Risk of Massive Quake Under Central New Zealand Rises – But Still ‘Very Unlikely’

Scientists have tried to assess what impact slow-slip events under the lower North Island following the Kaikoura Earthquake will have on the probability of another massive quake but acknowledge it is “a work in progress”.

The estimated probability of a massive earthquake under central New Zealand has risen due to a rise in slow-slip activity, but such a quake remains “very unlikely”.

GNS Science said the chance of a quake as big or bigger than the magnitude 7.8 Kaikoura Earthquake in the coming year had now been put at 5 per cent – up from 3 per cent. That is about six times larger than the probability before the November 14 Kaikoura quake.

The rise in the probability was due to ongoing slow-slip events following the earthquake on a large area of the tectonic plate boundary underneath the North Island, a GNS blog said.

Due to the large extent of slow-slip events, the adjusted forecast also covered a larger area than the standard aftershock area, taking in the lower half of the North Island and upper South Island.

“There are several faults in these areas capable of large quakes, including the subduction zone and crustal faults like those that ruptured during the Kaikoura earthquake,” GNS said.

Slow-slip events cannot be felt by humans or picked up by GeoNet’s seismographs. They do result in the same amount of land movement as in a quake but the movement takes places over weeks or months.

It is suspected clusters of normal earthquakes can be associated with slow-slip events, but the link is not well understood.

Since the Kaikoura quake slow-slip events have been detected in Gisborne, Hawke’s Bay, Kapiti and Manawatu.

The slow-slip events had made the job of calculating the likelihood of future large aftershocks trickier, GNS said.

The slow-slip events were happening along the fault between the Australian and Pacific Plates, known as the Hikurangi subduction zone.

Movements along the fault were equivalent to a magnitude 7.0 earthquake in the Hawke’s Bay-Gisborne region, and magnitude 6.9 earthquake in the Manawatu-Kapiti region.

“We have observed many similar slow-slip events in these areas of this size, but this is the first time we’ve observed slow-slip occurring simultaneously in multiple areas around the North Island in the 15 years we’ve been detecting them,” GNS said.

“This is also the first time we’ve been able to observe slow slip in New Zealand after a magnitude 7.8 earthquake, so it’s possible this is a normal pattern after such a large quake.”

The Hawke’s Bay-Gisborne slow-slip event lasted about a week, and it was mostly finished. Slow-slip events in that area were less than 15km deep.

There were a number of earthquakes offshore from Porangahau in the southern Hawke’s Bay in the 2-3 weeks following the Kaikoura Earthquake that were likely triggered by the east coast slow-slip event, GNS said.

The slow-slip event beneath Kapiti and Manawatu appeared to be ongoing at a relatively steady rate since the Kaikoura quake. Slow-slip events in those areas tended to be 25 – 45km deep.

Hundreds of slow slip events observed at subduction zones around the world had not triggered larger, damaging quakes, GNS said. “So, if slow-slip events do trigger large damaging quakes, it is very rare indeed.”

Experts from GNS and Victoria University had evaluated many strands of evidence to determine the likelihood of a quake equal to or larger than the Kaikoura Earthquake. They also consulted international experts who studied slow-slip phenomena in their own countries, GNS said.

“This is our first run at including slow-slip into the forecasts, and it is probably the first attempt worldwide to implement this, so it is definitely a work in progress and our estimates have large uncertainties.”

In the lower North Island, slow-slip events happened slightly deeper than the part of the subduction zone where the plates were stuck together, GNS said. Those stuck zones were thought to periodically rupture in large earthquakes.

During slow-slip events, stresses were applied to the stuck-plate zone. That increased stress happened during all slow-slip events. The number of earthquakes during some slow-slip events could increase, but didn’t always happen.

MICHAEL DALY
Stuff.co.nz