Probabilistic assessment of vent locations for the next Auckland volcanic field event

Auckland, New Zealand's most populous Region is centred on the Auckland volcanic field (AVF), which contains an estimated 49 small-volume basaltic volcanoes. Although these volcanoes are considered monogenetic, a number of centres may have been active either simultaneously or within a short period of time. After reviewing the characteristics of the field, known vent locations and a provisional eruption sequence were used to estimate likely eruption locations for the next event. The term event is used here to describe a short period of activity comprising one or more eruptions. Point pattern analysis and temporal consideration allow vents related by location and age to be grouped, suggesting that as few as 18 separate events may have produced the AVF. Probability distributions describing previous event attributes were used to generate Monte Carlo simulations, each defining likely vent locations for the next AVF event. Simulations were based on the assumption that distance from Rangitoto, the youngest volcano in the field, to the next event is best approximated using distances between the centroids of previous consecutive events. The maximum extent of the field is constrained only by faulting on its eastern side. The simulations show that 69% of events involved at least one eruption centred either on land or within 1 km of the coast. These events would result in substantial damage to buildings and infrastructure, significant business interruption and, potentially, loss of life. A simple risk assessment is presented considering only spatial eruption probability contingent upon a future event and building density within a 2 km grid. Work in progress aims to develop a more realistic assessment of risk for the region.