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Scientists zero in on the causes of slow-slip earthquakes - 16/11/2020

Unravelling the causes of slow-slip earthquakes under the seafloor east of the North Island may lead to more accurate forecasts of big quakes and tsunamis generated by the Hikurangi subduction zone, scientists say.

That’s the premise of a three-year research project led by GNS Science designed to reveal the inner workings of the enigmatic plate boundary fault viewed by scientists as the largest source of earthquake and tsunami hazard in New Zealand.   

The discovery 20 years ago of slow-motion earthquakes where plate boundary faults release pent-up strain slowly – over days to months – rather than seconds in a conventional quake, has revolutionised seismology and the understanding of fault mechanics.

Slow-slip diagram

Slow-slip quakes are semi-predictable and occur off the North Island’s east coast every few years. No-one feels them when they are happening and their driving force remains unclear.

The project is designed to detect subtle physical changes inside the fault prior to slow-slip earthquakes to reveal the mechanisms that regulate their timing.

“This will clarify if there are observable physical changes within the fault that could enable the development of more accurate forecasts of when the fault might fail, in both slow and, possibly, fast slip quakes,” project leader Laura Wallace of GNS Science said.

Dr Emily Warren-Smith and Dr Laura Wallace

Dr Emily Warren-Smith and Dr Laura Wallace from GNS Science

Tantalising evidence has emerged in recent years that a build-up of water pressure near the fault exerts a major control on timing of slow-slip quakes in New Zealand.

GNS Science seismologist Dr Emily Warren-Smith said if this build-up influences slip timing, then monitoring the accumulation of water in the fault may enable better forecasting of slow, and possibly, fast earthquakes in the future.

But Dr Wallace said it was possible that fluid pressure changes inside the fault might be a symptom of slow-slip earthquakes rather than a direct cause. Alternatively, there might be other processes such as the steady build-up of stress from tectonic plate motion that controlled the tempo of slow-slip quakes.

The project is aiming to resolve this dichotomy by mounting a large-scale deployment of undersea and land-based monitoring instruments in southern Hawke’s Bay and Wairarapa regions to monitor changes before, during and after a regularly recurring slow-slip event expected offshore in this region sometime in the next two years.

Dr Wallace said the project would forge new ground in the field of seafloor geodesy and help put New Zealand at the forefront of global efforts to monitor offshore faults that can generate large quakes and tsunamis.

“The project will lead to new evidence-based information that will help significantly in planning and preparedness and make New Zealand safer and better able to recover after a major earthquake.”

Collaborating on the project are Dr Emily Warren-Smith and Dr Katie Jacobs, both of GNS Science, and Dr Martha Savage of Victoria University of Wellington, along with scientists from Japan, the United States and Canada.

The project has been awarded funding of $960,000 over three years by the Marsden Fund which is administered by the Royal Society Te Apārangi.

Link from NZ Herald:
New Zealand’s mysterious ‘silent quakes’ may help forecast future tsunamis – NZ Herald, 16 November 2020