Appendix B – Hazard Summaries Local Source Tsunami Overview Tsunami are high crested waves generated by sudden fractures of the sea floor caused by earthquakes of magnitude greater than 6.5 (Richter Scale) or by submarine volcanic eruptions or landslides. In the Marlborough District tsunami have been reported in 1848 and in 1855 but little damage was reported as the area was sparsely populated. However this record does not provide a realistic picture of past tsunami inundation in Marlborough due to the short period of time able to be considered by a historical record, and it only records those events which happened where people were at the time. At least nine tsunami have inundated the Marlborough Coast in the last 5,000 years according to the available historic and palaeo tsunami records. Current palaeodata has provided new evidence to suggest that at least 5 locally generated catastrophic salt water inundations with wave heights greater than 5 metres, and probably greater than 10 metres have impacted the Cook Strait region in the last 5,000 years. The most likely source of these near source tsunami is the Hikurangi trench. Marlborough District could be affected by either locally generated or distant tsunami generated along the South American coast on the eastern side of the Pacific Plate. A tsunami that crosses the Pacific is generally associated with wave heights up to 5 metres across a broad front. Locally generated tsunamis are associated with waves of the order of 10 metres in height along a narrower front, perhaps as little as 20 kms. Hazard Likelihood A study by GNS Sciences (William Power 2013) indicates that the District could be (E) affected by tsunami from both near and distant sources and return periods were calculated for different scenarios. Waves potentially up to 12 metres were classified as a 1:2,500 years tsunami event (an internationally adopted standard) with an 84% confidence level. Waves between 3-4 metres were classified as a 1:500 years event. Smaller waves (mostly from distant sources) are a regular occurrence in NZ and most are 1-3 metres in height. Hazard Social: Consequence (5) • In a worst case scenario with little time for warnings to be communicated and a large number of residents living in remote areas it is highly likely that such a tsunami would result in multiple fatalities. • Public health issues with water contamination and a lack of essential services such as power. • Psychosocial issues would expect to be dramatically increased. • Many people from outside the District will be trapped in Marlborough. Built: • Picton and Havelock ports and large areas of the townships could be seriously damaged. • Blenheim sewerage ponds could be inundated. • Electricity supply could be seriously compromised including some Transpower Marlborough Civil Defence Emergency Management Plan, 2017-2022 Page 86 20180727060856 15662 INFO webrefs.c: line 118: webrefs age number set in contents.xml /home/webapps/asp_au/data/asp/publications/nz-marlborough-district-council/july-2018/marlborough-civil-defence-emergency-management-group-plan-2017-21/publication/contents/contents.xml