Strong crust cause of big Sth Island quakes

CHARLES ANDERSON
Last updated 05:00 27/11/2013

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Future large earthquakes in the southern South Island could see many thousands of residents there suffer the same kind of powerful aftershocks experienced by Cantabrians over the last three years, new research shows.

Investigations have found the layer of hard rock beneath Canterbury that caused some of the strongest earthquakes of their type recorded may extend as far as Oamaru.

The research, led by GNS Science seismologist Martin Reyners, showed Canterbury's unusual rock structure produced some of the most violent vertical ground accelerations recorded in an earthquake.

The rock body, from below the ocean, also occurred throughout Wellington and the East Cape, but was deeper below Canterbury.

What was different about Canterbury's quakes was the rocks got stronger below 10km and continued to a depth of about 30km, Reyners said.

"What you end up with is a tight system so when the fault moved, the rock around it couldn't go anywhere because it was welded to this really strong [rock] layer."

A normal fault might have cracking within 100 to 200 metres of it, but in the Canterbury earthquakes the damage was 5km wide.

"That's huge," Reyners said. "Essentially we didn't have that weak crust shock absorber."

The researchers were determining how widespread this system was in the South Island.

"This is important for defining the earthquake hazard for people living between mid-Canterbury and Southland," Reyners said.

However, he believed it could extend to Oamaru because of a sequence of earthquakes in 1876 that had similar characteristics to Canterbury's quakes.

"It just means that if earthquakes happen in the area they are going to be very energetic but the flipside is that because the crust is so strong it takes a lot of strain to build up before an earthquake."

The repeat time for earthquakes along the Greendale fault was between 22,000 and 27,000 years.

"The people in Canterbury were very unlucky to be around when it went . . . There will be few other places in the world where a similar earthquake sequence might occur."

The researchers had initially set out to determine the three-dimensional structure of the crust to about 35km under Canterbury by using a technique called seismic tomography - similar to a medical CAT scan or ultrasound.

He said the research could change the way seismologists predicted aftershocks.

The unusual delay between the September 2010 and February 2011 quakes also may have been caused by a "strength recovery" required for the crust following the cracking following the September quake, Reyners said.

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- Fairfax Media

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