The Hikurangi subduction zone: a credible magnitude 8.9 earthquake and tsunami scenario. Video / East Coast LAB
Mysterious, silent earthquakes could be key to forecasting the next catastrophic rupture on New Zealand's largest fault: the gigantic Hikurangi Subduction Zone.
A newly-funded, million-dollar study will draw on a trove of data and sophisticated modelling to unlock the hidden relationship between "slow-slip events" and swarms of seismic earthquakes they've been linked to along the sprawling plate boundary.
Although they've only been discovered in relatively recent times, slow-slip events – essentially quakes in slow motion - have been associated with some of the planet's most devastating natural disasters.
They've been found to precede Japan's 9.1 Tohuku earthquake and tsunami in 2011, the 8.1 Iquique earthquake in Chile in 2014, and a 7.2 shake off the coast of Mexico the same year.
Only last year, researchers reported how the slowest quake ever recorded - lasting 32 years - eventually led to the calamitous 1861 Sumatra earthquake in Indonesia.
Yet, because of their relatively regular frequency here, scientists consider them part of normal behaviour in our subduction zone – and recording one didn't mean a major rupture was on the way.
They tended to occur at shallow depths off Gisborne and Hawke's Bay, and at deeper levels observed off the Manawatū and Kāpiti regions, releasing pent-up energy equivalent to a magnitude 7.0 earthquake.
More specifically, they played out – sometimes over days, weeks, or even months – in an area where the subduction zone was transitioning from being "stuck" beneath the southern North Island, to an area where the subduction zone was "creeping" further north, around Gisborne and Hawkes Bay.
And because they happened too slowly to be picked up by seismometers – or to be felt by humans – they could only be recorded using special GPS equipment measuring the slow movement of land.
Sometimes, though, they've coincided with swarms of quakes – and even volcanic activity – including flurries of tremors recorded in separate East Coast episodes last year.
"This suggests that they have a tangible, but not well understood, influence on earthquake occurrence rate," Otago University's Associate Professor Ting Wang said.
"The quantitative relationship between slow-slip earthquakes and seismic swarms remains unclear, but it is paramount for short-term earthquake forecasts."
One intriguing question facing scientists has been why they appeared to occur in roughly five-yearly cycles along the subduction zone: before a pair of events in 2021, they'd been observed in 2016, 2011 and 2006.
Possible explanations include the slow-slip area regularly reaching some form of threshold after being constantly loaded with stress by plate motion, or being primed by a build-up of water around the fault zone.
Perhaps the bigger question was their potential to influence the next "big one".
"Recently published research suggests a 26 per cent chance of rupture in a magnitude 8 or larger event beneath the lower North Island within the next 50 years," Wang said.
"Such an earthquake would generate strong ground shaking and tsunami damage throughout the country."
Headline projections in one EQC-commissioned report estimated worst-case scenario impacts from a one-in-500-year event could include 33,000 fatalities, 27,000 injuries and $45b worth of property loss.
As it stood, Wang said, these monster events were inherently difficult – some may argue impossible – to predict.
Still, there was the tantalising possibility that, buried within years of seismic data, we might find tell-tale signals and patterns.
That's just what Wang and a team of experts from GNS Science and Massey, Otago, Victoria universities aim to do using the first analysis of its kind in the world.
With a mix of modelling techniques and machine-learning methods, the team planned to first catalogue Hikurangi slow-slip quakes and seismic swarms from geodetic and seismic data.
Next, they'd model the statistical relationships between each – and thus develop new ways to forecast not just slow quakes, but great ones.
"A systematic statistical analysis with a long sequence of data is needed to find a quantitative relationship between [slow quakes and earthquake occurrences]," Wang said.
"We'd like to find out the occurrence patterns of these, and see if there are possible precursory signals in these patterns for great earthquakes along subduction zones."
One of New Zealand's leading experts in slow-slip events, GNS Science geodetic scientist Dr Laura Wallace, will also be involved in the three-year programme.
"We know from observations in New Zealand and globally that slow slip events do influence the likelihood of future earthquakes," Wallace said.
"Due to the great datasets we now have access to, the time is right to address this important problem.
"Monitoring slow slip events is one of the best ways we have to keep our finger on the pulse of the Hikurangi subduction zone, and this work will enable us to use these observations to keep tabs on any changes in the likelihood of large future earthquakes."
The programme is being supported with a million-dollar grant through the Ministry of Business, Innovation and Employment's Endeavour Fund.
