Sinking seamounts off Hawke's Bay could lift quake risk, research finds

Earthquakes and slow slips events off the East Coast of New Zealand may be influenced by mountains that lie on the ocean floor of the fault.

Research co-authored by GNS Science, and published in Nature Geoscience magazine this week, found that the mountains pulled into a subduction zone create brittle and quake-prone earth.

A subduction zone, present off Hawke's Bay, is where one tectonic plate dives under another and is also where the largest and most damaging earthquakes can occur.

The Hikurangi Subduction Zone is NZ's largest and most active fault and runs along the East Coast. GNS says this new research could help scientists better understand how seamounts on the sea there could influence its behaviour.

The researchers integrated data from samples of subducting rock and sediment around seamounts, like these cores which drilled from the Nankai Trench offshore Japan in 2000. Photo / Demian Saffer, PSU.

GNS Science's Susan Ellis said the research used cutting-edge computer modelling techniques to simulate what happens when seamounts enter a subduction zone.

"When a seamount sinks into a trench, the ground ahead becomes brittle and prone to earthquakes because the water is squeezed out," Ellis said.

"This brittle rock can be a source for earthquakes.

"But in its wake, the seamount leaves softer, wet sediment, which can help dampen or slow down subduction slip."

Ellis said the weakened rock could be an important factor in slow-slip events which were like earthquakes, but happen silently and slowly over weeks or months.

"Our findings show scientists need to carefully monitor what happens around a subducting seamount, so we can better understand where future quakes might occur."

The study suggested subduction of the undersea mountains could influence where earthquakes and slow-motion earthquakes (slow-slip events) occur on subduction zones.

Slow-slip events are earthquakes which occur over weeks to months, and are not felt on land as pressure is released slowly, rather than suddenly like an earthquake.

The predictions from the model agreed with the locations of offshore tremors and slow-slip events observed on the Hikurangi Subduction Zone offshore Gisborne, where one of these large undersea mountains is subducting.

This study was undertaken in collaboration with scientists from Pennsylvania State University and the University of Texas.

It was funded by the United States National Science Foundation and an MBIE Endeavour funded project led by GNS Science to understand the seismic and tsunami hazard posed by the Hikurangi Subduction Zone.

Tsunami hikoi

Hawke's Bay residents are being urged to use the ninth anniversary of the 2011 Japan earthquake and tsunami on March 11 to practise their evacuation routes.

Hawke's Bay Civil Defence Emergency Management Group manager Ian Macdonald encouraged families, communities, businesses, schools and childcare centres to identify their nearest safe zone and practise their "tsunami hīkoi" to higher ground or inland during the week of March 9 to 15.

Macdonald said although rare events, earthquakes and tsunami had the potential to devastate coastal communities.

"In the 2011 Japan earthquake and tsunami, it was the people who acted on the earthquake as their warning and evacuated to safe locations who survived," he said.

"That's one of the reasons Hawke's Bay Civil Defence Emergency Management Group places a strong emphasis on preparing for a tsunami evacuation as part of the anniversary of this event."