Periodic mega-tsunami in the southwest Pacific: physical and human impacts
by
Edward Bryant
School of Geosciences, University of Wollongong

The modern world appears secure in its knowledge of hazards. However tsunami today occur at a frequency and magnitude that is less than the prehistoric record. The size of some of these past events in the Australian region, as shown by geological evidence, is beyond the capability of earthquakes, while their regional extent rules out submarine landslides. This evidence consists of dump deposits on headlands 40-130 m in height, boulders 6 m in diameter stacked in aligned piles in front of cliffs at sheltered locations, bedrock sculptured into forms similar to those carved by catastrophic floods from ice sheets, and deposition of strandlines up to 35 km inland. Only meteorite or comet impacts with the ocean can generate tsunami big enough to produce these features. Radiocarbon dating along the Southeast Coast of Australia indicates that these events have occurred at a periodicity of around 400-500 years throughout the Late Holocene.

One recent tsunami around AD 1500 stands out. This event affected over 400 km of the Australian coastline. It is also recorded on the East Coast of New Zealand as well as on Lord Howe Island in the middle of the Tasman Sea. Aboriginal and Maori legends allude to a cosmogenic source for this event or one like it. The Maori legends can be dated around AD 1500 based upon the evidence for occupational burning on the South Island. In additional, cultural changes by Aborigines in Australia after this time lend support for a substantial tsunami. Presently, the point of impact responsible for the disaster has not been determined, but it probably lies southeast of New Zealand. The size of the meteorite or comet also remains problematic with the pattern of coastal erosion being better explained by a multiple fragmenting object.

The imprint of this most recent event is not ubiquitous. Headlands protruding seaward onto the continental shelf show more dramatic signatures of catastrophic erosion than coastline lying within broad, shallow embayments. By extension, islands rearing from the seabed should show similar if not greater effects. The steeper and higher the incline, the more dramatic the erosion. Lord Howe Island may illustrate this latter aspect. If the periodicity of cosmogenic tsunami in the southwest Pacific region is valid, then present coastlines here exist tenuously within a narrowing temporal window devoid of mega-tsunami given the timing of the last major event in AD 1500.

Date received: March 1, 2002