Eruption recurrence rates in a basaltic volcanic field based on tephra layers in maar sediments: Implications for hazards in the Auckland volcanic field

Abstract

Long-term eruption recurrence rates in monogenetic basaltic volcanic fi elds are difficult to assess because of low eruption frequencies, but they are important because of the spread of human infrastructure into such fi elds. Auckland City, New Zealand, is built on the Auckland volcanic fi eld, a young (<250 ka) basaltic volcanic fi eld. In the absence of abundant material for radiocarbon and isotopic dating, an eruptive chronology based on basalt tephra layers deposited in maar lakes was developed. Interbedded, welldated tephra layers from silicic vol canoes some 220-270 km to the south were used as age constraints. The basalt tephra layers reveal a pattern of activity not evident from the temporal-spatial distribution of volcanic landforms. Twenty-four basalt tephra layers over the last 80 k.y. represent an average frequency of one per 3.5 k.y. Recurrence times vary from <0.5 k.y. to 20 k.y. and show no temporal trend. The tephra record shows a major "fl are-up" in explosive activity at 32 ± 2 ka during heightened activity at 34-24 ka. This was related to a period of simultaneous eruptions from several volcanoes across the fi eld revealed by paleomagnetic and isotopic ages. In contrast, the fi eld has been relatively quiet during the last 20 k.y., punctuated by the construction of a shield volcano at 0.7 ka. Thus, the surface manifestation of magmatism varies greatly with time, complicating long-term volcanic hazard forecasting. Despite the high risk from future basalt activity because of proximity, ash fall from distant andesite volcanoes is the most frequently occurring volcanic hazard in Auckland City.

A model to assess tephra clean-up requirements in urban environments

“…Over the last 190 ka (G. Leonard, pers comm, 2016), the AVF has produced over 1.7 km 3 of eruptive deposits (Kereszturi et al 2013). The annual probability of an eruption within the AVF ranges from 0.03-0.08% depending on assumptions within different probabilistic hazard models (Molloy et al 2009;Hurst and Smith 2010;Bebbington and Cronin 2011). However, activity has been clustered through time, with repose periods of <0.5 k.y.…”

“…to 20 k.y. (Molloy et al 2009;Hopkins et al 2015). Probabilistic hazard models indicate that in Auckland the annual probability for 1 mm tephra thickness from all sources (local and distal) for ≥ VEI 4 eruptions is 0.9% (Jenkins et al 2012) and for >10 mm from all sources (local and distal) is approximately 0.02% (Jenkins et al 2012) to 0.03% (Hurst and Smith 2010).…”

Paleomagnetic record of basaltic volcanism from Pukaki and Onepoto maar lake cores, Auckland Volcanic Field, New Zealand

“…Estimates of the average sedimentation rate for the Onepoto sequence have been provided by Shane & Hoverd (2002) and Shane & Sandiford (2003). These data made it possible to correlate part of the Onepoto core to the upper part of the older core from Pukaki maar (Shane & Hoverd 2002) and then to correlate these to the recently studied cores from Pupuke, Orakei, and Hopua maars (Molloy et al 2009). …”

“…A summary of the stratigraphy and chronology of silicic tephra in New Zealand was published by Froggatt & Lowe (1990). Additional tephrostratigraphic information was recently obtained from geochemical analyses of cores from maar craters in the AVF (Sandiford et al 2001;Shane & Hoverd 2002;Shane 2005;Molloy et al 2009). However, little information is available about the history of the basaltic eruptions in the AVF, and there is little evidence of a spatial or temporal trend in this activity making it difficult to predict future eruptions (Shane & Smith 2000;Edbrooke et al 2003).…”

Evaluating the impacts of volcanic eruptions using RiskScape

“…However, the rate of eruptions isn't constant, with notable periods of temporal clustering: half of the known eruptions happened in the last 60 ka, with a temporal clustering of eruptions around 30 ka . For crosshazard comparative purposes, the recurrence rate is between 500 and 20,000 years (Molloy et al, 2009). A further challenge is that there are no definitive spatial or volumetric trend for the location or size of AVF eruptions (e.g., Bebbington and Cronin, 2011;Le Corvec et al, 2013;Bebbington, 2015).…”