A decade of earthquakes

Ten years after the Darfield earthquake, three seismologists from GeoNet reflect on a decade of big earthquakes and what we've learnt from them.

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.

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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.

Darfield earthquake

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 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…

Go to this episode on rnz.co.nz for more details