Beneath the waves off New Zealand’s east coast, scientists have uncovered a hidden network of ancient faults that may hold the key to understanding mysterious slow-motion earthquakes.
A new international study, published in Science Advances, identified hidden fault structures called polygonal fault systems (PFS) as a major influence on the behaviour of the north Hikurangi subduction zone.
Earth Sciences NZ marine geologist Philip Barnes said the discovery would help explain why slow earthquakes occurred where they did.
“It also shows that these events may be influenced by the reactivation of old fault structures that formed much closer to the surface than the present depths of the subduction zone.”
In the Hikurangi subduction zone off the east coast of the North Island, the Pacific plate went below the Australian plate.
While the southern section of this zone remained locked and could produce massive earthquakes over magnitude 8, the northern part behaved differently.
It regularly produced slow slip events, movements that unfold over days to months, releasing tectonic stress without sudden shaking.
These slow slip events did not cause violent shaking themselves but could increase stress on nearby faults and trigger more damaging earthquakes, Barnes said.
“Understanding what controls them is vital to improving earthquake and tsunami warnings.”
The study, a collaboration between Chinese and American researchers as well as Earth Sciences NZ (formerly NIWA), used data from the International Ocean Discovery Program and the high-resolution three-dimensional NZ3D seismic survey conducted off Gisborne.
Fault systems were mapped in “unprecedented detail” with high-resolution 3D seismic imaging, deep-sea drilling data from the International Ocean Discovery Program, and advanced computer modelling.
The faults formed over millions of years during sedimentation long before and initially far from the subduction zone, Barnes said.
“But as the seafloor is dragged into the subduction zone during the convergence of the tectonic plates, they can be reactivated and evolve into major thrust faults. Our analysis also shows they provide important pathways for fluids, which play a major role in fault slip.”
Connecting fault structure and fluid migration offered new insight into one of the key processes thought to trigger slow earthquakes, he added.
“Until now, we lacked the imaging resolution to link these features directly to slow slip behaviour. This study changes that and gives us a new lens to better understand subduction zone dynamics.”
Barnes said it was a “major step forward” in understanding the geological processes happening beneath our coastlines.
“With better models and better data, we are now in a stronger position to understand how subduction zones work.”