The shaking's settling down

MICHAEL DALY
Last updated 14:36 26/07/2013
Geonet

Animation of Cook Strait earthquakes since 19 July. Bathymetry courtesy of NIWA.

Relevant offers

With just two earthquakes above magnitude 4 in the 24 hours to early this afternoon, the aftershock sequence from the Cook Strait quakes is showing signs of quietening down.

Despite that, the risk of another big shake remains heightened, with GNS Science estimating the probability of another quake above magnitude 6 to be about four times greater than normal.

As at midday today, the probability was 2 per cent for a quake of that size in the next 24 hours, 8 per cent in seven days, and 27 per cent in a year.

The Cook Strait sequence started with a 5.7 quake on Friday last week centred 30 kilometres east of Seddon in Marlborough, while the largest was Sunday's 6.5-magnitude quake centred 25km east of Seddon.

"Basically the aftershock sequence is starting to quieten. That is kind of what we would expect," GNS seismologist John Ristau said.

That did not rule out the possibility of another big quake. "It's small but not zero," he said.

"You are more likely to get one of these big aftershocks, another big earthquake, very soon after the first big earthquakes. The longer you go without getting one, the less likely it is you are ever going to get one."

It was difficult to say when seismicity in the area would return to normal.

"It might still take a few weeks to go back to background levels of seismicity, providing we don't get another big earthquake to rejuvenate the aftershock sequence," Ristau said.

In the absence of another big shake, people might stop noticing the aftershocks within the next week or two.

People in Wellington might also be less likely to feel the aftershocks as the epicentres had been migrating closer to Seddon.

Before 2pm today, the last aftershock above magnitude-4 was a 4.2 at 9.48pm yesterday, which followed a 3.8 15 minutes earlier. There was also a 3.9 about 7.40pm and a 4.2 just before 2pm yesterday. Geonet data showed there had been 60 earthquakes with a magnitude between 4 and 4.9 since 7am on Friday last week.

Ristau said a magnitude-4.2 quake about 11am today centred in Cook Strait west of Paekakariki was completely different to the sequence off Seddon.

Ad Feedback

- Stuff

Comments

Special offers
Opinion poll

Is the region better served by having multiple events over one weekend or spread out throughout the year?

Multiple events

Spread out over year

Vote Result

Related story: (See story)

Featured Promotions

Sponsored Content