Understanding New Zealand's largest fault - the Hikurangi subduction zone

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RNZ rnz.co.nz

Geologists have been probing New Zealand's largest earthquake fault and have installed two underwater earthquake observatories to try and understand slow slip or 'silent' earthquakes.

The drilling ship and a team of international scientists are between expeditions to our biggest fault line.
Source: 1 NEWS

The Hikurangi subduction zone is a sleeping geological giant that stretches down the east coast of the North Island. It is where the Pacific tectonic plate is being pushed under, or subducted, beneath the Australian plate.

The Hikurangi subduction zone produces some of New Zealand's biggest earthquakes and tsunamis, but while some of these large magnitude events happen suddenly, others happen over days or even months. These slow slip earthquakes were only recently discovered, using precise GPS technology that can measure tiny movements of the earth's crust.

NIWA's Phil Barnes says a slow slip event can release slip, or movement along the fault, similar to large magnitude earthquakes.

"But because it's released quite slowly, it doesn't release all that seismic energy that in terms of ground motion and seismic waves that pass through the earth make the ground shake," Mr Barnes says.

"So it's relieving stress on the plate boundary fault by allowing it to move and creep along in little bursts of activity."

GNS Science's Laura Wallace says three slow slip earthquakes occurred along the subduction zone or related faults following the magnitude 7.8 Kaikōura earthquake in 2016. The slow slip events measured up to magnitude 7.3.

Geologists around the world are keen to find out why and how slow slip earthquakes occur on subduction zones, and what implications they might have for sudden large magnitude earthquakes that can produce damaging tsunamis.

An international expedition, led by Laura Wallace and Demian Saffer from Penn State University in the United States, has just returned from a two-month expedition to the Hikurangi subduction zone aboard the drilling ship JOIDES Resolution, as part of the International Ocean Drilling Programme (IODP).

The expedition drilled more than a kilometre beneath the seabed, collected sediment and rock cores at four sites, including the deformation zone where the plate boundary is at the sea floor.

The cores contain alternating layers of mud and sands, or turbidites. These are generated by turbidity currents or density flows, which are underwater avalanches of sediment-laden water. One such underwater avalanche occurred in the Kaikōura Canyon following the 2016 earthquake and travelled for hundreds of kilometres along the Hikurangi Trough.

The turbidites in the cores are interspersed with layers of white and pink volcanic ash blown from large volcanic eruptions in the Taupo Volcanic Zone.

Ms Wallace explains that the cores will help geologists understand the kind of material that is being sucked into the subduction zone and might help explain why faults behave the way they do.

"It's a way of sampling the rocks that might be involved in slow slip events," says Mr Barnes.

Mr Barnes says one of the cores was drilled from the top of a seamount - an extinct underwater volcano. When these rocky lumps are subducted at the plate boundary they can behave quite differently to the smoother sediments around them. Mr Barnes likens it to trying to push a lump of dirt under a carpet; it can take a while for the lump to get squashed and flattened.

Spy in the fault

Two sub-sea floor observatories were installed in the slow-slip region. For the next five years they will continuously collect chemical information as well as data on temperature and pressure. The observatories will be able to listen to the creaks and groans of the plate boundary and nearby faults, and the geologists hope that they will be recording during future slow slip events, which occur quite regularly.

One of the observatories is on a major active fault that splits off from the Hikurangi megathrust fault, Wallace says. The scientists were able to position instruments so that they sit below, within and above the fault.

The second observatory is closer to land, above the area where the large slow slip event took place following the Kaikōura earthquake.

The scientists will return in five years and use a remote operated underwater vehicle to retrieve the data.

Underwater earthquake observatories have been established in three other places in the world, Saffer says. Two are near Japan, in a similar geological setting, and Saffer says they were able for the first time to collect very precise data on an unexpected set of slow slip events.

"The idea behind these observatories is that they are much more sensitive than a lot of other ways that you can sense or measure the deformation, or the contraction or expansion - the so-called 'creaks and groans' - of the crust."

"We're hopeful that in New Zealand we won't just be able to monitor the slow slip events that we know are happening," says Saffer, "but be able to see with more sensitivity what's happening in the lead-up to those events and following those events."

Drilling into a volcano

The JOIDES Resolution is currently drilling into an active volcano, Brother's volcano, on the Kermadec Arc.

The Brothers volcano expedition is the fifth of six two-month-long expeditions the JOIDES Resolution is undertaking around New Zealand.

IODP is an international research collaboration that conducts seagoing expeditions to study Earth's history and dynamics recorded in sediments and rocks beneath the ocean floor.

New Zealand participates in IODP through a consortium of research organisations and universities in Australia and New Zealand, including GNS Science, NIWA, University of Auckland, Victoria University of Wellington, and University of Otago.

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