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Extra Instruments Deployed to Capture Aftershock Data

Extra Instruments Deployed to Capture Aftershock Data

Scientists are deploying extra seismic instruments in Marlborough to enable more accurate measurement of the aftershocks off the coast of Seddon.

They will deploy nine instruments in coastal Marlborough over the next few days to boost the quality and quantity of data being recorded by the existing national network of instruments.

The battery-powered instruments, owned and operated by GeoNet which is funded by EQC, will be left to record for at least two weeks before the data is retrieved and analysed.

“The more accurate data will help in understanding the ruptures that are occurring and how they are linked to nearby faults off the coast of eastern Marlborough,” said seismologist Stephen Bannister of GNS Science.

It was unclear if the earthquakes were occurring on a known fault or if they were occurring on an, as yet, unidentified fault, Dr Bannister said.

By the end of this week, seismologists expect to have a clear understanding of the size and geometry of the fault that ruptured on Sunday. This will then indicate the level of stress change that has occurred on neighbouring faults
– those within a 15km radius of Sunday’s epicentre.

The faults in Cook Strait have been well mapped by NIWA. In 2008 NIWA prepared a 36-page report on Cook Strait faults as part of the multi-agency It’s Our Fault project. It shows a busy network of faults on the seabed.

There is a small possibility that some of the faults in this region could pose a tsunami threat, so scientists today issued a reminder that if people near the coast feel strong earthquake shaking for 30 seconds or longer, they
should self-evacuate and move to higher ground.

Seismic engineers are poring over data recorded from networks of instruments installed in Wellington buildings.

The instruments were installed over the past few years, and Sunday’s magnitude 6.5 quake was their first major test.

Each of the six structures has instruments placed at different levels to record how various parts of buildings perform during earthquakes. The project jointly involves GNS Science, EQC, the Department of Building and Housing, structural engineering agencies, and university engineering schools.

Knowledge gained from the recorded data will help to ensure that design standards of modern buildings can cope with the stresses imposed by strong earthquake-shaking. The buildings are the BNZ building, Wellington Hospital,
the Majestic Centre, a high-rise accommodation hall at Victoria University, and the Thorndon flyover.

Meanwhile, GNS Science has revised its aftershock probabilities for central New Zealand as follows. The figures have declined sharply since Sunday and show for the next seven days there is:

An 87% probability of a magnitude 5.0 to 5.9 aftershock – compared to 99% on Sunday
A 19% probability of a magnitude 6.0 or higher aftershock – compared to 30% on Sunday

And for the next 12 months, the figures show there is:
A 99% probability of a magnitude 5.0 to 5.9 aftershock – unchanged from Sunday
A 39% probability of a magnitude 6.0 or higher aftershock – compared to 45% on Sunday.

This last probability figure is five times higher than the equivalent figure prior to the sequence starting last Friday.
However, this figure is expected to decline sharply over the next few weeks.

The zone covered by these calculations is a large box extending from Nelson in the west to Riversdale Beach on the Wairarapa coast in the east, and from Masterton in the north to Clarence in the south.


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