An international team of geoscientists has unleashed a full-fledged onslaught to understand New Zealand’s biggest earthquake and tsunami hazard.
On 8 March, the JOIDES Resolution drill ship will begin a two-month expedition to bore deep into the Hikurangi subduction zone off the east coast of New Zealand’s North Island. There, the Pacific plate of Earth’s crust dives, or subducts, beneath the Australian plate. The grinding of these geological titans has the potential to unleash a magnitude-9 earthquake and accompanying tsunami.
The drilling effort is part of a broader project to better understand the danger of the Hikurangi. “It’s a major earthquake and tsunami hazard to the largest population centres, and it’s not very well understood,” says Laura Wallace, a geophysicist at the GNS Science research institute in Lower Hutt, New Zealand, and co-chief scientist of the upcoming cruise. The expedition will also give researchers the chance to probe the fault’s role in a type of slow-motion earthquake.
Whatever the scientists find will help inform their understanding of seismic processes in other parts of the world with similar geologic settings, says Susan Schwartz, a geophysicist at the University of California, Santa Cruz.
Work kicked off in October, when researchers sprinkled nearly 300 seismometers in a dense array near the town of Gisborne on the North Island. Around the same time, two research vessels — the US Marcus Langseth and New Zealand’s Tangaroa — deployed seismometers on the ocean bottom and blasted sound waves at them to visualize the ocean crust. Then, in December, the JOIDES Resolution did some initial drilling at three sites off the coast of Gisborne, to prepare for the bigger expedition that will kick off this week.
Anatomy of a danger zone
Together, the studies aim to build a detailed picture of the guts of the subduction zone. It is perhaps the largest geophysical-research effort in New Zealand’s history, says Stuart Henrys, a geophysicist at GNS Science who led the deployment of the land seismometers. The governments of New Zealand, the United States, the United Kingdom and Japan are helping to fund research on the fault over five years.
One thrust of the work is to determine whether, and how often, the Hikurangi might rupture in quakes as large as magnitude 8 or 9. A section of the fault offshore Wellington is geologically locked and does not move, whereas a more northern part, near Gisborne, moves slowly. The seismic studies should help to illuminate the behaviour of rocks on either side of the fault and how that influences earthquake risk in both regions, Henrys says.
Another big question is the role of ‘slow-slip’ events akin to slow-motion earthquakes, in which the action unfolds over weeks or months, rather than seconds or minutes. Geologists aren’t sure how slow-slip events influence the risk of larger quakes along a fault, but the Hikurangi is a natural laboratory for exploring that, Wallace says. Researchers can access it relatively easily, because offshore Gisborne the subduction zone experiences the shallowest slow slip in the world, just a few kilometres below the seafloor.
The Hikurangi usually sees slow-slip events once every year or two — including an episode triggered in November 2016 by the magnitude-7.8 Kaikoura quake on the South Island1. “It basically lit up the subduction zone in slow slip,” says Wallace. What scientists learn about slow slip at the Hikurangi could help them to better understand earthquakes in other slow-slip regions, including those off the coasts of Costa Rica, Mexico and Japan.
Drilling for answers
The JOIDES Resolution expedition aims to drill three holes into the area where the Pacific and Australian plates collide. This is likely to reveal what types of rock lie on either side of the Hikurangi fault, information that would allow researchers to better understand the physical properties of the place where earthquakes are generated.
For instance, a thick layer of sediments covers the deep-diving Pacific crust. “Getting our hands on those sediments before they are subducted will give us important insights into the frictional properties of rocks in the slow-slip zone,” Wallace says. Drillers will need to penetrate to 1.5 kilometres beneath the sea floor for scientists to truly understand this subducting crust and its role in Hikurangi quakes, says Nathan Bangs, a geophysicist at the University of Texas at Austin who led the Langseth cruises.
The drill team will install long-term observatories in two of the boreholes, roughly 400 metres beneath the sea floor, to monitor how pressure and temperature change during slow-slip events.
Nature 555, 295-296 (2018)