Two University of Waikato scientists believe they have found a fault line running under Hamilton city.

Waikato scientists believe they have uncovered a previously unknown fault line running straight through Hamilton.

The discovery has prompted experts to brief local leaders, and with the previously closest fault thought to be the Kerephi fault on the Hauraki Plains, it could mean a reassessment of the city's current low risk of earthquake rating.

But don't go diving for cover just yet - the fault may have last moved about 200,000 years ago and liquefaction - the silty ooze that left much of Christchurch a quagmire following the 2010 quake - is thought to be the biggest risk.



Supplied Cracks and soil layer movements in a construction site in northeast Hamilton got scientists wondering about a fault.

Discoveries of historic liquefaction, then underground cracks in a construction site in northeast Hamilton tipped the University of Waikato science brains off about the possible fault.



Drs Willem De Langeand Vicki Moon have since used land features to work out where it might run, De Lange said.

"We've identified several [faults], we think, but the one we're most confident in runs from Temple View, it runs essentially along the river at Days Park... and then out to Gordonton," he said.

"So that affects essentially the northwestern side of the city."

They need to do two to three more years of research to prove it conclusively, but they've already eliminated several possible causes for what they've seen.

As a scientist, it's an exciting discovery.

"But as someone who lives in Hamilton I'm not losing any sleep over the hazard," Moon said.

De Lange adds: "If it's moving every 100,000 years it's not going to have much effect at all."

However, if liquefaction had happened in the past it would happen again, they said.

Hamilton is generally seen as low-risk, for example on the National Seismic Hazard model, which works off a basis of data on known, active faults.

That included "assumptions" about the Hamilton area, De Lange said.

"By actually finding the faults we may find that this is worse than they assumed or equally we could find out that it's not as bad as they assumed."

The Earthquake Commission (EQC) said in a statement that the finding would not change its levy for Hamilton residents.

"The finding of a possible fault in Hamilton does not change the EQC levy for people living in the Waikato. Wherever a customer lives within New Zealand, EQC's cover costs 15 cents (plus GST) for every $100 of home or contents fire insurance a customer has," said in a statement.

The annual cost per house and contents was capped at $180 plus GST.

Insurance Council NZ spokesperson Sarah Knox said it was up to the council's individual members to decide on whether the potential fault affected their premiums.

The council would not be issuing any advice on the matter to its members.

But De Lange said finding a fault in Hamilton wasn't a surprise because it had always been thought some were in the area.

"We just never had [found one] over the last 40 years of looking," he said.

"[The Hamilton Basin] is filled with soft sediment that easily deforms and has been highly modified... so we've never been able to find them."

That also meant it was more realistic to talk about a fault "zone" than a line in terms of the latest discovery.

De Lange and Moon suspect what they've found moved within the last 200,000 to 250,000 years, at least.

"It may well be a lot younger than that," Moon said.

New Zealand faults have to stay still for 125,000 years before they're classed as inactive, and there's a reason the period is so long.

"There has been a history of severe earthquakes occuring in New Zealand in places where we haven't identified faults before. So there is a degree of caution," De Lange said.

Christchurch's September 2010 Darfield eathquake is an example.

Hamilton City Council and Waikato Regional Council - and the scientists themselves - are keen to keep residents from panicking about the possible Hamilton fault.

It was "business as usual", regional council's regional hazards team leader Adam Munro said, but they would take the information into account.

"This is not about pressing panic buttons. It's just about understanding more about the geography that we live within," he said.

"We're not saying we're going to have a huge event in the next day or so. We're saying this is just is what it is and the fault, if it is a fault, has always been there and it warrants further research."

"You can't say categorically anywhere in New Zealand is safe from earthquakes. We straddle two tectonic plates."

The regional council had part-funded the liquefaction research leading to the discovery and planned to support the University of Waikato in its next steps.

Research to provide "much more conclusive evidence" would take about two or three years, Munro said.

Hamilton City Council mayor Julie Hardaker said there would be no changes to council process until further research was done.

"Their research to date is about a possible fault line. And they emphasised that to us: a possible fault line. They don't have absolute certainty on that yet but there's certainly - in geological terms - evidence surrounding that" she said.

"At this stage until there's a lot more work done... There's no change to our low risk earthquake status and in fact the way we are approaching anything."

Council chief executive Richard Briggs said the context was that the scientists had found a point where there was a possible fault and used deductive reasoning to see where its route could be.

The liquefaction had happened within the last 19,000 years.

And the Hamilton area had always been known to have some liquefaction risk, Hardaker said.

How did they find it, and what now?

A series of needles in the haystack led to University of Waikato scientists Drs Willem De Lange and Vicki Moon finding what appears to be a Hamilton fault.

"[It] runs from Temple View, it runs essentially along the river at Days Park... and then out to Gordonton," De Lange said.

It started with an discovery of two sites of historic liquefaction by masters student Melissa Kleyburg in the Hamilton and Hauraki areas.

"I didn't expect her to find any... and she did," Moon said.

That liquefaction - when sediment bubbles up to the surface due to ground movement - happened within the last 19,000 years ago.

That prompted the scientists to put the word out to Waikato contractors, asking them to get in touch if they saw anything in their work that could indicate a fault.

Just before Easter, at a construction site in northeast Hamilton, someone did.

A series of almost-vertical cracks and a broken line of red soil caught the scientists' attention.

The red soil had dropped about half a metre over the course of the four metres examined.

Moon and De Lange believe it moved at least within the last 200,000 to 250,000 years.

They looked at other possible causes but a fault started looking increasingly likely.

"There are just too many coincidences," Moon said.

Looking at topographical features, such as the path of the hills around Hamilton, they mapped out where they think the fault runs.

Questions and more research work remain, so De Lange and Moon are applying for funding from EQC.

"The aim then is to dig a trench through it, which means we can really figure out the size of the fault, its displacement and, more importantly, when it moved and how big it was."

De Lange and Moon are also keen to hear from geotechincal engineers and engineering geologists who think they may have found signs of a fault.

What did they find?

Liquefaction in two Waikato sites (one in Hamilton, one in Cambridge)

Signs of what they think is a fault in northeast Hamilton: cracking and movements in the soil layers.

A suspected path for the fault: from Temple View, through Days Park at Chartwell, out to Gordonton.

They believe it has moved in at least the last 200,000-250,000 years.

More research is needed to conclusively prove whether this is a fault, and if it is active (has moved in the last 125,000 years).

Facts on faults

NZ faults are active if they have moved in the last 125,000 years.

Hamilton is classed as having a relatively low earthquake risk on the National Seismic Hazard model.

Soft sediment in the Waikato makes it hard to identify faults in the area.

The Kerepehi Fault in the Hauraki Basin was previously Hamilton's closest known active fault. It has moved about four times in the past 10,000 years.