Darfield quake the impetus for many science advances - 03/09/2015
This week marks the fifth anniversary of the magnitude 7.1 Darfield earthquake in Canterbury. It caused widespread damage and power outages in Canterbury and while there were no fatalities, two people were seriously injured.
Scientists describe the location of the earthquake as ‘a bit surprising’, although it was consistent with the known fault structure of the region.
Prior to the earthquake, Christchurch was seen as a place of moderate earthquake hazard. Scientists also knew that the bedrock under Canterbury was dominated by very old east-west trending faults, and they had studied the east-west trending Ashley fault just north of Christchurch for many years.
The period between 1888 and 1940 in Canterbury was moderately busy for earthquakes. Following that, there was a relatively quiet period for about 50 years with very few quakes and this probably led to the public perception that the region had very low risk of suffering earthquake damage.
This changed dramatically in the pre-dawn hours of 4 September 2010, when the Darfield earthquake jolted many South Islanders awake.
The major energy release occurred on an unknown, or buried, fault – now known as the Greendale Fault. Quite early in the investigation, it was clear to scientists that the Greendale rupture was a very rare event because there was no evidence of surface rupture across the Canterbury plains for at least 15,000 years.
The main priorities for scientists immediately after the Darfield quake were to map the fault, understand the complexity of the rupture, and accurately record the rich aftershock sequence.
“It turned out to be a very complex set of near simultaneous ruptures involving a network of up to eight small adjacent faults,” said Kelvin Berryman, head of the Natural Hazards Research Platform and leader of the science liaison with Canterbury response agencies in the months following the Darfield quake.
Recording aftershocks is best done by a deploying a temporary (weeks to months) network of seismic instruments and GNS Science, through the GeoNet project, put considerable effort into deploying a dense network of seismographs and GPS instruments in Canterbury.
“From an early stage, it was fairly clear that aftershocks were concentrating at both ends of the Greendale Fault - near Hororata and near Rolleston,” Dr Berryman said.
The large amount of energy in the Darfield aftershock sequence led scientists to believe that another magnitude 6-plus earthquake was always a possibility somewhere in the aftershock zone.
The strength of ground-shaking in central Christchurch from the Darfield quake coincided roughly with the strength estimated as a 500 year return period event that is the basis for earthquake factors in the building code for residential and normal commercial building.
Subsequent research showed a strongly directed pulse of energy eastward along the Greendale Fault toward Christchurch, but this was not known for a couple of months until scientists had fully analysed the data from aftershocks and the main shock.
“Other than the liquefaction-induced damage, which was extreme in some places, the damage to buildings was generally less that might be expected for a 'code level' event.”
Damage was largely restricted to chimneys and weak un-reinforced masonry buildings.
Services such as electricity, water, and transport were quickly restored, and this helped form an attitude of being resilient to earthquakes. Similarly it promoted a strong 'no problem' attitude in the weeks following the Darfield quake, Dr Berryman said.
As recently as the early 1990's several magnitude 6 quakes occurred in the lower Buller Gorge, more than 60 years after the initiating event in 1929
The main scientific results stemming from the Darfield quake have been an understanding of the complexity of the rupture and the extent and severity of liquefaction damage, even though it was only a forerunner of the damage from the subsequent quakes on 22 February 2011 and 13 June 2011.
“The Darfield earthquake also showed us that rare events - maybe once in 20,000 years - have to happen some time. Low hazard does not mean no hazard.”
The long series of aftershocks provided new insights into that way stress is transferred in the earth’s crust. Scientists now have a better understanding of the 'influence distance' from one earthquake to another.
“Scientists are pretty certain that the Darfield quake triggered the magnitude 6.3 Christchurch earthquake on 22 February 2011. But the research on this wasn’t immediately clear in the weeks following the Christchurch quake.
After the devastating February 2011 earthquake in Christchurch, science and engineering efforts re-doubled and resulted in a wider and deeper understanding of hazard impacts and the challenges of the recovery process that lay ahead.
Being prepared should be a personal, family, community and business priority. Businesses that develop a good risk management approach are invariably more successful in 'peacetime' as well
A major area of research in the wake of the quakes has been on societal and business impacts. Case studies have identified the characteristics of community and business resilience that go beyond ‘earthquake specific’. This understanding has applications not just in New Zealand but also internationally.
Dr Berryman said a major outcome from the extended quake sequence has been the importance of a good scientific basis for management of response and recovery.
National conversations about earthquake-prone buildings, maintaining in-ground infrastructure and communication, and increasingly the consequences of climate change, have all been driven from the Canterbury experience.
There has also been a huge amount learned about liquefaction and this was being incorporated into risk management around New Zealand.
“Another post-Darfield feature has been an excellent joining together of science and policy in relation to natural hazard risks in New Zealand.”
The Canterbury experience has confirmed that cohesive communities where people know their neighbours, where the vulnerable are identified and supported, and where community leaders are empowered, result in a high level of resilience to whatever nature throws at us.
Earthquake activity in Canterbury is still about 10 times higher in the aftershock region than it was prior to 4 September 2010.
“The region is likely to continue to experience aftershocks for many years. Scientists draw parallels with the Buller region which has had an elevated level of earthquake activity since the 1929 magnitude 7.8 Murchison quake.
“As recently as the early 1990's several magnitude 6 quakes occurred in the lower Buller Gorge, more than 60 years after the initiating event in 1929.”
The fundamental message to Canterbury residents is still – ‘be prepared’. The level of earthquake hazard in Canterbury is similar to that faced by many New Zealanders in places like Wellington, Napier, Marlborough, and the South Island’s West Coast.
“Being prepared should be a personal, family, community and business priority. Businesses that develop a good risk management approach are invariably more successful in 'peacetime' as well.”