Central NZ tsunami risk may be higher
"We are not out of the woods."
The major November 14 earthquake may have given a crucial hint that central New Zealand is more vulnerable to tsunami than previously thought.
Scientists are fascinated by the complexity of the magnitude 7.8 quake and the rising number of faults that ripped apart.
Ten onshore faults are now believed to have been involved and it is possible as many as 15 to 18 fault segments broke.
Now, initial analysis of the subsequent tsunami suggests there may be more faults offshore than realised.
These could be sources for tsunami that would affect the Marlborough, Canterbury, Wellington, Wairarapa and Hawke’s Bay coasts.
GNS Science geophysicist Dr William Power said the November 14 quake provided an ‘‘interesting puzzle’’ in terms of where the tsunami was generated.
‘‘We know the Needles Fault ruptured offshore and we strongly suspect the Hundalee Fault ruptured offshore, as we know it ruptured to the coast.
‘‘It seems likely both would have produced deformation of the sea bed. There’s an open question about whether it was only those two offshore faults involved or whether there are other faults not discovered or evaluated that may have been involved.
‘‘An interesting puzzle here is that if you look at the Kaikoura tide gauge and what happened on that morning, the water level drops steadily for 25 minutes before coming up again to a first peak about 40 minutes after the quake.
’’Those times are quite long. To me that implies there is quite a large area of deformation offshore, suggesting more faults may be involved than those two,’’ he said.
Along the coast close to the Needles and Hundalee faults, the tsunami had likely been onshore within a few minutes of the shake.
‘‘In Kaikoura, the first big wave peak is after 40 minutes. That is a long time for Kaikoura, suggesting uplift over a wider area offshore than we might expect from the Hundalee and Needles faults alone. How much further the offshore uplift extends is still something to be worked out.’’
Scientists were still collecting and interpreting tsunami data, including changes to the natural environment, such as transported fish and shellfish, and debris across roads.
Power said the tsunami might have reached 4 metres in some areas of the Kaikoura coast. However, the fact the quake struck close to low tide and that the land in the area was uplifted at the same time reduced the extent of inundation.
‘‘A better understanding of the fault movements offshore in the Kaikoura earthquake will help us better evaluate the tsunami risks in central New Zealand.’’
Environment Canterbury (ECan) geological hazard analyst Helen Jack said there was a ‘‘slightly increased risk of another tsunami anywhere along that coast’’ because of the higher probability of another large quake running offshore.
‘‘The important message is that we are not out of the woods.’’
It was not yet known how the earthquake, and the rocks and sediment still sliding into the sea, might have affected the stability of the Kaikoura canyon a few kilometres offshore.
An underwater slip into this trench poses a major threat to coastal communities south of Kaikoura. Any tsunami generated could be several metres high and could flood Goose Bay within two minutes, and Oaro and South Bay within about five minutes.
‘‘It could be several metres [high], depending on how much [water] is displaced or how it is displaced. If it (the sediment) moves in a sloppy way, it won’t generate a tsunami, but if a whole big slab of rock came down in one go, it would. There’s so much uncertainty.’’
Local communities around Kaikoura were generally well prepared for tsunami, she said.
‘‘There’s always been a tsunami risk along the Kaikoura coastline. That hasn’t stopped people visiting this unique area before and it shouldn’t now. People just need to be prepared.’’