Prau Bay to Pigeon Bay: Searching for tsunamis on the seaward side of Lytelton Harbour

Three embayments, Pigeon Bay, Port Levy and Purau Bay, along the northern side of Banks Peninsula and within the outer entrance to Lyttelton Harbour were investigated for evidence of past tsunamis.We identified three palaeo- and as many as four historical tsunamis that have inundated the area within the past 1400 years. The two earliest events and the 1960 tsunami were found only at Purau Bay and Port Levy, with the 1604AD and 1868AD occurring at all three sites. Evidence for the 2010 Chilean and 2011 Japan tsunamis was found at Port Levy and at Purau Bay we also noted details of the March 2014 flood. The latter, while not technically a part of this study, proved useful as evidence of a non-marine catastrophic event.The main tsunami hazard for the area is strongly tied in with seismic activity along the subduction zone of northern Chile/southern Peru. Unfortunately, little is currently known about the palaeoseismicity of that region and inferences of past events have been made from a range of data. This report extends our current understanding of past tsunamis in Lyttelton Harbour by adding three pre-1868AD events; Event 1: 660-775AD, Event 2: 1300-1400AD and Event 3: 1604AD.This suggests that an event of similar or larger magnitude to the 1868AD tsunami probably occur at least once every 350 years or so. Relatively smaller ones such as the 1960 Chilean tsunami have undoubtedly affected Lyttelton Harbour as well, but while potentially catastrophic today if they arrived close to high tide, they are unlikely to have left an enduring recognisable sedimentary signature. Based upon the number of smaller historically documented tsunami, though, and the marked growth in coastal development (residential and port infrastructure, etc.), it is likely that at least one of these small events will cause significant damage in the intervening period between the larger ones.This study shows that a record of past tsunamis can be found in some of the embayments of Lyttelton Harbour and a more extensive survey will doubtless uncover more events. It is interesting to note that at least two of the new events reported here occurred within the time of Māori settlement and we strongly recommend that efforts be made to carry out discussions with tangata whenua in order to extend, add value and richness to the findings detailed in this report