Volcanic ashfall in New Zealand – probabilistic hazard modelling for multiple sources

Abstract

The airfall ash component of the New Zealand Probabilistic Volcanic Hazard Model (PVHM) estimates the likelihood of volcanic ash deposits of any given thickness at any site, based on the frequency-magnitude relations of all the significant volcanic sources in New Zealand and the wind distribution statistics. The main source of error in these models is the uncertainty in the historic ash production records of the volcanoes. Another source of error is that the current wind pattern may not be the same as during the period that the observed ash record was deposited. The spatial distribution and magnitude/frequency curves of volcanoes are very different from those of earthquakes, and this is reflected in the hazard models. Whereas a typical probabilistic earthquake hazard model will have the expected acceleration increase by a factor of 3 to 5 between 500 and 10,000 year return periods, the expected ash thickness can change by up to a factor of 1000 over these periods. The Auckland Volcanic Field is now occupied by New Zealand's largest city, so the comparatively small and infrequent eruptions potentially represent a major social and financial hazard. Our modelling of lake core ash records in the Auckland area suggests that the known vents and eruptions probably represent less than half of the total volcanic activity from the field, and that the hazard from local events is comparable to that from more distant sources. An area needing improvement is the hazard in Auckland from local eruptions, with more ash thickness data and better basaltic eruption models.