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Geologists have discovered an active fault north of Wellington which they say gives an improved insight into the earthquake hazards facing the region.

The Otaki Forks Fault, as it has been named, runs for 70km between Upper Hutt to the Mangahao River valley east of Levin.

Geologists had long been curious about the suspicious-looking surface feature of unclear origin, but until this year it had not been subjected to field studies.

With the help of funding from the Earthquake Commission, geologists John Begg and Russ Van Dissen of the Institute of Geological & Nuclear Sciences Limited (GNS) studied the feature to find out if it was an active fault, and to assess the size and frequency of earthquakes it could generate.

By digging a trench across the feature, they found evidence that it had ruptured either two or three times in less than 10,000 years, each time producing a large earthquake.

“ The fault has produced horizontal displacements of several metres which means it’s capable of generating an earthquake of magnitude 7 or greater,” Mr Van Dissen said.

“ Our assessment is that it ranks with the Ohariu Fault, west of Wellington, in terms of its ability to produce a major earthquake every 1000 to 4000 years.”

While the threat from such a fault might seem remote, Mr Van Dissen pointed out that the devastating earthquakes in Kobe in 1995 and in Taiwan last year both occurred on faults that had recurrence intervals of 1000 years or more.

Further field studies would be needed to determine if the Otaki Forks Fault ruptured along its entire length, or if sections ruptured independently.

“ Our purpose in publicising this is not to scare people, but to reinforce the message that taking sensible precautions is important.”

It was unclear if the southern end of the fault joined the Wellington Fault or the Ohariu Fault, west of Johnsonville. If it joined up with either of these two faults, it would imply that they are more active than previously thought.

Mr Van Dissen said GNS was continually improving the knowledge of New Zealand’s earthquake hazards and an important part of that process was characterising known faults.

“ Advances in this area will ultimately help save lives and reduce property damage in the event of an earthquake.”

It was difficult to recognise some faults because evidence had been removed from the landscape by high erosion rates and human settlement.

The discovery brings to seven the number of major faults in the Wellington region, all capable of generating an earthquake of magnitude 7 or greater. In addition, there are at least 20 “second order” faults also capable of producing a damaging earthquake.

The major faults in the Wellington region are the Wellington Fault, Shepherds Gully Fault, Ohariu Fault, Northern Ohariu Fault, Wairarapa Fault, and the Otaki Forks Fault. The Subduction Thrust, about 30km below Wellington, is also a potential source of damaging earthquakes.

The Wellington Fault is the most active of the surface faults with a recurrence interval of about 600 years. It last ruptured between 400 and 500 years ago.


For more information contact:
John Begg, Geologist

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