New Zealand seismic activity

Time to prepare for Alpine Fault quake

The faultline behind the swarm of quakes in central New Zealand may be much longer than previously realised and therefore capable of larger quakes.

Stuff,

23 July, 2013

The National Institute of Water and Atmospheric Research is heading into Cook Strait tomorrow to map the area around the swarm of earthquakes that has been rattling Wellington and Marlborough since Friday.

It is hoped the work will identify the faultline from which the quakes have been generated.

GNS Science seismologist John Ristau said that, because the quakes appeared to be happening between the known Vernon and Needles faultlines, it seemed increasingly likely that the London Hill fault was to blame.

In 2003 a Marlborough District Council geotech report described London Hill as a one of "several relatively small faults near the east coast between Seddon and the Waima River".

Dr Ristau said it now appeared the faultline might be much longer than originally thought, which meant it could be capable of creating much larger earthquakes.

"That means it's actually considerably longer than initially thought ... the larger a faultline is, the larger the earthquake."

It was important scientists were able to determine exactly which fault the earthquake had occurred on, as it would allow them to establish whether other nearby faults could now produce large earthquakes as a result.

It appeared the level of stress had not greatly effected the other Wellington faults, but there were a few lines in the Marlborough region that were capable of creating quakes of magnitude 7 or greater.

"If they increase stress, it could trigger a similar-sized earthquake or even a much larger earthquake ... hopefully by the end of the week we'll be able to be a lot more definitive."

Niwa research ship Tangaroa was diverted from survey work in the nearby Pegasus Basin overnight and will spend part of today measuring whether the earthquakes have triggered any landslides in the Cook Strait Canyon.

"We have previously identified an area of potential instability in the middle of the canyon and this will give us the opportunity to see if there have been any changes," marine geologist Scott Nodder said.

An area crossing the earthquake epicentre will also be surveyed to see if the sea floor has changed. Dr Ristau said that mapping would help determine the faultline.

The probability of another big quake, of magnitude 6 or greater, has fallen to 4 per cent within the next day, 13 per cent in the next seven days and 33 per cent in the next year.

The probabilities would continue to fall the longer the region went without a large event, Dr Ristau said.

Aftershocks were likely to continue for about a week.

EARLIER CONCERN OVER IMPACT ON ALPINE FAULT

Yesterday, GNS Science seismologist Stephen Bannister said scientists could not rule out the possibility that the quakes could stir up other faults and "kick off" the Alpine Fault.

He said the biggest misconception the public had when it came to earthquakes was that small ones minimised or took "the edge off" the possibility of a large one occurring.

However, GNS Science spokesman John Callan said the recent swarm of quakes east of Seddon was "not increasing or decreasing the risk of a quake on the Alpine Fault".

He said the quakes were too far away to affect the Alpine Fault, which stretches 600 kilometres from Milford Sound along the western Southern Alps to Marlborough and is the on-land boundary between the Pacific and Australian tectonic plates.

Recent research led by GNS Science found it last ruptured 296 years ago, and it predicted a 30 per cent chance of a big quake along it in the next 50 years.

The average interval between large quakes on the fault was 330 years.

In the past 8000 years it had ruptured 24 times and caused magnitude-8 quakes, including four in the past millennium. The longest gap between major quakes was 510 years and the shortest 140 years.

"An earthquake on the Alpine Fault in the near future would not be a big surprise. Equally, it could be many decades away, based on its past behaviour," Callan said.

"There is no better time than the present to prepare for the next quake on the Alpine Fault. The more thoroughly we prepare, the lower the eventual impact will be."

He said work was under way to prepare to drill a deep borehole into the fault early next year to study processes taking place at depth inside a major plate boundary fault.

GNS Science is jointly leading the Deep Fault Drilling Project with Otago and Victoria universities, and it involved 22 organisations from eight countries.

