The year the earth shook
ADAM DUDDING
Quake experts are suddenly in the spotlight. From the top, Mark Quigley, Euan Smith, Jason Ingham and Helen Anderson.
What our scientists have learned from the Canterbury earthquakes.
Soon after the magnitude 7.1 earthquake struck Canterbury on September 4 last year, earth scientists raced to inspect the damage to the city and surrounding countryside.
They jumped in choppers to get the big picture, and drove along newly crooked roads with compass and tape measure in hand to get the small picture. At first they came from the University of Canterbury, but they were soon joined by researchers from GNS Science and Niwa, from universities around the country, from around the world.
Over the following days and weeks they buried suitcase-sized accelerographs to capture aftershock information. They pointed radars underground and flew light planes back and forth over the twisted land, packed with devices to measure changes in the earth's magnetic field, gravitational tug and elevation.
They downloaded pre-quake satellite radar imagery of the region and compared it to post-quake. They mapped landslides and flooding and tree damage and noted where farmers' fences had developed a kink. They hatched plans to dig a dirty great trench across the faultline, in which they will read the entrails of deformed rock and sediment to divine the past behaviour of the land.
Meanwhile, earthquake engineers roamed the ruins of buildings, trying to work out why some brick buildings performed so terribly despite being reinforced to withstand a shake.
It was an exciting time. This was the first major quake in New Zealand in the internet age, and because of the far-sighted work of now-retired Christchurch civil engineer John Berrill, a network of dozens of low-cost seismic sensors had been in place since the mid-1990s. Unlike the ghastly February aftershock, this quake caused no deaths and little serious injury, so there was not much to dampen the scientific enthusiasm.
When that fatal February aftershock did strike, the mood was naturally far more sombre, but in due course scientists realised they had even more and better data than from September, because more than 100 additional acceleration sensors had been newly installed.
As a result, says Victoria University geophysics professor Euan Smith, the Canterbury tremors collectively are New Zealand's best-observed earthquakes ever. Among the surprises in the data was the extraordinarily high vertical accelerations of the ground during the February quake, and the colossal quantities of liquefaction. In due course, says Smith, the crunching of that data will inform building codes, emergency preparedness and town planning.
Science is at the heart of deciding what to do next in Canterbury – for instance, one of the reasons it has taken so long to decide which suburbs will be condemned is the complexity of predicting future vulnerability to liquefaction.
The detailed analysis will continue for years. But in the meantime, here are a few of the lessons that science has learned.
LESSON 1
Suddenly, being an earthquake expert is cool
Mark Quigley, senior lecturer in active tectonics and geomorphology, University of Canterbury:
"I have been asked to talk about earthquakes almost every day since September. People have an infinite number of questions for me. The most popular is always the hardest to answer – `Are we done with these earthquakes yet?'
"Our undergraduate geology courses have always been popular at UC, [but] we had to limit enrolments for the first time this year. Our first-year lectures have been well attended and the students can't get enough about earthquakes; this has been a positive amongst the various hardships associated with these quakes."
Jason Ingham, Associate Professor of Structural Engineering, University of Auckland: "I've received numerous invitations to speak at international conferences, to things like Rotary clubs. Everyone wants to talk about earthquakes, so friends and family are a lot more interested in what I do than ever before."
LESSON 2
Charlatans, and the credulous, will always be with us ...
Mark Quigley: "I continue to receive the odd email about the connections between earthquakes and military-sponsored scientific investigations, such as the HAARP [High Frequency Active Auroral Research Program]. Some people are concerned that military organisations may be triggering these earthquakes, which is absolute nonsense.
"And, of course, there are always people who think they can predict earthquakes. This has always been the case following major earthquakes around the globe, yet the media still takes delight in elevating the profile of these individuals no matter how many times scientists invalidate their claims, or the `predictions' turn out to be false alarms."
LESSON 3
... but some dumb questions aren't actually that dumb
Helen Anderson, seismologist and former chief executive of the Ministry of Research, Science and Technology:
"After September 4, I got asked a couple of times whether it was true that earthquakes mostly happened at night, before I realised that it was then that people mostly felt the smaller ones, because they were lying down and it was still. You can feel an earthquake down to around magnitude 3 then, as compared to around magnitude 4 moving around, and magnitude 5 or higher when driving.
"Those seem to be dumb questions but actually they are important in helping people make sense of what's going on. Similarly there are lots of good graphics around to show people how aftershocks die off with time – helping people understand that helps dispel the kind of scaremongering that goes around, about this fault or that fault being due to go."
LESSON 4
We know a lot more about landslides
Euan Smith:
"Considerable effort has been put into mapping and monitoring of rock falls and landslides using cutting-edge techniques such as laser scanning, high-resolution photography and high-precision GPS. We have gained fundamental knowledge into the nature of these processes as a result."
LESSON 5
An aftershock can be much worse than the quake it follows
Euan Smith:
"That an aftershock should cause very much more damage to people and property than the main event is unprecedented worldwide as far as I know. It is having substantial impacts on the insurance industry, which are still being worked out. And emergency managers have learned about the size of the threat aftershocks may pose to emergency and recovery workers."
