Scientists finally have proof that central New Zealand could be ticking down to a highly damaging “megathrust” earthquake.
Earlier research has suggested the seabed between the Wairarapa and Marlborough is capable of generating magnitude 7-plus quakes.
Now researchers have found solid geological evidence that an area off the coast of Wairarapa and fringing Cook Strait causes “megathrust” quakes and tsunami similar to, but probably smaller than, the devastating magnitude 9.0 March 2011 event in Japan.
The study area was salt marsh flats on the edge of Big Lagoon near Blenheim
The work, out Tuesday morning, highlights the active threat the southern Hikurangi margin – where the Pacific Plate is being dragged down below the Australian Plate – poses to life and livelihood from Hawke’s Bay south to the Wairarapa, Wellington and Marlborough.
The Alpine Fault, which extends further south from that plate boundary, is also a hazardous feature that will generate a magnitude 8 quake when it ruptures, possibly some time in the next 50 to 100 years.
To look for evidence of past earthquakes on the margin, the researchers performed a painstaking examination of the geologic layers contained within a salt marsh at Big Lagoon in the southeastern Wairau River valley on South Island.
The scientists, from GNS Science, the University of Texas and Geomarine Research, have calculated that in the past 1000 years two subduction quakes of at least magnitude 7 occurred – one between about 880 to 800 years ago and the other between 520 and 470 years ago.
“This is the first evidence that the southern Hikurangi margin ruptures in large (7-7.9) to great (8+) earthquakes, and the relatively short time interval between the two events has significant implications for seismic hazard in New Zealand,” they said in Tuesday’s Bulletin of the Seismological Society of America.
A map showing the area when the Australian and Pacific tectonic plates collide. Researchers warn a ‘megaquake’ of magnitude 8 or more could occur in this area.
They cited an earlier paper that said for a magnitude 8.9 Hikurangi subduction quake, losses in the Wellington region alone were estimated to be about $13 billion, with about 3550 deaths and 7000 injuries.
Their findings would allow better modelling of the impacts and help communities prepare to cope with such an event, they said.
The Hikurangi margin, which runs from east of East Cape to offshore of the Marlborough coast, is one of the few subduction zones around the Pacific that has not generated a “great”, above magnitude 8, quake in historic times.
Kate Clark, GNS Science
Jamie Howarth, William Ries and Delia Strong, of GNS Science, using a piston corer to recover sediment cores from salt marsh at Big Lagoon, Blenheim, to determine the dates of the last megathrust earthquake off the Wairarapa coast.
Data shows that in the southern Hikurangi margin the Australian and Pacific plates are locked and accumulating strain where they meet, about 25km beneath Wellington and Blenheim.
Previous research suggests this locked patch between Cook Strait and Cape Turnagain could generate a quake of between 8.5 and 8.7 magnitude.
In their search for subduction-quake evidence, the researchers used a salt marsh on the edge of Big Lagoon near Blenheim to recover sediment that could be aged by radiocarbon dating.
They collected 48 sediment cores, from 0.5m to 2.2m deep. Analysis and dating of the buried soils in the cores showed there had been two occasions of sudden subsidence of the lagoon in the past 1000 years, indicative of two large quakes.
The older event was accompanied by a tsunami at least 3.3m high that swept more than 360m inland.
There was no evidence of a tsunami hitting Big Lagoon in the more recent quake, although there were tsunami deposits around Cook Strait, at Abel Tasman and on Kapiti Island about the same time as that event, researchers said.
Lead researcher Dr Kate Clark, of GNS Science, said the findings did not greatly change the actual level of risk to people in central New Zealand.
The National Seismic Hazard Model used a recurrence interval of 550 to 1000 years for a magnitude 8.1-8.4 quake but the researchers had found an actual interval of about 350 years between the two quakes.
While that was different, it was not an irreconcilable difference, given the average was only based on two events.