Photo / GNS Science
Photo / GNS Science

'27%' chance of NZ copping 7+ quake this month

The probability of another plus-7 magnitude quake within the next month has fallen to 27 per cent - but the chances of multiple large aftershocks between 5.0 and 5.9 happening each day remains a virtual certainty.

A new aftershock forecast published by GeoNet today - the first since Monday's quake magnitude was upgraded from 7.5 to 7.8 - estimated an average three 5.0 to 5.9 quakes within the next 24 hours, along with an average 16 within the week and 32 this month.

There was a 29 per cent probability of one or more 6.0 to 6.9 quakes within 24 hours, a 77 per cent probability for the week and a 95 per cent probability across the next 30 days.

But there was just a 3 per cent probability of another quake measuring over 7.0 in the next day, compared with estimated odds of 14 per cent this week and 27 per cent this month.

The "extremely" likely scenario was that aftershocks would continue to decrease in frequency - and in line with forecasts - over the next 30 days.

"Felt" aftershocks would occur from Monday's 7.8 epicentre near Culverden, right up along the Kaikoura coastline to the Cape Palliser and Wellington area.

This included the potential for aftershocks of between 6.0 and 6.9.

Less likely was an earthquake smaller than Monday's mainshock, measuring between 7.0 and 7.8 - numerous mapped faults in the Marlborough or Cook Strait areas were capable of such an earthquake.

"This scenario includes the possibility of an earthquake in the Hikurangi Subduction Zone," GeoNet spokeswoman Sara McBride said.

Earthquakes there or in the Cook Strait had the potential to generate localised tsunami.

"The Hawke's Bay earthquake sequence in 1931 provides an analogy to scenario two, as a M7.3 aftershock occurred approximately two weeks after the initial M7.8 earthquake."

An extremely unlikely scenario - with a chance of less than 1 per cent for the next month - was that recent earthquake activity would trigger an earthquake larger than Monday's 7.8 mainshock.

"This includes the possibility for an earthquake of greater than M8.0, which could be on the 'plate interface' - where the Pacific Plate meets the Australian Plate," McBride said.

"Although it is still very unlikely, the chances of this occurring have increased since before the M7.8 earthquake."

She reminded people that if an earthquake was too strong to stand up in, or lasted longer than a minute, they should move inland or to a higher point immediately, and not wait for a siren or further information.

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