A review of palaeo-tsunamis for the Christchurch region, New Zealand

Recent earthquake activity in and around the Christchurch region, New Zealand, has raised concerns about tsunamis inundating the coastline. While the occurrence of historical tsunamis is well documented, information about palaeo-tsunamis is poorly understood. This review draws together data from numerous sources to outline our current understanding of palaeo-tsunamis for the region. Six probable and one possible Holocene palaeo-tsunamis are identified and proxy data are summarised with reference to all potential near-contemporaneous sites throughout New Zealand. The events cover a period of around 6500 years, occurring around 6250-6500 yr BP, 4800-5350 yr BP, 4050-4450 yr BP, 3400-3700 yr BP, 2200-2300 yr BP, 1250-1350AD and in 1604AD. The relative exposure of Christchurch city to tsunamis has however, fluctuated over time. This is most notable over the past 2000 years or so since the Brighton spit extended south to close off the bay, then retreated and formed again causing the Avon-Heathcote Estuary (lhutai) to change from open to closed bay and then from open bay to estuary. The relative extent of the tsunami hazard is therefore controlled to a certain extent by the presence or absence of the spit, which is in turn governed by interactions between seismic activity and fluvial processes. Potential palaeo-sources vary from local (subduction fault rupture, nearshore submarine landslide, gas hydrate release, seamount flank collapse) to distant (subduction fault ruptures in Central and South America) candidates. In most cases more than one potential source was identified for each palaeo-tsunami. However, it appears that the most likely sources for most if not all of the palaeo-tsunamis were from Central and South America as opposed to more local alternatives. The identification of palaeo-sources was largely based upon historically-documented events and palaeoseismic research and as such was almost entirely limited to known local and distant fault ruptures. A review of other potential palaeo-sources with tsunamigenic potential reveals numerous additional candidates that need to be investigated in more detail. (C) 2012 Elsevier Ltd. All rights reserved.