'Silent' quakes spare region

CHRIS HYDE
Last updated 12:00 30/07/2013
Fault lines

AT RISK: Graphic showing Manawatu's fault lines.

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There is a magnitude 7.5 earthquake brewing near the Manawatu Gorge.

And if it wasn't for "silent" earthquakes, there would probably have been one of similar size under Palmerston North nine years ago.

Faults flow like lines across a palm through Manawatu's landscape and a now-retired earth sciences expert, emeritus Professor Vince Neall, wants residents to have the facts about them.

With the 6.5 magnitude Cook Strait earthquake still fresh in the minds of lower North Island residents, people in Manawatu needed to realise a big shake of our own was not just possible but inevitable, Dr Neall said.

Perhaps the biggest threat to the region lies 1.5 kilometres south of the Ballance Bridge in the Manawatu Gorge. Here, the Wellington fault does something "rather weird" - it splits.

As the North Island's largest and most active fault crosses the gorge it becomes two - the Ruahine and the Wellington.

Each has the potential to cause a magnitude 7.5 earthquake.

Based on data that looks back 2000 years, both fault lines are expected to deliver one every 500 years.

The last earthquake on the Wellington fault took place 300 years ago at Kahuki, west of Pahiatua. It is thought the magnitude was 7.5.

"It's the active faults that are a major concern to us," Dr Neall said.

"The Wellington fault is probably the one we have to watch out for most here.

"When you drive from Woodville and you turn right and you come up the Saddle Rd, that initial climb takes you right over the scarp of the Wellington fault.

"Even though it's on the eastern side of the range, as the crow flies it's not that far from Palmerston North."

The Manawatu region was one of the most active for earth movement in New Zealand but we were largely spared from big shakes by what were known as "slow-slip events" or "silent earthquakes", Dr Neall said.

According to GNS Science, one of the longest and best documented silent earth quakes in New Zealand took place from January 2004 to June 2005 beneath the Manawatu region.

This moved GPS sites near Ashhurst, Whanganui and Dannevirke 10 millimetres to 30mm to the east.

Scientists believe this was a result of about 350mm of slip on the plate boundary beneath Manawatu.

"Had it all occurred at once we would have had a 7.5 earthquake right under Palmerston North," Dr Neall said. "May all our earthquakes be slow moving."

Further southwest is the Ohariu fault which runs on the western edge of the Tararua Range near Levin.

It is deep but active.

Scientists did not know exactly what happened to it once it got nearer to Palmerston North, Dr Neall said.

To the east, the Waewaepa fault is active and has the potential to shake large areas, and even further east, veining across Wairarapa are the Alfredton and Waipukaka faults.

It is one of these that is thought to have caused the 1934 Pahiatua earthquake, which measured 7.6 and was probably 20 kilometres deep.

It killed one person.

Active fault lines and the anticlines - ridges of rock - caused by their movement have shaped Manawatu's landscape dramatically.

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The Manawatu River would reach the sea near Tangimoana if it weren't for the Mt Stewart anticline, Dr Neall said.

- Manawatu Standard

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