New fault found in Wellington Harbour

NIWA scientists have found a new active fault in Wellington Harbour after analysing data from a recent marine survey.

The fault, referred to informally as the Aotea Fault, runs in a northeasterly direction for about 2km in Lambton Harbour, petering out about 1km east of Westpac Stadium. 

NIWA marine geologists Drs Philip Barnes, Scott Nodder and Susi Woelz mapped the fault using geophysical techniques as part of the It’s Our Fault research programme led by GNS Science. 

This multi-year project is a comprehensive study of the likelihood of large earthquakes in Wellington and their impacts on people and the environment, and aims to make the Wellington region more resilient to earthquakes.

The Aotea Fault formed a scarp about 1 m high on the seafloor and is thought to be capable of moderate to large earthquakes in the order of magnitude M6.3 to M7.1. It has not yet been identified on land but Drs Barnes and Nodder believe it may extend to the south coast of Wellington and beyond into Cook Strait. They believe at least two significant earthquakes have occurred on the fault in the last 10,000 years.

The Aotea Fault is part of a series of several dozen geological faults in the Wellington region, many of which are considered capable of generating a strong earthquake.

GNS scientist Russ Van Dissen said the fault did not appreciably increase the overall ground-shaking hazard in Wellington. 

“Any ground-shaking that this fault could produce is already considered in Wellington’s seismic hazard calculations. So it is already accounted for in the building code,” he said. 

When seismologists estimate seismic hazard for a region they typically add ‘floating faults’ to their calculations as their knowledge of all earthquake-producing faults is imperfect. 

This ‘headroom’ in the hazard calculations allows for faults that may be hidden or have poor surface expression. 

Dr Barnes said the newly identified fault was a good example of such a fault which scientists occasionally found when analysing geological data.

“We are making this information known to the public on the basis that it is important to keep people informed about research into geological hazards.”

Dr Nodder said finding this fault was perhaps not a big surprise as it fitted into the overall pattern of faulting in the Wellington region. It demonstrated why ongoing fault studies were important. 

“Knowing more about Wellington’s geological faults has major benefits in terms of being better prepared and making the city more resilient to earthquakes,” he said.

“It is important to work out its exact location and ground rupture size so this can be incorporated into land use and engineering strategies to mitigate this hazard,” Dr Nodder said.

In the meantime, ongoing research for Greater Wellington Regional Council is also being carried out on the Evans Bay Fault, which is a possibly active fault north of Rongotai in central Evans Bay, first identified over three decades ago.

It’s Our Fault is funded by the Earthquake Commission, the Accident Compensation Corporation, Wellington City Council, Wellington Region Emergency Management Group,  and Greater Wellington Regional Council. 

Lambton Harbour. [NIWA]