A record of local storms and trans-Pacific tsunamis, eastern Banks Peninsula, New Zealand

Abstract

This study of five sand units at Lavericks Bay, New Zealand, reports on the sedimentary evidence for three trans-Pacific tsunamis and two local storms. The 1868 Arica, 1877 Iquique and 1960 Valdivia tsunamis from Chile were the largest distantly generated events in New Zealand’s history but have never before been identified at the same location. It is also the first time that the 1877 Iquique tsunami deposit has been found in New Zealand. Two further sand units were identified as local storm deposits laid down in 1869/1870 and 1929. The identification and chronology of these events were established through the use of geochemistry, palynology, diatoms, charcoal abundance and historical documents. Their relative magnitudes were estimated through the use of grain size parameters and lateral extent of the recognisable sand layers. The recognisable sandy tsunami deposits extend about 60% of the inundation distance, while the storm sediments are finer and less extensive. There were two notable geochemical differences between the storm and tsunami deposits. Both storm deposits had lower concentrations of marine proxy elements associated with lower Ca–Ti and Sr–Ba ratios. Other differences were noted between some of the tsunami and storm deposits such as rip-up clasts and sediment characteristics, but these were by no means unequivocal. It is possible that geochemistry may prove to be the only proxy capable of not only differentiating effectively between storm and tsunami sediments but also identifying the maximum inland extent of a deposit and of inundation. It is the ability to better understand the nature and extent of such catastrophic events through these subtle differences in event characteristics that will help improve risk management for coastlines around the world.