Wairarapa quakes not done yet

By don.farmer@age.co.nz -
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Deep-seated aftershocks predicted to die away

CREAK: Cracks in the Masterton-Castlepoint road after Monday's earthquake.
CREAK: Cracks in the Masterton-Castlepoint road after Monday's earthquake.

Expert updates in the wake of what is now being called the Eketahuna earthquake predict the most likely scenario is for aftershocks to drop away and remain relatively deep over the next month but geologists have not ruled out another quake of similar - or larger - size striking within the same timeframe.

Latest data from GNS Science contains three scenarios, based on the understanding of tectonics, data from the January 20 earthquake, historical observations and statistical models.

The first, which scientists claim is by far the most likely outcome, rated at 90 per cent over 30 days, is for aftershocks to gradually die away and remain deeper than 25km.

The second, rated at 9 per cent within 30 days and described as "very unlikely", is for another earthquake of between 6 and 7 on the Richter scale happening at the same depth of the aftershocks in a nearby part of the Pacific plate or perhaps centred at a shallower depth (less than 25km) in the overlying Australian plate.

If that occurred, there it is likely to be stronger shaking at the Earth's surface than was the case on January 20.

GNS Science know of two examples in the east of the North Island where a large earthquake was followed by another of similar or greater magnitude.

A 6.5 quake in June, 1942, centred near Masterton was followed by two others - a 6.5 that August and a magnitude 6 in December.

In February, 1990, a magnitude 5.9 struck near Weber/Porangahau followed by a 6.2 in May in the same vicinity and strong enough to damage buildings.

The third and most unlikely scenario according to geologists - less than 1 per cent within 30 days - is for a larger quake than magnitude 7 to hit on the plate interface between the Pacific and Australian plates.

Although "extremely unlikely", GNS Science has hedged its bets by saying an earthquake of that size can occur anywhere in New Zealand at any time.

The chances of one occurring have been "temporarily increased" by the Eketahuna earthquake.

Re-capping on the Eketahuna quake which struck just before 4pm on January 20, centred 10km east of Eketahuna, GNS Science has received over 9000 reports from people who felt the quake along with multiple reports of damage.

Most of the aftershocks are now happening in the crust of the Pacific plate, which is diving below the Australian plate. The Eketahuna earthquake occurred in a seismically active area that extends from Wellington to Gisborne and which scientists describe as a geologically complex area where the Pacific tectonic plate is being subducted beneath the Australian plate at a rate of about 42mm per year.

GNS Science research indicates a slow-slip event (SSE) was thought to have affected the stress on the faults associated with the Weber 1990 earthquakes.

There is a ongoing SSE beneath the Kapiti coastline that may be causing changes in stress beneath the Tararua and Wairarapa districts, and GNS research into SSEs and their relationship to earthquakes is ongoing.

Meanwhile, the Ministry of Civil Defence says the best thing to do during an earthquake is to "drop, cover and hold". Director John Hamilton said "if you are inside move no more than a few steps and do not try to run outside. Stay indoors until the shaking stops and you are sure it is safe to exit."

In most buildings in New Zealand, it is safer to stay where you are until the shaking stops, Mr Hamilton says.

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