Kaikoura earthquake shake greater than Christchurch

Publish Date
Monday, 10 April 2017, 8:26AM

Scientists have revealed incredible new insights into the 7.8 Kaikoura Earthquake - including the most violent ground shaking ever recorded in New Zealand.

Readings taken at the North Canterbury town of Waiau during the November event proved a new record for vertical ground acceleration - reaching 3g, or 30 times the force an airliner passenger feels at take-off.

Just-published research has detailed the incredible ground motion generated by the midnight quake, which triggered a tsunami and thousands of landslides, caused billions of dollars of damage, shifted the South Island and left two people dead.

Scientists inspect ground raised into a wall near Waiau by the November 7.8 M earthquake. Photo / Kate Pedley, University of Canterbury
Scientists inspect ground raised into a wall near Waiau by the November 7.8 M earthquake. Photo / Kate Pedley, University of Canterbury

While the November 14 quake erupted near the small North Canterbury town of Culverden, scientists say it was Waiau, about 22km away, that bore a New Zealand record for the most severe upward ground shaking.

GeoNet typically calculates the ground motion its seismometers detect with what's called peak ground acceleration, or PGA, a measure often compared to acceleration due to gravity.

One instrument at Waiau, since confirmed to have been functioning correctly, showed a maximum vertical acceleration of approximately 3g - eclipsing the previous national record of 2.2g produced by the February 2011 Christchurch Earthquake.

Damage to the Waiau School pool observed immediately after the 7.8 Kaikoura Earthquake. Photo / File
Damage to the Waiau School pool observed immediately after the 7.8 Kaikoura Earthquake. Photo / File

By comparison, passengers on airliners feel accelerations of around 0.1g on take-off, while someone in a car speeding up from 0km/h to around 100km/h in 10 seconds would experience 0.25g.

GNS seismologist Dr Anna Kaiser, the lead author of the new study, said someone riding a rollercoaster would feel forces greater than 1g; but they'd of course be strapped in.

Waiau School principal Mary Kimber said the power of the quake, which shook bricks from her home and ripped shelves off their bolts, was "terrifying".

"I was actually cowering on the floor ... people say, why didn't you get under the doorway? Well, you couldn't move, it was just too strong."

Waiau Pub co-owner Lindsay Collins, who was in Christchurch for all of the major Canterbury quakes, said the shake was the most frightening he'd experienced.

Damage inside the Waiau Lodge Hotel following the earthquake. Photo / Belinda Feek
Damage inside the Waiau Lodge Hotel following the earthquake. Photo / Belinda Feek

"Everything just went everywhere: I was knocked off my feet and everything was knocked off the walls," said Collins, whose century-old pub sustained extensive damage.

"I managed to get out of the bedroom after falling over bits and pieces, I got my son out of his room, and we stood there for a good minute while the shaking was going on ... it felt like a train."

A picture taken immediately after the quake shows a large landslide blocking State Highway 1, near Kaikoura. Photo / File
A picture taken immediately after the quake shows a large landslide blocking State Highway 1, near Kaikoura. Photo / File

Elsewhere, PGA readings of more than 1g, and therefore higher than that due to gravity, were recorded more than 100km away at Kekerengu (up to 1.2g) and Ward (up to 1.3g).

The total energy produced by the entire quake was equivalent to that of 400 atom bombs, or enough to power every South Island home for a year.

Its 8000 kiloton TNT equivalent was around 11 times more than the 2010 7.1 Darfield earthquake - and about 60 times more than the twin quakes that shook Marlborough in 2013.

This article was first published on NZ Herald and is republished here with permission.