Could devastating faultline have been detected sooner?

PAUL GORMAN
Last updated 16:32 17/02/2012
port hills
SHAKY LAND: The Port Hills and the Summit Rd looking east. DAVID HALLETT/ Fairfax NZ

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The Earth

The scourge of Christchurch: Liquefaction Mark Quigley's long, cold year Could devastating faultline have been detected sooner? Why did the Port Hills faultline rupture?

The February 22 earthquake occurred on a previously unknown fault below the Port Hills. Science reporter PAUL GORMAN asks what clues to its existence the fault may have given before the devastating quake and whether forecasts and warnings would have been of any use anyway.

Many Christchurch people thought it was the magnitude-6.0-plus aftershock they were warned could be waiting in the wings.

Four days after the major 7.1-magnitude earthquake ripped across the region, the initial resolve of Cantabrians was waning and adrenaline was running low following seriously sleepless nights.

Bleary-eyed Christchurch residents faced the chilly, drizzly dawn of Wednesday September 8, 2010, with the same mixture of fear and anticipation they had experienced each morning since the initial shock on Saturday.

As the suburbs were waking up, Mayor Bob Parker was in a briefing at Civil Defence headquarters at the Art Gallery.

Heathcote dairy owner Mahesh Patel was on his way to open up his shop, while down the road Valley Inn Tavern owner Dean Calvert was boxing-up glasses that had somehow survived the September 4 quake.

At 7.49am and 58 seconds, a piece of bedrock under immense pressure about 7km below The Tors, just west of the Lyttelton road tunnel, slipped suddenly.

It moved by no more than about 20cm, but sent violent shock waves racing through the ground beneath the city in seconds, finishing off buildings damaged four days before, forcing the closure of the road tunnel and distressing many residents, especially in the southern and eastern suburbs, who said the quake was more powerful than Saturday's big one.

GeoNet received nearly 1000 "felt" reports of the quake, with seven rating its felt intensity as Modified Mercalli Scale (MM) 8  "heavily damaging". Twenty-three categorised it as MM7, "damaging", and 122 as MM6 or "slightly damaging". It was felt as far away as Greymouth and Oamaru.

The aftershock brought down chimneys and widened cracks in the Harbourlight Theatre and Empire Hotel over the hill in Lyttelton. It sealed the fate of the Valley Inn Tavern.

Calvert ran from the building into the brick-strewn street as part of a wall and balcony collapsed, and said he would never go back in. Patel served a few customers and said he was almost too afraid to enter his dairy again.

In central Christchurch, deputy chief reporter of The Press Warren Gamble was just arriving at work. As the quake hit he watched the copper dome on the historic building wobble alarmingly back and forth.

The Art Gallery was evacuated. A clearly shocked Parker went on radio saying his "guts were churning".

"When will this thing end? It is like living in a maelstrom. This is a hammer blow to the spirit of a lot of people," he said.

Shortly after, Civil Defence said on the 8am Radio New Zealand news they thought it was the magnitude-6.0 quake some had talked about.

But it wasn't. In fact, its magnitude was barely over five  5.022 to be precise  making it only the 39th largest in magnitude terms of the sequence to date.

It was a foreshock for a much larger and deadly quake to strike two and a half-months later, though nobody at the time, scientists included, could have known for sure it was a warning of something far worse to come.

February 22's magnitude-6.3 event originated in almost the same location, perhaps a little further west towards Mount Vernon, but was shallower, at 5km deep.

September 8's aftershock tipped-off earthquake scientists that there were faults below the Port Hills and pinpointed the approximate area of the Port Hills Fault at depth.

But even armed with that knowledge, it was impossible for scientists to know at the time it wasn't just another one of hundreds of post-September 4 aftershocks and was instead signalling there would soon be a more extensive rupture there.

On the day, GNS Science natural hazards manager Kelvin Berryman told The Press the quake was unlikely to have been generated on any extension of what later became known as the Greendale Fault.

"All earthquakes are on faults, but some faults are bigger than other faults. This [Lyttelton] fault may only be 20cm long. It might just be an adjustment in a small piece of rock."

The aftershock zone was extending further east and west due to the redistribution of ground stress, Berryman said then.

With the benefit of hindsight, it's worth asking if there were any other pre-February 22 clues for scientists, other than the September 8 aftershock, about what might be ahead.

As we learnt recently, GNS Science began producing its own aftershock forecasts after the September earthquake.

Even the broadest forecasts did not become particularly widely-known general knowledge until a week or two before February 22.

It then took pressure from this newspaper to get the more detailed probability forecasts into the public domain, but that did not happen until May last year.

In a paper in the New Zealand Journal of Geology and Geophysics last month, GNS Science said its Short-Term Earthquake Probability (Step) computer model "forecast an earthquake of the size of the 22 February earthquake with a high probability".

"In the week prior to the 22 February earthquake, Step estimated an approximate 25 per cent chance of a magnitude-six or greater earthquake occurring in the general aftershock zone of the Darfield earthquake in the next year," the paper said.

That message, if it was conveyed, does not appear to have been conveyed very well, judging by the way the February 22 quake shocked the population, which admittedly had become complacent about the chance of a large aftershock after so many months.

This kind of forecast, covering the Canterbury aftershock region from west of Hororata to east of Christchurch, is so approximate that it cannot give any more than rough guidance and alert the public to taking basic precautions, such as stocking up their emergency kits.

To be useful in terms of saving life and livelihood, a warning has to be able to pinpoint the approximate times and locations of an event, as in the MetService's strong wind and heavy rain and snow warnings.

These have a generally good level of success and are accurate about 85 per cent of the time.

Quake forecasting is, unfortunately, still in its infancy. Understandably, scientists do not want to panic the public when they know their forecasts are still only general indications and stand or fall on the whims of the ground beneath us.

Until there is a string of successes from predictions of specific earthquakes it is unlikely the scientists will have the confidence to issue formal quake warnings.

It is also unlikely the public will have the confidence to follow any warnings, without such a track record, and to take extreme steps when they are issued, such as evacuating their homes.