An extensive network of seismic instruments, including six "down-hole seismometers" and about 40 surface instruments was recently installed near the planned drilling site so scientists could record the normal level of small quake activity.

The first stage was completed in 2011 with two holes drilled 101 and 105 metres into river terraces next to Gaunt Creek near Whataroa in South Westland.

West Coast tourist town Franz Josef straddled the fault ,but residents were unfazed by quakes in far away regions, such as Wellington, Franz Josef Development Society Incorporated chairman Marcel Fekkes said.

"Everyone is so used to the thing being here, we don't think about it much,'' he said.

"You can't live having fears like that really. When you think about it, the whole country is rather volatile from natural hazards."

Many Franz Josef residents have been fighting Westland District Council plan to establish fault-avoidance zones, which would impose significant restrictions on the township. Submissions closed last September, but a date for public hearings has yet to be set.

EQC BRACED FOR CLAIMS

The Earthquake Commission is bracing itself for an expected flood of claims from property owners throughout the upper South Island and Wellington region.

However, the commission says the quake swarm will not affect the processing of unsettled claims in Canterbury, while Prime Minister John Key says the country can "in principle" afford another earthquake.

EQC has received more than 350 claims after the quakes centred east of Marlborough that shut down central Wellington yesterday and left nerves across the region frayed.

It is already handling 459,198 claims from the Canterbury quakes.

Commission customer services general manager Bruce Emsom said most of the claims were for minor quake damage.

He said there was sufficient cover through levies, reinsurance and the Crown guarantee to provide for all valid claims.

EQC would bring in more field staff, if required, to deal with the influx of claims, so Cantabrians still waiting for their claims to be settled should not be affected.

"There is unlikely to be any impact on processing existing claims from the Canterbury quakes," he said.

Labour's EQC spokesman, Clayton Cosgrove, said that with the commission struggling to keep up with its workload it was important the Government moved swiftly to ensure it had extra resources and personnel so that claims resulting from the Wellington quakes could be dealt with efficiently and without causing any delay to the processing of Canterbury claims.

"You don't want anyone's claim held up," he said.

"Three years into a major catastrophe like this, one would have thought EQC would be well-practised now and have the systems up to deal with this.

''You would hope the mistakes that have been made down here ... would not be replicated and they could expedite claims faster."

Meanwhile, a Christchurch City Council offer to send personnel to Wellington to help authorities assess the extent of the damage has so far not been taken up.

RESEARCH ON SEDDON FAULTS INTENSIFIES

GNS Science seismologist Stephen Bannister said yesterday that intensive research was being carried out to pinpoint where the recent swarm of earthquakes had come from.

It was possible the recent quakes had come from a fault called the London Hill Fault, a relatively small fault near the South Island's east coast between Seddon and the Waimea River, he said.

Scientists would deploy nine extra seismic instruments in coastal Marlborough over the next few days to enable more accurate measurements of the aftershocks.

Seismologists expected to have a clear understanding of the size and geometry of the fault that ruptured on Sunday by the end of the week, along with knowledge of the level of stress change that had occurred on neighbouring faults.

Bannister said the fault the quakes were coming from had the capability to produce magnitude 7s, especially in northern Marlborough, on the Awatere and Clarence faults.

"Yesterday's earthquake may have a flow-on triggering effect to other nearby faults in the vicinity," he said.

"We're quite confident that the earthquakes are not occurring on the major fault between the Pacific and Australian plate, which we call the ‘subduction interface'."

Bannister said aftershocks could reasonably be expected to go on for days. A similar cluster of quakes that occurred just north of Seddon in 1995 had continued for about 11 days.

Probability forecasts for aftershock sequences had been "strongly tested" by the Canterbury sequence, he said.

There was a small possibility that some of the "busy network" of faults in Cook Strait could pose a tsunami threat, prompting a reminder to those near the coast that they should move to higher ground if they felt a strong earthquake shaking the ground for more than 30 seconds, he said.