Darfield. Christchurch. Cook Strait. Kaikōura.
In the past decade, New Zealand has experienced four major earthquake sequences that have caused loss of life, as well as extensive damage to land and buildings.
In this Our Changing World earthquake special, three GNS Science seismologists from GeoNet recollect their experience of the Big Ones and talk about the lessons we have learned from these ten shaky years.
We really are the shaky isles here in New Zealand. We live in a very tectonically active part of the world and our history books are full of big earthquakes.
The largest on record – but certainly not the largest ever – was the magnitude 8.2 Wairarapa earthquake of 1855. Much of current downtown Wellington, as well as State Highway 2 from Wellington to the Hutt Valley, became useable real estate only after it was uplifted in that jolt.
The magnitude 7.8 quake in Murchison in 1929, was closely followed by our deadliest earthquake, with 256 deaths – that was the magnitude 7.8 Hawkes Bay earthquake of 1931.
Let’s fast forward past the magnitude 7.1 Inangahua earthquake of 1968, and skip several big Fiordland shakes to get to this week’s anniversary.
At 4.36am on the 4th of September 2010, Canterbury was rudely woken by a magnitude 7.1 earthquake, which would become known as the Darfield quake. It was a shallow quake, just 10 kilometres deep.
A seismic recorder located almost at the epicentre recorded the strongest earthquake ground-shaking ever recorded in New Zealand at the time, with the ground near the epicentre moving up 1.25 times the acceleration due to gravity.
The Darfield earthquake took everyone by surprise.
“We wouldn’t have really expected a major earthquake in Canterbury,” says GeoNet seismologist John Ristau. “The big thing we would have expected would have been an earthquake along the West Coast of the South Island, with the Alpine Fault.”
John was on duty at GeoNet that morning and he says that “when I first saw this [seismic station] map indicating everything was happening in Canterbury, around Christchurch, my first thought was ‘this can’t be right’.”
The Darfield earthquake happened on an unknown fault, hidden beneath a layer of thick sediments, which would become known as the Greendale Fault. It last moved about 18,000 years ago.
John says there are nearly 1000 faults in the Active Fault database, but there are many faults that we still don’t know about.
Seismologist Anna Kaiser says the Greendale fault is a strike-slip fault, which is a vertical fracture where the land on either side moves horizontally.
“It was a complex rupture … which actually started on a little subsidiary fault, just north of the [Greendale Fault],” says Anna. “The Greendale fault had a bilateral rupture, which meant the rupture propagated out to the east and to the west, sending some of the energy east towards Christchurch.”
About six faults in total moved during the Darfield earthquake, which triggered a long sequence of aftershocks.
Just after midday on the 22nd of February 2011, a magnitude 6.3 earthquake struck Christchurch, resulting in 185 deaths.
“I thought we’d dodged a bullet with the Darfield earthquake,” says Bill. “But [with Christchurch] I knew we hadn’t dodged the bullet. We knew the ground motions in Christchurch were very strong and we knew there was the potential for catastrophic damage.”
The Christchurch quake was part of the ongoing Canterbury sequence that started with the Darfield quake, and its ground-shaking set a new record in New Zealand.
It was caused by another unknown hidden fault, which didn’t rupture at the surface but which moved land both sideways and vertically.
John says it wasn’t a particularly large earthquake but all of its energy was directed at the city centre.
This effect is known as directivity. “It’s something like a snow plough building up energy in front of it,” says Bill.
“The earth … was breaking forward as the waves were travelling forward as well, so it just kind of piled up.”
“There’s also something called a buried thrust,” says Bill. “This is where one side [of a fault] moves up relative to the other side. So when the earthquake stops just shy of the surface then we have this effect where it causes elevated ground motion.”
Bill says that prior to this seismic hazard models were mostly concerned with horizontal ground motions, but now vertical ground motion is taken into account more.
There was also extensive liquefaction in Christchurch, caused by the strong shaking.
Cook Strait earthquakes
The magnitude 6.5 Cook Strait or Seddon earthquake happened on the 21st of July 2013, and the 6.6 Grassmere earthquake took place on the 16th August 2013.
These two large earthquakes followed a swarm of similarly large quakes that happened across several days. They were widely felt in Wellington, but they didn’t propagate north across Cook Strait and didn’t cause too much damage in the capital.
John says the Seddon and Grassmere earthquakes are considered a doublet. “These are two earthquakes that have similar magnitudes and source properties, and they occur close together in both time and location.”
These earthquakes were well recorded by seismic recorders which are placed in buildings and on bridges in Wellington, creating valuable datasets for seismologists and building engineers.
Just after midnight on the 14th of November 2016, a magnitude 7.8 earthquake struck the Kaikōura region.
The Kaikōura earthquake was one of the most complex earthquakes recorded anywhere in the world. It lasted nearly two minutes and involved more than 20 faults, jumping more than 20 kilometres between some faults as it moved to the north.
Ground motion was nearly 3G. And a large slow-slip earthquake was triggered on New Zealand’s largest fault, the Hikurangi subduction zone which is the boundary between the Pacific and Australian tectonic plates.
The shaking triggered a nearly 7-metre tsunami and a massive underwater landslide down the Kaikōura canyon. On shore there were thousands of large landslides.
Sections of coastline were uplifted by up to 6 metres, and in places there was up to 12 metres of displacement.
State Highway 1 and the main trunk railway were closed for nearly a year to allow for repairs, and a number of large buildings in Wellington were so badly damaged in the quake they had to be demolished.
Lessons for the future
Earthquake monitoring and science has come a long way in the past decade.
“Due to our increasing modern monitoring networks, we really get to see a picture of an earthquake,” says Anna.
“Whereas before we might have had a fuzzy blurry picture from a distance, we’re starting to see a close-up movie of what’s unfolding and we can reconstruct that after the event.”
The GeoNet seismologists interviewed for this Our Changing World story all say that the past decade, more than ever, has taught us to expect the unexpected when it comes to earthquakes.
“I’ve been in New Zealand long enough and seen enough earthquakes to not expect large earthquakes to be simple. I think every big earthquake we have, we take a step back and learn something from it.” Bill Fry.
While we still don’t know how to predict earthquakes exactly, the experts say we have a better ability to forecast the likelihood of an earthquake happening in different places, even if we don’t know the exact location.
“When you have as many active faults as we have in New Zealand, they may have long recurrence intervals – which means a long time before they have earthquakes on them – but you get enough of them and there’s always a good chance that one fault somewhere is going to rupture. You just don’t know,” says John.
“It’s kind of like buying a lottery ticket – your odds of winning are pretty low, but if enough people buy lottery tickets there is a good chance that somebody is going to win.”
The experts stress we should all be as well prepared as possible for future earthquakes.
“We need to treat every area in New Zealand as a potentially hazardous area,” says Bill, “and we need to make sure we have our precautions in place [including] plans and provisions.”
Listen to the full podcast to hear Anna, Bill and John discussing the big earthquakes of the past decade, their own experiences as duty seismologists and what we’ve learned.
Our Changing World’s earthquake collection
Our Changing World has covered earthquakes and earthquake science, and you can find many interesting stories from the archives in our earthquake collection – check it out.
A quick note on the word ‘magnitude’ which means slightly different things depending on which definition you’re using. Richter or local magnitude is commonly used while seismologists prefer moment magnitude – and there are plenty of others. So, for example, 6.2 and 6.3 are both correct magnitudes for the Christchurch earthquake.
Published at Wed, 02 Sep 2020 05:36:42 +0000
Source: Darfield, Christchurch, Cook Strait, Kaikōura: A decade of earthquakes