East coast a magnet for big quakes

Scientists believe the North Island's east coast has been hit by up to 20 magnitude-8 earthquakes in the last 18,000 years. Photo / Alan Gibson

Scientists believe the North Island's east coast has been hit by up to 20 magnitude-8 earthquakes in the last 18,000 years as they use historical findings to try to predict when future quakes may happen.

The National Institute Of Water and Atmospheric Research (Niwa) is looking into the past to see if, and how often, New Zealand has been the subject of earthquakes as large as the magnitude-9 that hit Japan in March last year.

Studies of sediment taken 60km offshore of Poverty Bay found 67 layers identified as having been triggered by earthquakes of a magnitude 7.5 or above.

The layers captured 18,000 years of sediment and suggested the large earthquakes occurred every 230 years, Niwa principal scientist Geoffroy Lamarche said.

Dr Lamarche said the study also showed some mudslides had occurred simultaneously along the entire length of Hikurangi Margin between Poverty Bay and East Cape, which was believed to have been caused by "great earthquakes" of a magnitude-8 or above.

"Up to 20 such events have been revealed by the sediment cores, suggesting that as many great earthquakes occurred in the northern Hikurangi Margin over the last 18,000 years, that is every 820 years," Dr Lamarche said.

Christchurch's devastating quake in September 2010 was a magnitude-7.1.

In 1931 a magnitude-7.8 quake shook the Hawkes Bay region and killed 256 people.

Dr Lamarche said scientists were still studying the relationship between submarine mudslides and earthquake magnitude.

He said an understanding of the past would help map the likelihood of future large earthquakes in the region.

Similar work is also being done in Fiordland.

The sediment off the coast of Poverty Bay was taken in 2006 by a French research vessel. Scientists took two 20m-long cores of sediment from the sea floor using the Calypso corer, the only one of its kind in the world.