North Island 'silent earthquake' risk studied

KIRAN CHUG

02/08/2011

The possibility of a magnitude nine earthquake off the east coast of the North Island cannot be ruled out by scientists planning a new research project to study tremors in the area.

Scientists are hoping to study the phenomena of "silent earthquakes'' which happen on subduction zones, the same plate boundaries where megathrust earthquakes, or those larger than magnitude nine, occur.

GNS Science geophysicist Laura Wallace said little was known about silent, or slow slip, earthquakes or their relationship with megathrust events, but researchers wanted to learn more.

"We know the slow slip events are happening around the edges of the area where megathrust earthquakes happen. They help to outline that area.''

Since scientists established their presence around New Zealand in 2002, slow slips have been observed in Hawke's Bay, Manawatu and on the Kapiti Coast.

A large silent earthquake began under the Manawatu land region in the middle of last year, and was now tapering off.

Between September and December there was about 15cm of movement.

"If [the] slip in the current Manawatu event had occurred in seconds - as in a normal earthquake - rather than over many months, it would be equivalent to about a magnitude 7 earthquake, similar to the September 2010 Darfield earthquake.''

Seismologists had been able to correlate the silent earthquakes with clusters of very small earthquakes in the same region.

For example, a silent earthquake off Mahia and Gisborne last year caused small earthquakes in the same area, Dr Wallace said.

"Elsewhere around the world, seismologists have also detected a chattery seismic 'tremor' type signal that occurs during slow slip events, most notably at subduction zones in Japan and western Canada.''

As yet, no temporal link between silent and megathrust earthquakes had been established.

This week about 70 scientists from 10 countries are gathering in Gisborne to discuss the best ways to study silent earthquakes, and it is believed ocean drilling techniques could be used.

A proposal is being worked on for a research programme off the coast of Gisborne, where the Pacific plate is sinking beneath the eastern North Island along the Hikurangi Trough at a rate of about 5-6cm each year.

The plate is sinking at a rate of about 2-3 cm per year off the Wairarapa coast.

Scientists have previously seen as much as 15 to 20cm of movement along off the Gisborne coast in as little as two weeks.

Dr Wallace said Gisborne presented one of the best opportunities in the world for the drilling research because the silent earthquakes were occurring at depths of 5-15km below the seafloor, making the shallow part of the slip zone accessible with modern drilling methods.

- The Dominion Post