Scientists study past earthquakes

Scientists are exploring methods to determine the extent and effect of earthquakes that rocked New Zealand before oral and written records were kept.

NIWA principal scientist Dr Geoffroy Lamarche says scientists want to know if, and how often, large earthquakes in the order of the one which shook Japan last year have occurred along the Hikurangi Margin off the North Island’s east coast.

A scientist stands in the rift that opened in a paddock near East Bank Road during the Edgecumbe earthquake. Photo: GNS Science.

“We have identified a large number of small submarine mudslides along the continental slope between Poverty Bay and East Cape, which have been triggered simultaneously by earthquakes of magnitude 7.5 and over,” says Geoffroy.

“We are still assessing the relationship between these submarine mudslides and earthquake magnitude.”

The last major earthquake to hit the Bay of Plenty was a magnitude 6.3 event which shook Edgecumbe in 1987.

Centred in the town, the shallow quake was among the worst in the North Island for decades, with about 50 per cent of homes in the region being damaged.

Kawerau was another nearby town affected and Whakatane was also badly shaken.

Geoffery says the frequency and severity of past events must be known before predictions can be made about the likelihood of large earthquakes in the future.

He says there is little information available about the occurrence of large events bin the past.

In 2006, on board the French research vessel Marion Dufresne, scientists took two 20-metre long cores of sediment from the seafloor, 60 kilometres off the shore of Poverty Bay.

The cores were collected using the Calypso corer, which is the only one of its kind in the world.

Using a 60 metre-long steel tube which was pushed into the sediment, the Calypso corer retrieved two core samples from the seafloor. Samples were collected along the length of the cores, analysed and carbon dated.

“What we found was overall, 67 layers were identified as being triggered by earthquakes of magnitude 7.5 or above in the Poverty Bay region over the last 18,000 years. These earthquakes occur every 230 years on average.

“We were able to demonstrate that some mudslides occurred simultaneously along the entire length between Poverty Bay and East Cape, in which case we believe this was the result of great earthquakes (>8).

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

He says there are other types of layers in the sediment that are caused by natural undersea sedimentation, flooding or volcanic activity.

“Scientists are able to tell these layers apart because they are composed of different sorts of material and fine-scale organisation of the sediment differ.

“From the two cores, we can recognise turbidites, which are fine submarine mudflows or landslides usually triggered by earthquakes, from those sediments delivered and deposited from rivers," says Geoffery.

"We can characterise the turbidites from the shells, and the minerals, present in these sediments. The type, size and organisation of minerals as well as their chemistry provide clues on the origin, timing and mechanisms of positioning of the landslides. There are differences between sediments that are gently deposited on the seafloor, as part of the natural sedimentation in the deep ocean, from that delivered from rivers into the sea.

"This is important to understand the origin of the sediments before we can date them."

The information the scientists have found will be very useful for earthquake hazard assessment in the future. Similar work is also being undertaken in Fiordland.

Funding was provided by the Ministry of Science and Innovation.