Tsunamis five metres high could inundate Napier's town centre after a severe earthquake, new GNS Science modelling shows.
The mapping also predicts properties more than two kilometres inshore face damage. Waves more than 8m high at the shoreline could reach up to 5km inland within minutes after a major shake, study co-author William Power said.
The seas would be affected by the quake within five to 10 minutes, with the first wave hitting after half an hour. "When it's close to New Zealand, you may see two, three, four significant waves - it's quite possible waves will carry on for several hours."
The city's port and airport would both be flooded, creating major difficulties for relief attempts.
The project, led by GNS Science PhD student Stuart Fraser, also looked at whether avoiding tsunamis by climbing high-rise buildings was a safe strategy. That was particularly important in Napier, where people might have to travel several kilometres to reach a safe point inland in a very short space of time.
More than 5000 people survived the waters resulting from the Japanese tsunami by taking refuge in high-rise buildings.
Fraser suggested certain buildings, tested for structural integrity and of sufficient height, could be designated as public evacuation alternatives in New Zealand's tsunami-prone areas.
The new information had been incorporated into Napier's disaster planning, Hawke's Bay Civil Defence emergency manager Ian Macdonald said.
"The wave height is slightly higher than what was originally modelled, but the inundation lines won't change significantly."
Predictions for the flooding of the port and airport mean alternatives such as the Hastings Aerodrome were now central to relief plans, Macdonald said.
Tsunami warning lines, similar to those in Wellington's suburbs, were a possible response when emergency planners consulted the community, he said. The models would also be used for town planning in the future, such as the placement of new subdivisions.
The modelling looked at a variety of earthquake possibilities - from a magnitude 7.7 to 9 - and their effect on the Napier coastline.
The most devastating would be a magnitude 9 rupture on the Hikurangi fault, currently under "significant elastic strain". This fault, to the east of Gisborne and several kilometres below the sea floor, is where the Pacific continental plate is being pushed under the Australian plate.
The Hikurangi fault is "tentatively" thought to produce earthquakes about every 70 years. The last shake was 67 years ago, in 1947.
Other offshore faults that ruptured, including the Lachlan fault, would also produce tsunamis - though these would probably only reach a maximum of 1.3km inshore. In this case, Napier city would experience waves 1m high.
The new study considered higher wave heights than in previous maps, Power said. "The Japanese tsunami demonstrated that there had to be a rethink of assumptions when we talk about worst case . . . We have to go back and start looking at the . . . tsunami data, the geological data, and re-evaluate our worst cases."
The first tsunami waves could reach the shore in just half an hour to 45 minutes, so emergency planners advise people not to wait for warning signals such as sirens or radio messages. If it is long or strong, then count that as a natural warning, and go. If a quake lasts longer than a minute or is so strong you cannot stand up, head for the nearest high ground, and stay there until you get the official all-clear. Bluff Hill is the closest area to Napier's city centre that will be safe from even the highest expected tsunami waves. Solid high ground is preferable, but an alternative is to use high-rise buildings of three storeys or more as a refuge. Source: GNS Science, Hawke's Bay Civil Defence Emergency Management Group
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