Team to monitor offshore quakes along 'locked' part of Hikurangi Subduction Zone

Researchers on board the Laurentian Fan offshore Nova Scotia. A similar international mission to deploy ocean-bottom seismometers will happen soon here. Picture by Graeme Cairns

An international team of scientists are setting off on a voyage to paint a clearer picture of our earthquake and tsunami risk from New Zealand’s largest and most active fault, the Hikurangi Subduction Zone.

New Zealand is dotted with earthquake detectors, called seismometers, that make up the GeoNet network. This network can detect even very small earthquakes, provided those earthquakes occur on land.

“We’re effectively blind to small earthquakes occurring offshore,” Victoria University of Wellington Te Herenga Waka and project team leader Professor Martha Savage said.

To solve that problem, the team will soon set off on the Niwa Research Vessel Tangaroa, to deploy ocean-bottom sensors that can detect offshore earthquakes, which would otherwise go unrecorded.

“Previous studies have investigated areas of the Hikurangi Subduction Zone where slow-slip events are occurring, relieving stress on the plate.” said Professor Savage “This is the first study to look for earthquakes on the locked part of the fault.”

The Hikurangi Subduction Zone is a plate boundary fault, where the Pacific tectonic plate dives down westward beneath the Australian tectonic plate. It runs the length of the east coast of Te Ika-a-Māui/the North Island, and comes as close as 40km off the Gisborne coast.

In parts, the two tectonic plates are slowly sliding past each other, releasing pressure, but others are locked together, stuck and building up pressure which could be released as a major earthquake.

“We’re expecting to see 10 times more earthquakes on the locked zone than are currently reported,” Professor Savage Said

“The behaviour of these more frequent small earthquakes can tell us more about the larger earthquakes that occur less often.”

On November 25, the team from Victoria University of Wellington Te Herenga Waka and GNS Science Te Pū Ao alongside the University of Ottawa and Dalhousie University in Canada, will set off from Wellington to deploy the ocean-bottom sensors off the coast of Wairarapa.

“We’re excited for this new collaboration between Canada and Aotearoa New Zealand researchers”  Dalhousie University Professor Mladen Nedimović said. “The instruments are brand new and this will be the first project that uses them to study earthquakes in a locked subduction zone.”

“Even if we find very few earthquake signals in the locked zone, that will still be a significant discovery,”  GNS Science Te Pū Ao researcher, Dr Emily Warren-Smith said.

“It helps confirm that our land-based observations have been right and that there is significant stress build-up occurring offshore.

“There are also several other faults in the overriding Australian plate, above the main subduction fault, which also have the potential to produce large, tsunami-generating earthquakes. Understanding their activity is another vital objective,” Dr Warren-Smith said.

The research was made possible by funding from Toka Tū Ake EQC, the Canadian Foundation for Innovation, and the University of Ottawa, and is a key step in forecasting earthquakes and tsunami from the Hikurangi Subduction Zone.

“Larger earthquakes from New Zealand and overseas that will be recorded on the ocean floor can also help us to understand the geometry of the subduction zone,” University of Ottawa Professor Pascal Audet said.

“Information carried by these seismic waves can also help us to better understand the likelihood of earthquakes, and how the movement of a future earthquake might cause a tsunami.”

Several students will be on board for the six-day voyage and will be documenting their experience through daily blogs and vlogs.

• You can follow their blogs and vlogs about what it is like to participate in a scientific sea voyage, at eastcoastlab.org.nz.