Estimating the source of a 3500–4500 year old tsunami in the South Pacific region based upon boulder transport modelling

Abstract Tsunami-transported boulders lie scattered across Okawa Point on Chatham Island, Aotearoa-New Zealand. These boulders, weighing up to 117 t, are distributed inland from coastal storm ridges, and are reported to have been transported by a tsunami 3500–4500 years ago. However, the origin of the tsunami responsible for this boulder scatter is unclear. In this study, boulder transport modelling shows that tsunamis associated with the 1960 CE Chile (Mw 9.5), 2009 CE Samoa (Mw 8.1), and 1865 CE Tonga (Mw 8.1–8.8) events result in only limited inundation at Okawa Point and cannot reproduce the observed boulder transport. In contrast, large-magnitude events from two source regions can lead to substantial inundation and boulder mobility at Okawa Point: (1) an Arica-type source region in southern Peru/northern Chile (larger than the 1868 CE Arica event, but it probably includes that rupture length within it) and (2) the Tonga–Kermadec Trench. Overall, an Arica-type earthquake with Mw ~ 9.2–9.5 provides the most plausible explanation for the palaeotsunami responsible for the boulder scatter, while an earthquake on the Tonga–Kermadec Trench remains a viable alternative candidate at sufficiently large magnitudes. Further discrimination between these sources will require additional well-dated geological evidence from Chatham Island and other South Pacific sites, alongside improved constraints on boulder provenance and pre-transport positions, and exploration of heterogeneous-slip rupture models.