NZ moving to 'period of more earthquakes'

There is a 32 percent chance of another earthquake with a magnitude of more than seven within the next month, GNS says.

New Zealand Crown Research Institute for geophysics and nuclear science, GNS Science said there had been more than 800 aftershocks since the 7.5 magnitude earthquake rocked the country yesterday.

Most have been centred 10km to 20km east of the Marlborough town of Seddon, but several strong aftershocks have struck close to the centre of Kaikoura.

GNS duty seismologist Natalie Balfour told Morning Report three 'severe' category quakes struck last night, measuring as high as 5.8.

"We've had about 832 was our last update of aftershocks since the main event," she said.

"And yes, things have been really rolling along, we get about 20 an hour.

"Over magnitude 5, since the main event, we've had about 40 of those."

Dr Balfour said the most likely scenario was still that aftershocks would decrease in frequency and intensity.

GeoNet predictions

• Scenario 1 - Very likely (80 percent and greater): A normal aftershock sequence that is spread over the next few months to years. Felt aftershocks (about magnitude 5 and above) would occur from the midnight quake's epicentre near Culverden, right up along the Kaikoura coastline to Cape Campbell over the next few months to years. This is the most likely scenario.
• Scenario 2 - Likely (60 percent and greater): In the next month, it is likely earthquakes of about magnitude 6 in the North Canterbury and Marlborough regions will occur, as well as potentially offshore in Southern Cook Strait and offshore Kaikoura.
• Scenario 3 - Unlikely (less than 40 percent): The least likely scenario is that in the next month, there would be earthquakes of about magnitude 7 on longer known faults in the Marlborough and Cook Strait regions.

Scientists are also poring over seismic, satelitte and GPS data to understand the complex rupture sequence that produced the roughly two minutes of shaking felt throughout the country.

GeoNet and GNS's Geohazards Monitoring head Ken Gledhill said the aftershocks seemed to be reducing but more could be expected.

"The overall numbers are trending dow, at about the rate we'd expect, so that's the good news at the moment. I suppose you've already mentioned what's the potential bad news is - there's still the potential of a larger earthquake and we just cannot rule that out.

"What we're seeing at the moment is a pretty normal aftershocks sequence."

"Bear in mind that bit of the country is a bit like a train wreck ... it's a real junction, where a lot of things happening."

He said he would describe the 7.5 event yesterday as a complex earthquake which involved several fault breaks:

• The Kekerengu Fault, which runs between the southeast and northwest through Ward to the coast, appears to have had up to 10m of slip
• A newly identified fault at Waipapa Bay
• Minor movement on the seaward segment of the Hope Fault which runs almost between Hokitika and Kaikoura
• The Hundalee Fault, which ends close to the Kaikoura coast

The aftershocks and shifts did seem to be moving north, he said, but there was no guarantee of where the next big one could hit.

"What we can see on the surface is four places where different faults, where some are already identified, have actually had major movement. Looking at the data we have, it does look like it changes its nature as it comes north.

"So the action moved north basically and it changes its character as it moves north."

"I think what the last few years has demonstrated is I would be very foolish to say the next big earthquake is going to be in Wellington - we frankly don't know.

What was becoming obvious was New Zealand was moving into a period of more earthquakes, he said.

"There's been periods in history - and most of us grew up in the period of 50 years or so when there was hardly any large earthquakes - our parents grew up when there was a lot of large earthquakes in through the '20s and '40s."