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Source: GNS Science

The serious and widespread damage to Christchurch’s built environment a decade ago has ushered in dramatic changes to the cityscape. Photo – Martin Hunter

As well as the fatalities, the earthquake generated widespread building damage, liquefaction,  landsliding, rockfall, and cliff collapse. 

GNS Science seismologist Anna Kaiser, who was involved in the scientific response to the event, says anniversaries of natural disasters can be extremely difficult for some.

“Our thoughts are with all those who lost loved ones, or were seriously injured, or acutely affected in other ways. The earthquake sequence brought permanent and massive changes to the lives of many South Islanders,” Dr Kaiser says.

“Fortunately, the courage and determination of Cantabrians has held strong and underpinned their remarkable resilience and ability to go forward.”

Graph showing earthquakes per day during Canterbury sequence

Prior to the Darfield earthquake, only a handful of earthquakes were recorded in Canterbury each year. However, the Darfield earthquake – and what was to follow – changed views about  vulnerabilities to earthquakes in low seismicity regions.

For science too, and allied fields such as engineering and civil defence, the Canterbury earthquake sequence sparked many changes that have become woven into everyday life.   

Arguably, the seminal moment was on 22 February 2011 with the rupture of a previously unknown fault under the northern slopes of the Port Hills. The extreme ground-shaking it produced delivered a devastating blow to Christchurch.

“For many New Zealanders it reinforced the fact that we live in a country where there are many unknown active faults. In the future it’s almost certain there will be a damaging rupture on another unknown fault somewhere in the country,” Dr Kaiser says.

Unusual combination intensified shaking

There were several factors that combined to produce extreme ground-shaking in the earthquake.

“Most important were the proximity to Christchurch, the amount of energy released, and the way it was directed towards the central city. The thick soils below Christchurch also played a role in amplifying and trapping seismic energy.”

The crust under Canterbury is known to be dense and it takes a lot of energy to break a fault in strong crust as the faults there are strong.

A fault break in strong rock will release more energy than a similar-sized fault break in weaker rock.

“Think about breaking a piece of Styrofoam™ and a piece of particle board of the same size – the particle board will release a lot more energy. Studies of Canterbury earthquakes have found that on average they release more energy than is typical for similar earthquakes in other parts of the world.”

Dr Kaiser says the fault under the Port Hills was sloping back like the back of a lounge suite and pointing straight at Christchurch.

The tragic events that the people of Christchurch experienced are not forgotten and the lessons learned can be used to reduce impacts and help save lives

John Ristau, GNS Science seismologist

“The fault ruptured upward to the north-west, stacking energy as it went and directing that energy towards the central city. It’s a bit like a snowplough piling up snow as it moves along.

“Normally, seismic waves attenuate quickly as they move away from the fault that ruptured, but in this case there was little chance for that to happen.“

The flow of information

The communication of science was an area that saw growth and development during the earthquake. Fellow GNS Science seismologist John Ristau says the event dramatically changed the focus of the earth science community from being mainly about the science. 

“Providing useful information to all our audiences is crucial and it’s something we are conscious of with every geohazards event.

“In this way the tragic events that the people of Christchurch experienced are not forgotten and the lessons learned can be used to reduce impacts and help save lives.”

Lengthy aftershock sequence

The earthquake is notable for its long – and continuing – aftershock sequence, due mainly to the to the strong and dense crust beneath Canterbury. There have been more than 11,000 aftershocks since February 2011, and more than 15,000 since the Darfield quake in September 2010.

Aftershocks are still occurring every couple of days, although many are too small to be felt.

GNS Science seismologist Matt Gerstenberger says the earthquake was extremely well documented by a network of seismic instruments around the greater Christchurch area and the rich data collected provided many fresh insights that changed global scientific thinking about earthquakes.

This new knowledge has fed into a range of mitigation measures and other initiatives that are helping New Zealand communities become more resilient to earthquakes.

“The rupture of an unknown fault west of Christchurch that hadn’t ruptured for at least 20,000 years and subsequent quakes in Cook Strait and Kaikōura have sparked major gains in the science of earthquakes and in resilience and preparedness.

“GNS Science now has a 24/7 operations centre – called the National Geohazards Monitoring Centre – that monitors data around the clock and provides rapid information on earthquakes, volcanoes, tsunamis, and landslides to support emergency responses and to inform the public.”  

Cumulative number of quakes during Canterbury sequence

Arrival of aftershock forecasting

Dr Gerstenberger also notes that the Canterbury sequence was the first earthquake in New Zealand in which aftershock forecasting was used. For the first 18 months after the quake, GNS Science provided weekly updates on the probabilities of aftershocks occurring in a given time period.

The figures are currently updated annually. Aftershock forecasts have been calculated for several major earthquakes since Christchurch.     

“They are useful for a range of end-users including engineers, infrastructure managers, the civil defence sector, the insurance industry, and government planning.

“The probabilities are fed into new building standards, so that our buildings will be more resilient to earthquakes in the future. They are also used by the insurance industry to compare risks from different hazards.

“For all sections of the community, it is useful to have a general indication of what we can expect to help with planning and preparedness.”

MIL OSI