New research details Kaikoura tsunami impacts
Two new scientific papers describe the potential severity of waves from locally-generated tsunamis off the Kaikoura coast.
One paper outlines modelling of three tsunamis triggered by submarine earthquakes of the 'maximum credible magnitude'. The second paper outlines modelling a one tsunami caused by a submarine landslide where a large area at the head of Kaikoura canyon gives way. Both describe potentially catastrophic waves reaching the shore in as little as a few minutes.
The papers are by a team of scientists from NIWA, led by Dr Roy Walters, and will appear in the March issue of the New Zealand Journal of Marine and Freshwater Research.
"Big tsunamis are extreme, and rare, events, but modelling them is an important part of getting prepared. The pattern of waves generated by undersea earthquakes or landslides is very complicated. To save lives, authorities need the best possible information on where and how far people should evacuate, where essential services should be located, the likely effect on roads and railways, and a host of other questions which our models can help answer.
"Our research also reinforces the need for people to be aware of tsunamis, and to get away from the water as quickly as possible if the feel a big earthquake or notice a sudden change in sea level," Dr Walters says.
While the specific results are of intense interest here, the techniques involved are attracting wider scientific attention. "Work on the 1998 Papua New Guinea tsunami indicates that submarine landslides are a much more significant source than previously thought," says Dr James Goff of NIWA.
Funding for this research came from the Foundation for Research, Science & Technology, the Marsden Fund, and Environment Canterbury (ECan).
Summary of findings
Kekerengu Bank Fault (magnitude 7.1-7.3)
About ten minutes after the earthquake, the tsunami reaches the Clarence River where the wave is about 5 metres above sea level. Because of the type and orientation of the fault involved, the sea does not rush out first. The highest part of the wave arrives later at Parikawa and Kekerengu. A number of reflected waves get trapped or "stuck" near the coast and move to the south and north. These waves keep hitting this whole shoreline for over 3 hours. The largest wave to hit Kaikoura (about 2.5 metres) arrives about 1.5 hours after the earthquake.
Conway Ridge Fault (magnitude 6.6-6.8)
The main part of the tsunami reaches shore between the Waiau River and Oaro about 7 minutes after the earthquake. The highest point of this wave is about 2 metres above the sea level, with about 0.7 metre waves at Oaro, Goose Bay, and South Bay. North of the Kaikoura peninsula, the waves are generally small except at Kaikoura where the waves resonate or "amplify", reaching just over 1 metre about 50 minutes after the earthquake.
North Canterbury Fault (magnitude 6.8-6.9)
The main part of the tsunami reaches shore near Point Gibson, well south of Kaikoura, about 10 minutes after the fault ruptures with a maximum height of 3.6 metres. The first waves are generally small to the south of the Kaikoura Peninsula, but much larger waves follow – crest heights reaching 0.45 metres at South Bay. At Kaikoura, again waves resonate (amplify) in the bay. Oscillations of up to 0.5 metres high start about an hour after the quake and last several hours.
Submarine landslide from head of Kaikoura canyon
The moment the landslide happens would be dramatic for anyone in a boat off the coast. Water around the canyon head walls would appear to be sucked down creating a large hole with a pinnacle of water rising in the centre. Large waves radiate out, some following of the direction of the sliding mass of sediment below. However, the biggest waves radiate back towards Goose Bay and Oaro.
All this happens very quickly. The tsunami’s first wave arrives at Goose Bay just one minute after the landslide starts. The wave crest is about 13 metres above sea level. A smaller wave, slightly less than 2 metres high, reaches Kaikoura about 15 minutes after the landslide, and a sequence of waves arrive at the township for almost an hour.
How likely are such tsunamis?
Kaikoura is in a highly seismically active region, but not all earthquakes of sufficient magnitude to trigger tsunamis.
The recurrence intervals on the submarine faults in this study are long – in the order of tens of thousands of years. In general, the recurrence intervals for other faults in the area are much shorter. Submarine landslides in the canyon, however, are thought to occur about once every 200 years on average.
"There is plenty of anecdotal evidence of smaller landslides, and there’s even some archaeological evidence which suggests large events occur. A hunt for better field evidence would be very helpful. In the case of the landslide-generated tsunamis, we need to learn a lot more about the size of the landslides that can and have happened," says Dr Walters.
The scientists say there is some circumstantial evidence for prehistoric tsunamis along the Kaikoura coast but there is a lack of substantive historical data. Interestingly, there’s a local Māori legend of a taniwha "that lived at the exit of Kaikoura Lagoon (where Lyell Creek drains out adjacent to Kaikoura Township today). The taniwha (water) used to rise up and kill Māori warriors on the spit from time to time." The same taniwha was also said to be responsible for the death of a girl. Two girls walked from Omihi (Stream) to Oaro (River) to pick berries. They were so engrossed in this work that they did not notice that the taniwha has risen up and completely surrounded them by water – at the last minute one girl jumped to safety but the other was 'eaten'. Later, the dead girl was thrown back onto the shore.