Faults off West Coast capable of causing 7.8 magnitude quakes

Photo / File

Faults off the West Coast are capable of causing earthquakes up to magnitude 7.8, seismic research shows. They are the type that generate tsunamis and it's unknown when they will rupture.

There are 10 active marine faults over a 320-kilometre stretch from Hokitika to Farewell Spit, reveals a report from the National Institute of Water and Atmospheric Research (Niwa) for the West Coast Regional Council (WCRC).

Buller District Council emergency management officer Mike Casey said there had been a renewed level of interest in earthquakes since Canterbury.

It was good to see more research coming through that clarified existing beliefs around fault lines off the West Coast.

Niwa marine geologist Philip Barnes said the work filled a huge gap. It was a significant advance in understanding seismic hazard on the Coast. The region was vulnerable because of its infrastructure and its proximity to a fault-controlled coastline.

Niwa led the two-year mapping project for the WCRC, which wanted to assess the tsunami risk for its coastal communities.

Dr Barnes said it was impossible to provide that information without knowing whether there were active faults close to shore.

"The oil industry has been very active off the West Coast and has collected a lot of data about some of the faults over several decades.

"However, this is the first time the offshore data has been analysed to determine whether these faults are active and the potential earthquake hazard they pose."

In its report to the WCRC, Niwa said: "The faults are parallel to the coast within 30km of the shore; there are 10 marine faults in the 320km between Cape Farewell in the north and Hokitika in the south; these include five segments of the Cape Foulwind Fault, the Kahurangi and Kongahu faults, and three others referred to informally as the Farewell, Elizabeth, and Razorback faults; the faults range in length from 10km to 120km and are expected to generate earthquakes ranging in magnitude from 6.4 to 7.8; it is estimated earthquakes of these strengths would recur on individual faults between once every 7500 to 30,000 years."

Dr Barnes said while these faults were relatively large and capable of causing severe earthquakes, it was thought that they had extremely long recurrence intervals. This meant large earthquakes would be very infrequent.

However, it was impossible to tell when they might have ruptured before, and there were significant uncertainties in the rates of fault activity.

Scientists were a step closer to addressing the Coast's tsunami risk, but there was still a lot to be done.

The WCRC's planning and environmental manager, Michael Meehan, said the information would fit into work to be carried out in the future.

"The council thought it important to gauge what we are dealing with, and to help us decide where to from here," he said.

The West Coast had had two of New Zealand's most severe earthquakes in recent history: the Murchison earthquake in 1929, which measured 7.8, and the 7.1 Inangahua earthquake in 1968. The quakes were caused by ruptures on onshore faults that shared many similarities with the offshore faults.

- Westport News