm thickness is largely confined to the central North Island and dispersal is dictated mostly by day-to-day weather. Only major towns in the eastern North Island between Tauranga and Hastings can expect >1 mm of ashfall with probabilities between 10 and 30%.
However, with increasing eruption sizes (1-5 km3 magma), the probability of ash thicknesses to reach damaging levels (10-100 mm) becomes increasingly significant, especially in the Gisborne, Hawke's Bay, Bay of Plenty, Waikato and Manawatu regions. With increasing eruption size, ash dispersal becomes less dependent on weather, as the formation of a major umbrella cloud may force ash upwind or cross-wind. For the largest eruptions (50-500 km3), ash thicknesses associated with major damage (100mm) or severe structural damage (>300 mm) can be expected at high probabilities in most major towns or cities in the North Island, even as far away as Auckland or Wellington. Ashfall >1 cm in thickness may even occur in the upper South Island for these large eruptions at significant probabilities (>30%). Particular weather conditions result in increased probabilities of ashfall in different locations. Light winds associated with a high pressure system over New Zealand generally lead to the thickest accumulations of ash in the central North Island and the highest probability of ashfall at major cities. In contrast, strong dominant westerly winds lead to a significant amount of ash being distributed offshore into the Pacific Ocean.