Evolution of the Auckland Volcanic Field (New Zealand) boundary

Abstract. Monogenetic volcanic fields pose a significant risk to life and infrastructure when situated near urban regions. To place constraints on developing volcanic unrest scenarios and apply hazard mitigation measures, emergency planners and decision makers must consider where the next eruption may occur and the likelihood that it will be within the present bounds of the field. Utilising a high-precision chronology of eruptions, tested against a synthetically derived dataset, we examine the likelihood that the next eruption from the Auckland Volcanic Field (AVF) will occur within or outside of the present field boundary. We mapped the spatial-temporal growth of the field using a temporally evolving convex hull boundary, and the locations of subsequent eruptions were then tracked to test whether they fell within the convex hull boundary defined by the existing eruption centers. When considering eruptions since 63 ka, this retrospective approach reveals a probability of 71 % that the next eruption will fall within the current boundary of the field or 74 % when age uncertainty is considered. When a 3.5 km buffer is added to the convex hull, the probability increases to 98 %. Meanwhile, use of a synthetic dataset allows a forward looking analysis which suggests a 2 km buffer may be sufficient to capture at least 95 % of future eruption locations. We found that the AVF boundary grew in steps, where one to four new eruptions would occur outside the boundary with subsequent eruptions within the newly expanded boundary. Local scale spatio-temporal clustering also occurs with successive clustered centres aligning in a NNE/NE direction. We propose that this process relates to either changing magmatic source processes or crustal magmatic pathways that are available for use for multiple eruptions before closing off to new magma due to conduit cooling, solidification or changing tectonic stress patterns.