Sunday, May 15, 2016

New fault a threat for Tauranga

Tauranga is under threat from the formerly seismically sleepy Waikato, according to new research.
The region has been moved into the front row of major earthquake threats because of new research indicating the Hauraki Plains will get an earthquake as big as Christchurch's – sometime in the next few thousand years.

This video depicts the timeline of New Zealand's big earthquakes. Image and video: GetNetNZ/Youtube.
New research finds the Hauraki Plains, including Matamata and Te Aroha, will probably be struck by an earthquake with the same amount of shaking as the magnitude 6.3 earthquake in Christchurch in 2011.
Research by GNS Science researcher Pilar Villamor and Mira Persaud, of the Swiss National Science Foundation, brought about a reappraisal of the 80km-long Kerepehi Fault, which runs through the Waikato to the Hauraki Gulf.
They found the fault, which runs through Matamata into the water at the Firth of Thames, contributes is a seismic hazard to major North Island cities including Tauranga, Hamilton and Auckland.
The Kerepehi fault can produce earthquakes with magnitudes anywhere from 5 to 6.3, 6.5 or even 7.2, if all the fault segments rupture at the same time, says Pilar.
The longer a fault rupture, the larger the magnitude that is associated with it.
“We don't know because the data we have is not strong enough to tell us whether that is happening but we also don't have strong evidence that it is not happening. You play devil's advocate with your own data.”
The best evidence they have is one of the fault segments ruptured sometime between 5000 years ago and 1700 years ago.
“So if it's closer to 1700 years ago, then we have still a long recurrence to go,” says Pilar.
The Kerepehi fault is similar to faults in Rotorua and the Bay of Plenty, where fault segments that have ruptured through soft materials on the surface merge into a single fault at depth.
“The best example is actually Edgecumbe in 1987. The major fault that ruptured then was probably about 18km long in depth. But when you look on the surface, the part of the fault that ruptured on the surface ruptured in patches.
“So along that fault line you have patches of 3km, than a gap of a few kilometres, then a patch of 8km. So if you go a thousand years after and you map those traces, you will see disconnected traces on the ground but they connect in depth.
“To get a good feeling about what is going on you have to dig a lot of trenches across most of the traces and that is expensive.”

Kerepehi fault gains prominence. 
The research on the Kerepehi fault has been ongoing for many years as funds have been made available.
It was boosted recently by the access to higher resolution Lidar data, and the arrival of Mira Persaud doing post-doctoral research and who wanted to do more fieldwork.
The trigger bringing forward the publishing of the paper was a special issue of the NZ Journal of Geology and Geophysics and is available online.
A lot of the data is used in the national hazard model for New Zealand, and is incorporated into the model used by the building code to decide what sort of levels of the strength the regions of the country need.
“So even if the data was not fully published, it was being used,” says Pilar.
Scientist use aerial photographs to track fault traces, preferably old photos, taken before building and pine trees obscured much of the landscape.
“You can still see a lot of fault traces you cannot see these days on google maps,” says Pilar.
The higher resolution Lidar data has enabled them to find many more fault scarps that they could see with the aerial photography.
“When the fault scarps are very small or depending on the day the photograph was taken, where the light was coming from and the shadows, whether it was a sunny day or not some photos are not that good for certain fault scarps,” says Pilar.
“And if they are very small like a little bump in the ground of half a meter you might not see it on the aerial photo. So with this technique of lidar we are now starting to detect a lot of interesting traces along the country.”
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