Predicting the big one
A scientific breakthrough has been hailed as a “first step” in eventually being able to forecast when a major earthquake could happen off the Gisborne coast.
The Hikurangi subduction zone runs the length of the North Island’s East Coast, passing as close as 40km offshore of Gisborne.
Subduction zones are associated with the world’s most powerful earthquakes, as well as slow-slip events, also known as silent earthquakes.
However, a new study involving monitoring equipment installed on the seafloor near Gisborne and the East Coast has given scientists a better understanding of the way plate tectonic stresses build up and are dissipated on the Hikurangi subduction zone.
The study’s findings are expected to bring science a step closer to being able to forecast the likelihood of damaging earthquakes and tsunamis on this major fault.
Led by GNS Science, the 18-month study involved detailed analysis of several hundred earthquakes between Hawke’s Bay and East Cape. It produced the first direct physical evidence of the way stresses change before, during and after slow-slip events or SSEs.
SSEs are a feature of this region, occurring on the subduction zone every 12 to 18 months.
The research paper — titled Episodic stress and fluid pressure cycling in subducting oceanic crust during slow-slip — was published in the journal Nature Geoscience this week. It clarified the mechanisms at play deep inside the fault by focusing on a zone between 15 and 35 kilometres below the surface where the Australian and Pacific tectonic plates meet.
Lead author and GNS Science seismologist Emily Warren-Smith said the study showed it was possible to keep track of stress changes at the plate interface by accurate monitoring of earthquakes.
It included data recorded by the national seismic and geodetic network operated by GeoNet alongside 20 ocean bottom seismometers deployed off the east coast of the North Island in 2014-2015 as part of collaborative research with organisations in Japan and the United States.
Before this research, scientists had hypothesised a build-up of stress and pressure from fluids on the deep parts of faults may trigger them to slip, but physical evidence to support this was lacking.
The study found a certain type of small earthquake started occurring in the subducting (or downgoing) plate in the lead-up to SSEs.
“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,” Dr Warren-Smith said.
The study concluded that highly pressurised fluid released from the subducting plate travels upward and lubricates the interface between the two plates. That initiates a SSE where a large patch of the fault moves slowly and benignly for weeks or months and then stops.
Deformation during the SSE then acts to temporarily relieve the pressure before the process starts over again.
Repeated cycles of this pressure build-up and release seem to control how often slow-slip earthquakes occur 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,” said Dr Warren-Smith.
“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.”
Dr Warren-Smith said the findings had international significance and would open up new avenues of research at other plate boundary zones around the world.
“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 near New Zealand but worldwide.”