Slow slip events on subduction faults are bursts of movement on the fault that last weeks to months rather than minutes as in large earthquakes.
Scientists learned of their existence only about 15 years ago and are trying to solve the mystery of how they happen.
They have been described as the most significant seismological discovery in recent years.
Expedition co-leader and Niwa marine geologist Dr Philip Barnes says while slow slip events have been recorded around the world, the examples in the northern part of the Hikurangi subduction zone are special because they happen in relatively shallow depths beneath the sea floor where data can be collected to help reveal how they work.
“Right now, we can only speculate about what’s driving them. For a slow slip event to occur, you require the fault to move a little and slowly, without the movement advancing into a normal earthquake rupture.
“Since their discovery, scientists have come to understand that there is a whole continuum of seismological processes between constant creep on a fault and big earthquakes, and slow slips are one of those,” Dr Barnes said.
Understanding the relationship between slow slip events and earthquakes is an important aspect of this work.
Last year’s Kaikoura earthquake triggered a large slow slip event off the east coast, covering an area of more than 15,000 square km.
The slow slip event took place in the region of the planned expedition and should shed new light on why this occurred.
Drilling dataThe scientists will collect drilling data up to 1200m beneath the sea floor at three sites that range in depth from about 1000m to 3500m. During the drilling, instruments will record physical information about the rock types thought to host the subduction fault, and the properties of faults that splay from it to reach the sea floor.
The second focus of research on this expedition will study the underwater Tuahine landslides, led by Dr Ingo Pecher of the University of Auckland. Dr Pecher says scientists want to learn about the relationship between gas hydrate — a solid ice-like substance formed under the seafloor where gases are trapped in water — and creeping.
“Scientists have thought for more than 30 years that this ice-like substance stiffens the sea floor, whereas its dissociation, for example melting, might lead to submarine landslides.
“Recent evidence discovered east of Gisborne, however, suggests that hydrate itself might be responsible for creeping of submarine landslides. We plan to study how gas hydrates might cause such slow deformation,” Dr Pecher said.
The expedition is the first of two aboard the scientific drilling research vessel JOIDES Resolution, operated by the National Science Foundation.
On the second Hikurangi expedition, funded by the International Ocean Discovery Program (IODP), core samples will be taken and bore hole observatory instruments placed in two of the drill holes that will continue to collect data, for a number of years.
It leaves in March and will be led by Dr. Laura Wallace of GNS Science and Professor Demian Saffer of Pennsylvania State University.
Dr Barnes will also be on board the second voyage and said both expeditions represent a massive financial investment in New Zealand science by the international IODP consortium.
“This region, offshore of Gisborne, has been recognised by the international science community as the best location on earth to use available ocean drilling technology to improve understanding of what causes slow slip events.
“Together, the data and sea floor observatories will contribute a great deal to understanding fundamental subduction fault processes. Hopefully they will also underpin future improvements in hazard assessment for New Zealand, leading to a more resilient society,” said Dr Barnes.
All data collected will be publicly available from mid-2019.
New Zealand participates in IODP through a consortium of research organisations and universities. The Australia and New Zealand IODP Consortium (ANZIC) is supporting participation of New Zealand scientists and outreach officers on both expeditions.
