New information from international scientists on the Hikurangi subduction zone

RESEARCH VOYAGE: The RV Roger Revelle. Picture supplied

New information from international scientists' voyage to New Zealand's largest fault shows earthquakes here are affected by extinct, undersea volcanoes called seamounts.

Christine Chesley, from Columbia University, said the research was getting closer to finding out how a subducting seamount might affect the structure and distribution of fluids in subduction zones, which impacts the types of earthquakes that occur at the Hikurangi subduction zone, which runs as close as 40 kilometres off the Gisborne coast.

Ms Chesley said the research showed seamounts offshore of the East Coast were helping to create the slow-motion earthquakes.

Subduction is when two tectonic plates meet and one dives beneath the other.

Slow slip events are common along the Hikurangi subduction zone, where the Pacific Plate is subducting under the Australian Plate.

As the name suggests, the energy isn't discharged through a sharp jolt — as for an earthquake — but slowly over weeks to months.

Seamounts come in a range of sizes and shapes — from bumps and ridges to mountain-like features that would have once risen higher than Mt Ruapehu under the sea.

The scientists used a special technique that allowed them to figure out the electrical conductivity of rocks beneath the seafloor.

“Seawater is an excellent conductor compared to most seafloor rocks, and so this method allows us to see where the water has infiltrated,” Ms Chesley said.

“By placing receivers on the seafloor offshore from Gisborne and towing an instrument that generated electromagnetic fields just above the receivers, we were able to calculate the conductivity of a seamount on the seafloor.”

The team found that the seamount was a lot more conductive than they were expecting, and consistently so.

This meant the seamount was holding a lot of seawater inside of it like a sponge.

“We also found pockets of rock in the Australian Plate that were conductive just above a subducting seamount.

“These conductive zones happened to overlap the location of recent seismic activity.”

“These findings mean that seamounts carry water like a sponge, so when they subduct they can probably crack and crush pieces of the upper plate and potentially release water into these cracks. This water transfer could be related to some slow slip earthquakes.”

The research is part of research into slow-slip earthquakes and fluid flow at the Hikurangi subduction zone and is a key finding from the voyage of the R/V Roger Revelle (oceanographic research vessel) in 2019.