Towards the calibration of tsunami models in the Auckland region using paleotsunami deposits

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Clark KJ, Hayward B, Morgenstern R. 2020. Towards the calibration of tsunami models in the Auckland region using paleotsunami deposits. Lower Hutt (NZ): GNS Science. 70 p. (GNS Science report; 2020/34). doi:10.21420/Q9J2-5727.

The Auckland region does not have a historic record of any significant tsunamis, but it is exposed to a number of local, regional and distant tsunami sources. Of these potential sources, the largest tsunamis are expected to be generated by large earthquakes on the Kermadec Trench. Tsunami models of large Kermadec Trench earthquakes (M 8.5–9.4) suggest parts of the eastern coastline of the Auckland Region could be exposed to tsunamis with wave amplitudes of up to 10 m on Great Barrier Island and 1–5 m on the mainland coastline. The primary objective of this EQC project is to contribute to the calibration of tsunami modelling by identifying potential paleotsunami deposits in the Auckland region that can inform us of the run-up heights, inundation distances and recurrence of tsunamis in the prehistoric period (pre- AD 1850s). This report presents a review of existing paleotsunami information in the Auckland region, methods and results of new field studies on paleotsunami in the Auckland region (primarily on Great Barrier Island) and outlines future steps toward a better understanding of tsunami hazard in Auckland and the upper North Island. We reviewed evidence for paleotsunamis at 18 sites within the Auckland region and our review found three sites (Tawharanui, Whangapoua Beach and Harataonga Bay) have robust evidence of paleotsunami. The dating of the inferred paleotsunamis at all sites is relatively poor, and it is currently hard to evaluate if there are temporal correlations (similarities in age) between the records. To undertake new field studies of paleotsunami we evaluated 12 coastal areas and selected sites based on previous paleotsunami research, tsunami modelling, suitability of the coastal depositional environments and accessibility of the site. Field reconnaissance was undertaken at 8 coastal sites but only two had likely evidence of past paleotsunamis, these were the previously identified sites of Whangapoua Beach and Harataonga Bay (Tawharanui could not be revisited). Further data were gathered at Whangapoua Beach and Harataonga Bay to better constrain the age of the inferred paleotsunami deposits. Most sites on the Auckland mainland did not have ideal depositional environments for capturing and preserving paleotsunami sediments, so the lack of paleotsunami information on the mainland is more a reflection of the environment than lack of past tsunami inundation. We recommend future research to review paleotsunami records in the neighbouring regions of Northland and Coromandel Peninsula, coupled with multidisciplinary paleotsunami field studies with iwi and archaeologists. We also recommend a general investigation to evaluate the potential of sheet gravels within sand dunes as paleotsunami indicators, and tsunami modelling at specific sites to understand local effects on tsunami amplification. Further field studies at three sites on Great Barrier Island and two sites on the Auckland mainland are recommended in order to improve the age precision and reliability of the Auckland paleotsunami record. The coastline of Auckland and its neighbouring regions offer our most promising sites to better understand the size and frequency of large to great Kermadec Trench earthquakes and this information could be of critical importance for understanding tsunami risk in New Zealand. (auth)