Sedimentary differences between the 2002 Easter storm and the 15th-century Okoropunga tsunami, southeastern North Island, New Zealand

Abstract

Internationally, a key problem in reconstructing chronologies of coastal hazards is the ability to distinguish between storm and tsunami deposits. This situation has been exacerbated by the low number of known locations where both types of deposit occur along the same stretch of coastline. The sedimentological characteristics of a tsunami and a storm deposit laid down on the same stretch of coastline on the southeast coast of the North Island, New Zealand, are distinctly different. The 15th-century tsunami was probably caused by fault rupture in the Cook Strait region, whereas the Easter 2002 storm was generated by a meteorological depression centred some 900 km to the southeast. The differences include areal extent, thickness, and grain-size characteristics. The tsunami deposit thins abruptly at the margins, fines inland, is more poorly sorted, has entrained rip-up clasts, and has an erosional lower contact, often with a buried soil. The storm deposit has a highly variable grain-size distribution with a marked coarsening at its landward extent, is better sorted, coarser, and has a sharp, non-erosional lower contact associated with buried vegetation and soil. The coarser grain size is probably the result of differences in sampling regime as opposed to wave energy. The storm deposit extends about 40 m inland as opposed to about 200 m for the tsunami. Variations in the preservation of evidence are a reflection of the age of deposition. Records of tsunamis and storms in New Zealand indicate that there are probably several coastal sites where both types of deposit can be compared.