Research reveals proof of 'megathrust' quakes in New Zealand

  • 18/05/2015

Two enormous "megathrust" quakes ripped through New Zealand in the past 1000 years, likely triggering tsunamis and registering a magnitude of more than 7.5, according to new research.

For the first time, scientists have found direct geological evidence of large subduction quakes in the Cook Strait-Marlborough area on the Hikurangi margin, where the Pacific and Australian tectonic plates collide.

There has previously been no record or evidence of a major subduction quake on the fault, but scientists have always assumed they can occur.

Sediments and microfossils extracted from samples collected in Big Lagoon, a large coastal lake east of Blenheim, show the area was rocked by quakes roughly 350 years apart, according to the research published this week in the Bulletin of the Seismological Society of America.

One quake occurred between 520 to 470 years ago, and the other 880 to 800 years ago.

Lead author Kate Clark, of GNS Science, says the quakes likely occurred about 10km to 30km beneath the Cook Strait seabed, and the older quake was accompanied by a three-metre-high tsunami that travelled about 360m inland.

Scientists can't say how big the quakes were, but quakes with similar impacts in comparable geological settings registered at a magnitude of more than 7.5.

The current seismic model estimates these types of mega quakes occur every 500 to 1000 years.

A 2013 study found that, in a worst case scenario, a 8.9-magnitude quake on the Hikurangi margin could cause 3350 deaths, 7000 injuries and lead to $13 billion of damage in Wellington alone.

The research has been welcomed by Kiwi scientists, who say it will be helpful for emergency management and civil defence planning.

Associate Professor John Townend, who's an Earthquake Commission fellow in seismic studies at Victoria University, said the research would allow New Zealand's seismic hazard model to be refined.

"In order to understand the hazard posed to New Zealand by future large earthquakes, we need to know when such earthquakes have occurred in the past and how big they were," he said.

"The challenge with this is that the earthquakes of most interest happen very infrequently."

What's a subduction quake?

  • Occur under the surface of the upper plate where two plates meet
  • More likely to trigger tsunamis, affect a larger area, and be larger in magnitude than other types of quakes
  • Responsible for some of the biggest quakes in the world, including the 9.3-magnitude quake that rocked the Indian Ocean in December 2004

(Source: Science Media Centre)

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