Predecessor to New Zealand's largest historic trans-South Pacific tsunami of 1868 AD

Multiproxy analyses of coastal sediments at Cape Pattisson, Chatham Island, identified evidence of two past tsunamis. The most recent event was the 1868 AD tsunami for which there is a wealth of historical evidence. We argue that the earlier event is most probably the 1604 AD South American tsunami. The chronology for these two events was established using palynological data from the Chatham Island sediments, and historical data from South America. It is unlikely that the exposed coastline of Cape Pattisson preserves evidence of earlier events, but given the historical and palaeotsunami records in South America, it seems likely that many earlier trans-South Pacific tsunamis would have struck the Chatham Islands and possibly mainland New Zealand. This is the first time that sedimentological evidence for a prehistoric trans-South Pacific tsunami has been documented in New Zealand, albeit on an outlying island. In the light of the findings on Chatham Island, a reassessment of the New Zealand palaeotsunami database indicates that there are several possible 1604 AD deposits on the east coast of both mainland islands. Further work needs to be done to determine whether these are indeed associated with the 1604 AD event. This use of data from a country with a relatively long historical record adds immense value to understanding the timing of palaeotsunamis in countries with shorter records. This technique offers an excellent opportunity to evaluate the magnitude and frequency of past trans-South Pacific tsunamis and to assess the risks posed to individual Pacific islands.