LESSON 6
Risk from unknown faults may be greater than we thought
Euan Smith:
"There has been a great deal of debate about the fact that neither the Darfield fault (now called the `Greendale fault') and the one that broke on February 22 were known prior to the earthquake.
"I think this will result in a reappraisal of earthquake hazard throughout New Zealand, because it would be prohibitively expensive to undertake studies to determine whether or not there were faults that showed no surface expression or signs of geologically recent activity. This would be true even if the investigations were confined to the vicinity of population centres.
"I think this issue will result in adjustments to models that probabilistically forecast the rate of earthquake occurrence region by region – ie, to give a probability that an earthquake of any magnitude would occur within a specified distance of a site (eg, Christchurch city) in a specified time (eg, 100 years). The models which currently exist have provision for the possibility of unknown faults. I think this provision will be increased in the future."
LESSON 7
We need better ways to reinforce old brick and stone buildings
Jason Ingham, Associate Professor of Structural Engineering, University of Auckland:
"We already knew that masonry buildings – including unreinforced stone, unreinforced clay brick, and reinforced concrete masonry – perform badly in large earthquakes.
"But the single biggest lesson from Christchurch has been the poor performance of adhesive anchors into masonry, which are meant to secure newly added structures to the original masonry. Where strengthening schemes have failed this has been the principal cause.
"The University of Auckland has worked with the University of Minnesota to obtain data on the performance of new anchors set into masonry.
"Some 250 tests – done on the walls of buildings scheduled for demolition – were performed using a range of proprietary adhesives and anchors.
"It didn't help that it started snowing in the middle of everything!
"The data is currently being compiled."
LESSON 8
Being an earthquake expert is no protection against earthquakes
Jason Ingham:
"My colleagues and I were in the February earthquake. I was delivering a seminar series around the country, with the topic of assessing and strengthening buildings and their foundations for earthquakes. I was supposed to be in and out of Christchurch in a single day. We had 84 people registered for the afternoon seminar – a good proportion of all the earthquake structural engineers in Christchurch. We were just preparing to begin when the earthquake struck. Our seminar was across the road from the Grand Chancellor hotel, which we had initially considered as a venue."
Mark Quigley: "We live in Avonside and have lost our house and land to the earthquake – we are in the red zone. We have lived without sewerage since February and have experienced significant challenges trying to balance immense workloads with coming home to a destroyed and largely vacated suburb. It has been the most challenging period of our lives, but it has been rewarding at times as well."
Euan Smith: "It has made me very much more sober about the impact of a large earthquake on Wellington. The Dominion Post's publication of precincts of `earthquake-prone' buildings was alarming. I had no idea (as a seismologist!) how big and bad the problem was of buildings erected prior to modern codes."
LESSON 9
There's always more research to do
Jason Ingham:
"Two projects that have not been done, but that I very much wish to do, are extracting timber diaphragms from masonry buildings scheduled for demolition, because the overall structural dynamics of masonry buildings are strongly influenced by the performance of the timber floors. And testing full-scale buildings before they are demolished, to learn details that can be used to strengthen similar old buildings throughout New Zealand."
LESSON 10
Science matters
Mark Quigley:
"The science is being recognised at the high levels of government. The answers to issues regarding city planning and rebuilding are at the interface of science, government, insurance companies, and public perception."
SORTING FACT FROM FICTION
Q – Was the Christchurch magnitude 6.3 quake in February an aftershock of the September quake or not?
A – It was an aftershock. The term refers to a quake smaller in magnitude than the preceding main shock, part of the sequence that closely follows it in time, in the region influenced by changes in crustal stress levels from it. The magnitude 6.3 earthquake fits all of these criteria.
Q – Why didn't scientists know about the faults that caused the two earthquakes?
A – Before September 4, there were no surface signs or seismicity of the Greendale Fault or the fault that generated the Lyttelton aftershock.
Q – Why wasn't some warning given about the possibility of a big and damaging aftershock?
A – Warnings were given over the risks from large aftershocks. The prediction of aftershock size is based on Bath's Law, which states "the average difference in magnitude between a main shock and its largest aftershock is 1.2, regardless of the main shock magnitude".
Q – Why did the February aftershock occur so long after September?
A – A six-month gap between a 7.1 and a 6.3 is not unusual.
Q – How predictable was February 22 – and what was the influence of the moon and tides?
A – The specific timing, location, or magnitude of earthquakes cannot be predicted. The moon can sometimes influence small earth tremors, but there is no credible evidence linking it to larger quakes.
Q – Why was the 6.3 so much more destructive?
A – It was close to Christchurch, and also shallower. The 6.3's energy was only about 1/20 of the 7.1 event, but it was about 6km from the CBD, while the 7.1 was about 44km away.
Source: Royal Society
– Responses compiled by Adam Dudding
WANT TO KNOW MORE?
Real-time info on quakes: www.geonet.org.nz
Quigley's personal website: www.drquigs.com
Time-lapse visualisation of where and when the quakes struck: www.christchurchquakemap.co.nz
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