5.1 jolt was a 'classic' aftershock, scientist says

The 5.1 quake, which struck 10km north-east of Kaikoura at 11.23pm on Wednesday, was among around 12,000 aftershocks that have come in the wake of the November 14 Kaikoura Earthquake. Picture / Geonet

The late-night quake that shook Marlborough and the lower North Island on Wednesday was a "classic aftershock" that fell within the bounds of what scientists have forecast following November's 7.8 event.

The 5.1 quake, which struck 10km north-east of Kaikoura at 11.23pm, was among around 12,000 aftershocks that have come in the wake of the November 14 Kaikoura Earthquake.

Despite more than 2000 people reporting feeling it, GNS Science duty seismologist Caroline Holden said there was little unusual about the quake, which struck at a depth of 16km.

"To me it just seemed like the kind of classic aftershock we've had in the last two months following the Kaikoura Earthquake."

Its rupture mechanism - strike-slip faulting - was consistent with the previous big quakes recorded in seismically-busy Marlborough.

While it would be difficult to pinpoint what fault the quake occurred on, Holden estimated the rupture would have covered a distance of around a kilometre.

"Compared to what New Zealand can produce, that's just a crack, really."

The location was within the area of an observed swarm of aftershocks and fell within the range of aftershock forecasts.

"But there hasn't been anything bigger than magnitude 5 recently, so it was a bit of a reminder," she said.

"The bottom line is that even small ones like magnitude 5 events, because of their proximity to populations and because they are quite shallow, are always going to be felt."

Central fire communications said about 12.15am they had not received any earthquake-related calls or any reports of damage in the Wellington region.

A southern fire communications spokesman said they had not been called out to any emergencies relating to the latest quake either.

While central ambulance communications also said they had not received any reports of injuries.

People talking about the quake on social media reported feeling it in Paraparaumu and Wellington.

One woman said: "Felt like a friendly [earthquake], gently rocking Wellington at bed time. Seemed to go on a good few seconds though."

Latest aftershock probabilities issued by GeoNet show there is now a 25 percent chance of one or more magnitude 6.0-6.9 earthquakes occurring within the next month - a decrease from 54 percent from the previous forecast last month.

The probability of a quake measuring over magnitude 7 was just three per cent this month - and 12 per cent this year.

That compared with an 89 per cent chance of one or more quakes above 5.0 this month - a prediction already borne out with Wednesday's quake - and between seven and 29 such-sized events this year.

Probabilities had fallen over time, although the sequence could well carry on for years.
Since November 14, more than aftershocks had measured between 4.0 and 4.9, 53 ranged between 5.0 and 5.9, and four were greater than magnitude 6.0.