Step closer to forecasting massive hikurangi subduction zone quake

Tuesday, 21 May 2019

Scientists believe new research will help in the quest to be able to forecast major earthquakes and tsunamis on the Hikurangi subduction zone, off the North Island's east coast.

The researchers emphasise they won't be able to make a forecast any time soon, but work on understanding the zone has shown them how they can take an important step along the way.

The Hikurangi plate boundary is the place where the Pacific tectonic plate is subducting, or diving beneath, the Australian tectonic plate. Subduction zones are a type of fault and are responsible for the largest and most powerful earthquakes and tsunamis in the world, such as Sumatra in 2004, Chile 2010, and Japan 2011.

According to GNS Science, the Hikurangi subduction zone is potentially the largest source of earthquake and tsunami hazard in New Zealand, but there is still much to learn about it. Scientists from around the world are in the midst of an extensive multi-year research programme aiming to better understand it.

**READ MORE:

* Scientists prepare for Hikurangi subduction zone faultline to rupture in future

* Scientists drilling into the Hikurangi trench hope to find a 'hotbed' of quake, tsunami data

* North Island's east coast sliding away in large slow-motion earthquake**

Research published this week in the journal Nature Geoscience analysed in detail several hundred earthquakes between Hawke's Bay and East Cape to produce the first direct physical evidence of the way stresses change before, during and after slow-slip events.

Slow-slip events happen on the subduction zone every 12 to 18 months.

The study clarified the mechanisms at play deep inside the fault by focusing on a zone from 15-35km below the surface where the Australian and Pacific plates meet, GNS said.

The study showed it was possible to keep track of stress changes at the plate interface by accurate monitoring of earthquakes, lead author and GNS seismologist Dr Emily Warren-Smith said.

The study found that a certain type of small earthquake started occurring in the subducting (or downgoing) plate in the lead up to slow-slip events.

'This type of small earthquake can only happen when there is a lot of fluid around at very high pressure. They started disappearing once the slow-slip event finished, suggesting the pressure had dropped again,' Warren-Smith said.

The study concluded that highly pressurised fluid released from the subducting plate travelled upward and lubricated the interface between the two plates. That initiated a slow-slip event where a large patch of the fault moved slowly and benignly for weeks or months and then stopped.

Repeated cycles of this pressure build up and release, seemed to control how often slow-slip earthquakes happened on particular parts of the subduction zone.

'Quantifying how stress and pressure is accumulated and released around the plate boundary interface during slow-slip cycles is crucial for understanding the physics of subduction systems and contributing to the forecasting of earthquake and tsunami hazards in these areas,' Warren-Smith said.

'We argue that increased monitoring of earthquakes around the Hikurangi subduction zone, including a permanent network of offshore instruments, will accelerate the development of reliable forecasts of potentially damaging subduction earthquakes.

'We are still a long way from being able to forecast when a major earthquake will occur on the Hikurangi subduction zone, but this is an important step along the way.'

The findings had international significance for research at other plate boundary zones around the world, Warren-Smith said. 

'Now we know what signals to look for, we can start to examine other subduction faults to see if they behave the same way, not just in New Zealand, but worldwide.'