November 14 earthquake a big shock with big surprises

Monday, 13 November 2017

Drone footage from different sources reveals the extensive damage caused by the devastating 7.8 magnitude North Canterbury earthquake in November 2016.

When the first pictures of the impact of the November 14 earthquake started emerging, a stunned world realised just how astonishing an event it had been.

Great gashes were ripped through the landscape, giant landslips had come down, a 'wall' more than three metres high rose across a field, there was land where there used to be sea, roads were ripped apart, and large new lakes formed where rivers had been dammed.

Then scientists started talking about the number of faults involved, culminating in a paper in March that said the quake ruptured at least 12 major crustal faults plus another nine lesser faults.

The so-called wall of Waiau - a stark example of the power of the Kaikōura Earthquake..

Starting in North Canterbury, the rupture moved north for more than 170km, straddling two distinct active fault domains - the North Canterbury fault zone and the Marlborough fault system. On the Kēkerengū Fault, pieces of the Earth's crust were displaced relative to each other by up to 25m at a depth of about 15km.

**READ MORE:

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

* Kaikōura an example of how large quakes trigger slow-slip events at a distance

Parts of the uplifted Kaikōura coast looked like an alien world.

* GeoNet posts video showing the uplift of Kaikōura 's coastline**

Parts of the South Island moved more than 5m closer to the North Island and there was extensive uplift along the coastline, with one fault-bounded block being pushed up 8m.

The study's authors, led by GNS Science geodesy specialist Ian Hamling, said the complex quake defied many assumptions about the degree to which earthquake ruptures were controlled by individual faults.

'The number of faults was certainly a big surprise,' Hamling said.

One intriguing aspect of the quake was the way it looked to have jumped as much as 15km between some faults, when global studies of past quakes had shown ruptures were generally stopped if faults were separated by more than 5km.

Hamling said data showed there were actually hidden faults at depth that essentially connected the rupture from south to north.

Dust created by a strong after-shock hangs above the Clarence River which was blocked, causing a huge dam.

That did raise issues for earthquake hazard models - which estimate the strength of shaking that can be expected in an earthquake - because without those hidden faults the apparent gap would be too large to allow a thoroughgoing rupture.

'On top of that, the stress changes induced by the earlier part of the rupture also further promoted the northern segments to fail so it may be that the rupture could have jumped even if the hidden structures weren't there,' Hamling said.

Civil Defence warned in the days following the quake of a 150m high dam caused by the earthquakes which could rapidly fail, spilling water and debris from the new 'lake' upriver of Hapuku.

'It would be highly unlikely that something as complex would happen in the same location within our lifetimes. That said, there are other areas around New Zealand with complex networks of faults and we don't know if they could break in a similar manner.'

The magnitude 7.1 Darfield earthquake in September 2010 was another fairly complex event in a place where it wasn't expected 'so we are certainly still learning'.

He was working on a Marsden Fund project to try to integrate satellite radar observations with GPS to look for evidence of potentially hidden faults across the South Island.

There has been debate about how much of the earthquake's energy was produced by surface faulting and how much was produced in the subduction zone where the Pacific tectonic plate is being pushed below the Australian Plate.

Peter Turner believes this new canyon formed near Waiau following this months magnitude-7.8 earthquake is between 600 and 800 metres long and 50 metres deep.

'This is an interesting question, and still being debated in the community,' Hamling said. From his perspective, most studies from New Zealand and overseas pointed toward the subduction zone having a minimal component.

'Much of the work from overseas is using global datasets which, in my opinion, give a blurry view of what happened.'

Two cows and a calf on the Millton farm became world famous a year ago when they were marooned by the earthquake

Bubbles erupting from Kaikoura coastline is something of extreme scientific value - as well as being enchanting and beautiful, says scientist Matthew Hughes.

A simulation shows how the Kaikōura earthquake moved up the country before causing Wellington offices to shake, including a call centre in the Hutt Valley, caught on CCTV.

Footage from Kaikoura reveals dramatic changes to the coastline with the seabed rising up to 2m following the 7.8 quake.

Uplift of the Kaikōura coast described by GNS Science's Kelvin Berryman. (Video first published in December 2016)

All the local data pointed toward the subduction zone having a minimal amount of slip, about 10 per cent of the total energy release.

Paparoa Point, a favourite spot for campers, was transformed by the uplift caused by the Kaikōura Earthquake.

A farm cottage directly on top of the Kēkerengū Fault destroyed in the Kaikōura Earthquake.

State Highway 1, north of North of Waipapa Bay in the aftermath of the quake.

'Most of the global models cannot fit the observed ground displacements which we are able to fit with our attempts,' Hamling said.

'However, it is a tough question to answer conclusively and very hard to decompose the different signals generated by the shallow faults and any possible deep slip.'

Discoveries about the quake continue to be made, with research published in October showing the Kēkerengū Fault failed twice during the quake - the first time rupture reactivation has been identified with robust evidence on a crustal fault.

In the first rupture there was several metres of slip on the fault, GNS seismologist Caroline Holden said. Eleven seconds later there was an extensive rupture with up to 20m of slip.

Rupture reactivation during an earthquake likely happened more often than was thought. It had been suggested for the tsunami-triggering 9.0 magnitude Tohoku subduction earthquake off Japan in March 2011, and the 7.2 magnitude El Mayor - Cucapah Earthquake in Baja California in April 2010.

'More work is needed to understand what physical mechanism drives a fault to rupture multiple times in an earthquake,' Holden said.

One possibility at Kēkerengū was that the first rupture might have been triggered by seismic waves from the Humps-Hundalee ruptures. The second rupture could have been triggered by secondary and possibly later ruptures on other faults, such as the Jordan, Papatea and Kowhai.

Another possibility was that regions of the fault with lower strength could have ruptured first. That could have resulted in the nucleation of the second rupture on the stronger part of the fault, Once that broke it could have led to the large slipping and reactivation of the first rupture.

The November 14 earthquake also provided one of the best examples in the world of a large distant earthquake triggering a slow-slip event.

That happened between 250km and 600km away, on the shallow part of the Hikurangi subduction zone beneath the east coast of the North Island, off the Hawke's Bay and Gisborne coasts.

It was thought to be the first time scientists had recorded a large-scale slow-slip event triggered by passing seismic waves from a distant large earthquake, GNS Science geophysicist Dr Laura Wallace said.

While such events may be quite common, most of the areas where it might happen were offshore and did not have instruments close enough to the plate boundary to record what happened.

Slow-slip events are similar to earthquakes, involving more rapid than normal movement between two pieces of the Earth's crust along a fault. But unlike earthquakes, where the movement happens in seconds, movement in slow-slip events – also known as silent earthquakes – can take weeks or months.