Darfield quake 10 years ago revealed secrets

Ten years ago a magnitude 7.1 earthquake ripped a 30-kilometre gash across the Canterbury Plains, revealing previously hidden faults and changing our understanding of earthquake science.

Te Herenga Waka–Victoria University of Wellington Professor of Geophysics Martha Savage and a team of postgraduate students from the School of Geography, Environment and Earth Sciences were among those who wanted to make sense of what at that time was New Zealand’s most powerful onshore quake for nearly 80 years.

The 4.35am earthquake, centred about 40km west of Christchurch near Charing Cross on what was soon named the Greendale Fault, triggered other quakes closer to the city, including the deadly magnitude 6.3 quake on February 22, 2011, which killed 185 people.

It was later discovered it was about 30,000 years since the Greendale Fault had last ruptured and that movement on up to nine faults may have been responsible for the 4 September 2010 Darfield quake.

Professor Savage and her postgraduate students worked with GNS Science, the University of Auckland and the University of Wisconsin in the first two weeks immediately after the quake to deploy a dozen or more seismometers from the United States across the fault zone and the wider region to record aftershocks.

The study was funded by the United States National Science Foundation and EQC.

“We had a couple of seismometers on the fault itself, trying to measure waves propagating along it, and the others were further out.

“They were in place for almost four months. In fact, we pulled them out just a few weeks before the Christchurch earthquake, as they were only meant for ‘rapid response’ and we had to send them back to the US. Also, some farmers wanted to plough their fields that we had seismometers in.”

While Professor Savage continued teaching courses on the University’s Kelburn campus, her postgraduate students in Canterbury kept her informed of developments in the field.

In one of the subsequent research papers, she and colleagues looked at the aftershock sequence and its distribution across the Plains.

“We started with the earthquakes that GeoNet had located, then we added in data from all our stations, did some advanced techniques to get better locations and found they aligned really well on some new faults not known before.

“Our GNS Science colleagues determined that the main fault that ruptured seemed to have been triggered by another earthquake to the north. It was a complex event.

“We also saw the aftershocks died down much more rapidly at the end of the Greendale Fault in the Darfield area than at the other end, closer to Christchurch.

“We didn’t have any ability to predict that the February earthquake was going to happen, but it was definitely clear that the seismicity was not dying down as quickly over there as it was in the main aftershock area.

“Unfortunately we couldn’t analyse the data recorded on the instruments until we removed them, so we couldn’t have analysed that in time before the Christchurch earthquake.”

The researchers also found that ground stress around the Greendale Fault was oriented differently to what was expected, Professor Savage says.

“The different stress direction could have been caused by the earthquake relieving stress, which is expected but not often observed.

“Normally stress should be relieved after an earthquake, which changes the stresses around the centre of the fault, but sometimes it will enhance the stresses at the edges. That’s why you sometimes get an earthquake and then another earthquake a little way away a while later, like the Christchurch one.

“You can’t really tell people not to worry after an earthquake. That was a problem before the 2009 L’Aquila earthquake in Italy, when scientists told people things were going to be okay, but they weren’t.

“As a scientist you have to very careful not to make false reassurances—but you don’t want to make people scared either.”

The University team also found slight increases in seismic wave speed over time, possibly because the earlier shaking had made the ground more unstable and the ground gradually healed during the four months of deployment.

“We also found there were larger amplitudes from the surface waves than we had expected, which were amplified in the deep basins. It was a special phase, called a ‘higher mode’, that could cause extra shaking you wouldn’t have expected to see.”

The Darfield earthquake gave scientists the opportunity to push the boundaries of their specialist fields and add to knowledge about quake hazard in the region and in New Zealand, Professor Savage says.

“We discovered new information about how large earthquakes affect the Earth in terms of the occurrence of aftershocks, changes in the stress field and how the rocks heal from damage caused by the earthquakes.”