Kaikoura earthquake ruptured 21 faults - that's possibly a world record

Wednesday, 15 March 2017

At least 21 faults ruptured during the Kaikōura earthquake - a possible world record for a single earthquake.

Twenty-one faults ruptured during the magnitude 7.8 Kaikoura earthquake, scientists have found.

That probably approached or exceeded the world record for the number of faults to rupture in a single earthquake, GNS Science earthquake geologist Dr Kate Clark said.

Speaking at a presentation to mark four months since the November 14 event, she said 50 people from a range of New Zealand and overseas institutes had been working to measure the impact of the quake.

There had been about 180km of surface fault rupture in the quake, with the largest displacement across a fault being about 12 metres on the Kekerengu Fault.

The first indications on the day of the quake were that five faults had ruptured, Clark said. Even that number was a surprise.

**READ MORE:

The quake lifted the seabed near Kaikoura, changing the coast dramatically.

* Little Pigeon Bay cottage smashed by tsunami waves

* NASA photos show Kaikoura land raised by quake

The quake ripped up State Highway 1 north of Kaikoura.

* Finding a new normal in Kaikoura

* Before and after the quake**

Landslips triggered by the violent shaking blocked State Highway 1.

'We were all a bit bewildered, confused and also amazed . . . It was kind our first clue that this was probably going to be quite a complicated and unusual earthquake,' Clark said.

There had been about 180 kilometres of surface fault rupture in the quake, with the largest displacement across a fault being about 12 metres on the Kekerengu Fault.

The quake was probably part of the evolution of the boundary between the Australian and Pacific plates.

It happened in an area where the plate boundary was evolving, with the Hikurangi subduction zone to the north and the Alpine Fault – where the plates move side-by-side – to the south, Clark said.

'These sort of complex earthquakes are probably all part of how the plate boundary evolves.'

The tsunami caused by the quake was recorded on numerous tide gauges around the country – reaching Wellington Harbour in about an hour, Christchurch a couple of hours afterwards and the Chatham Islands four to five hours later, tsunami scientist Dr William Power said.

At the Kaikoura tide gauge the water level had started dropping immediately after the quake, and after 25 minutes was down about 2.5m – indicating a tsunami was under way.

Then over the next 15 minutes the water level had risen about 4m from its lowest point, Power said. A series of waves had followed over the next several hours.

Beyond that the overall water level rose about a metre from before to after the quake, as the gauge was lifted along with the seabed and surrounding land.

The biggest run-up height – rise in water levels – seen on land was between 6m and 7m at Goose Bay, south of Kaikoura.

At Little Pigeon Bay on the north side of Banks Peninsula, where a vacant cottage was inundated, the run-up height was about 3m, Power said.

Water had been squeezed into the V-shaped bay, with the inundation going about 140m to 150m into the back of the bay.

Inside the cottage, the water had been about 1m deep. An adult probably would have survived but an infant or person with limited disability probably would have been in great danger.

Engineering geologist Sally Dellow said about 200 landslide dams caused by the quake had been identified. The largest was probably on the Hapuku River in the Kaikoura Range, which contained about 12 million cubic metres of material.