Expedition to reveal Hikurangi Subduction Zone’s ‘hidden’ quakes

Earthquake-detecting ocean bottom sensors like these - pictured being deployed offshore in Nova Scotia in Canada - are about to be dropped off the coast of Wairarapa in a new international expedition. Photo / Graeme Cairns

Scientists are sailing into New Zealand’s largest fault zone to get a deeper understanding of its potential to trigger enormous earthquakes.

Over the past decade, major studies have unlocked a trove of fresh insights about our Hikurangi Subduction Zone – the vast boundary off the North Island’s East Coast where the Pacific tectonic plate dives beneath the Australian tectonic plate.

These have included the recent discovery of a giant reservoir of buried water thought to help hydrate the zone’s faults – and a host of new findings about mysterious and silent “slow-slip” quakes that could hold the key to forecasting the next major event.

The Hikurangi Subduction Zone spans an area stretching well north of East Cape to the South Island's east coast. Image / GeoNet

Scientists believe the subduction zone is capable of triggering “megathrust” earthquakes and tsunamis – recent examples include those in the Indian Ocean in 2004 and Japan in 2011 – and estimate a one-in-four chance of an 8.0 event beneath the lower North Island within the next 50 years.

While previous studies have focused on areas of the boundary where those slow-slip quakes quietly unfolded - helping to relieve pent-up tectonic stress over days, weeks and months - a new project marks the first to look at offshore quakes in a region where the plates are locked together.

These quakes, which scientists have been effectively blind to, will be observed using an array of ocean-bottom sensors about to be deployed off the coast of Wairarapa.

“We’re expecting to see 10 times more earthquakes on the locked zone than are currently reported,” said Professor Martha Savage, a geophysicist at Victoria University of Wellington, who’s leading the international team.

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

Slow-slip events occur in an area where the Hikurangi Subduction Zone is transitioning from being 'stuck' beneath the southern North Island, to an area where the subduction zone is 'creeping' further north, around Gisborne and Hawkes Bay. Image / GeoNet

Even if few quake signals were picked up in the locked zone, that’d still make for a “significant discovery”, said team member Dr Emily Warren-Smith, of GNS Science.

“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.”

The expedition, beginning later this month, is supported by funding from Toka Tū Ake EQC, the Canadian Foundation for Innovation, and the University of Ottawa in Canada.