Evidence of potentially massive quakes found under central New Zealand

Scientists have found evidence of the possibility of massive earthquakes capable of generating tsunamis in central New Zealand.

At least two large subduction earthquakes had occurred under central New Zealand in the past 1,000 years, with one occurring 520 to 470 years ago and the other 880 to 800 years ago, the government's Institute of Geological and Nuclear Sciences (GNS Science) said Tuesday.

Subduction quakes differ from normal quakes in that they occur on the under surface of the upper plate, where two plates meet, instead of on faults within the upper plate.

They were responsible for some of the biggest quakes and tsunamis in the world, GNS Science researcher Kate Clark said in a statement.

Recent examples included the magnitude 9 Tohoku earthquake and tsunami in Japan in March 2011 and the magnitude 9.3 Indian Ocean earthquake and tsunami in December 2004.

The older of the two earthquakes identified from sediment cores taken from Big Lagoon, on the northeast of the South Island, was accompanied by a 3-meter-high tsunami that traveled inland about 360 meters.

"The findings are significant in terms of understanding earthquake and tsunami hazards in the lower North Island and upper South Island," Clark said.

The evidence did not allow scientists to estimate the size of the two quakes, but quakes with similar impacts in comparable geological settings were in excess of magnitude 7.5.

Scientists were investigating other locations in the lower North Island to find further evidence of subduction quakes to help to provide a better picture of how big these quakes might have been and how they impacted the region.

Subduction quakes were already accounted for in the National Seismic Hazard Model, which fed into building codes and informed engineering standards, said Clark. Endi