Newly discovered earthquake faults near Wairarapa towns

A new study has uncovered earthquake faults in the Wairarapa valley – four of them close to some of the region’s towns.

By Sue Teodoro of Local Democracy Reporting

Wairarapa’s Emergency Management Office controller Simon Taylor said the findings were not a surprise and could help people better understand risks.

Wairarapa is no stranger to earthquakes, with New Zealand’s largest recorded quake since 1840 being the 1855 rupture of the Wairarapa Fault, measuring 8.2 on the Richter scale.

The earthquake findings, published in a paper from Earth Sciences New Zealand, were shared with Local Democracy Reporting under the Official Information Act.

The newly discovered Ruamahanga, Woodside, Carters Line, and Pāpāwai faults were close to population centres.

Earthquake geologist Genevieve Coffey, one of the authors of the paper, said the longest was the Pāpāwai Fault.

"It extends for a length of about 26km from Morison Bush, south of Greytown, towards the northeast and past Pāpāwai marae. It crosses the Ruamahanga River and is mapped to finish at the base of Fosters Hill," she said.

"The Carters Line Fault is 18km long and is located about 1.5km southeast of Carterton. It is mapped from just north of the Waiohine River, towards the northeast, finishing west of the Ruamahanga River close to Cornwall Rd.

"Branching off the Masterton Fault just south of the Waiohine River is the Woodside Fault. It’s shorter, with a length of 6km, and is mapped to extend into Woodside finishing about 1.5km from the centre of Greytown.

"Finally, the Ruamahanga Fault is the northernmost of these faults. It has a length of about 7km extending through eastern Masterton and to the north, finishing just past the Ruamahanga River."

She said Woodside was the most active of the faults studied, with a slip rate of 1.1mm a year.

"On average the Woodside Fault breaks in a large earthquake roughly every 770 years."

Coffey stressed the slip rate and recurrence intervals were estimates at this stage, and needed to be tested by field work.

She said faults were often made up of numerous fault lines, called traces.

"While we’ve known for a while that earthquakes can occur in Wairarapa with faults like the Wairarapa, Masterton, and Carterton faults already mapped, these newly mapped, more widespread traces tell us that there is a larger area of Wairarapa where earthquakes may occur and cause strong ground shaking."

She said the findings suggested the 1855 quake could have caused more ruptures across the region than previously thought.

"The area was also likely bush covered, so some fault ruptures likely went unnoticed."

Coffey and her colleagues plan to do fieldwork along the Masterton and Pāpāwai faults soon, to study the previous quakes.

She said it was possible the Pāpāwai Fault could produce the largest quake of those mapped.

"We chose the Pāpāwai Fault as it was one of the newly identified faults presented in the paper and we know very little about it. It is also the longest of the newly mapped faults, which means it may produce the largest (maybe around a magnitude 7) earthquakes out of the faults mapped."

Coffey continued: “In contrast, we’ve known about the Masterton Fault for a while, but past studies haven’t revealed much about its earthquake history. Because it passes through Masterton, it’s an important fault to revisit and we think we have a location where can learn a lot more.”

A finding of the study was that faults across Wairarapa likely ruptured with one another in multi-fault quakes, like the 2016 Kaikōura earthquake.

"It’s also important to remember that earthquakes trigger other hazards such as landslides, liquefaction and tsunami (when offshore) and there are some good lessons from the 1855 Wairarapa earthquake," Coffey said.

"Earthquake fault ruptures may also impact rivers such as the Ruamahanga and we’re hoping to do some work looking into this with mana whenua."

Wairarapa’s three councils set up a combined permanent emergency unit last year, the Wairarapa Emergency Management Office (EMO).

Taylor said mapping already showed significant faults in the district, often close to population centres.

"In fact, this helps us understand better the type and level of risk in the environment around us."

He said in a significant quake the Wairarapa Emergency Operations Centre would activate and could lead to a mayor, or mayors, declaring a state of local emergency.

"The Wellington Regional Emergency Management Office (WREMO) is likely to be involved, and if the event was greater than Wairarapa, a national response may be triggered."

He recommended people look at the WREMO website which had information on how to best prepare.

The authors of the Earth Sciences New Zealand paper, Newly discovered active faults in the Wairarapa Valley: Implications for multi-fault rupture and kinematics in the southern North Island, Aotearoa New Zealand, were: Genevieve Coffey, Nicola Litchfield, Regine Morgenstern, Rob Langridge, and Dougal Townsend.

– LDR is local body journalism co-funded by RNZ and NZ On Air.