Secrets of the subduction zone: is this how New Zealand plans ‘megaquakes’?

The Hikurangi Subduction Zone: A Credible 8.9 Magnitude Earthquake and Tsunami Scenario. Video / East Coast LAB

Mysterious, silent earthquakes could be key to predicting the next catastrophic rupture on New Zealand’s largest fault: the gigantic Hikurangi Subduction Zone.

A recently funded $1 million study will draw on a wealth of data and sophisticated modeling to reveal the hidden relationship between “slow-slip events” and the seismic earthquake swarms they’ve been linked to along of the sprawling plate boundary.

Although only discovered in relatively recent times, slow-slip events – essentially slow-motion earthquakes – have been associated with some of the most devastating natural disasters on the planet.

They were found to precede the 9.1 Tohuku earthquake and tsunami in Japan in 2011, the 8.1 Iquique earthquake in Chile in 2014, and a 7.2 tremor off the coast of Mexico the same year.

Just last year, researchers reported how the slowest earthquake on record – lasting 32 years – eventually led to the disastrous 1861 Sumatra earthquake in Indonesia.

Yet, because of their relatively regular frequency here, scientists consider them part of normal behavior 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 seen off the Manawatū and Kāpiti regions, releasing pent-up energy equivalent to a magnitude 7 earthquake. 0.

Specifically, they took place – sometimes for days, weeks or even months – in an area where the subduction zone was changing from being “stuck” below the south of the North Island, to an area where the subduction zone “crawled” further north, around Gisborne and Hawkes Bay.

Source/GNS Science
Source/GNS Science

Source/GNS Science
Source/GNS Science

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 that measures the slow motion of the earth.

Occasionally, however, they have coincided with earthquake swarms – and even volcanic activity – including bursts of tremors recorded in separate East Coast episodes last year.

“This suggests that they have a tangible, but not well understood, influence on the rate of occurrence of earthquakes,” said University of Otago associate professor Ting Wang.

“The quantitative relationship between slow-sliding earthquakes and seismic swarms remains unclear, but is critical for short-term earthquake predictions.”

An intriguing question for scientists is why they appeared to occur in roughly five-year cycles along the subduction zone: before a pair of events in 2021, they had been observed in 2016, 2011 and 2006.

Possible explanations include the slow slip zone regularly reaching some form of sill after being constantly stressed by plate movement, or being initiated by an accumulation of water around the fault zone.

Perhaps the biggest question was their potential to influence the next “big”.

“Recently published research suggests a 26% likelihood of a rupture in a magnitude 8 or greater 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.”

Key projections from a report commissioned by the EQC estimated that the worst-case scenario impacts of a 500-year event could include 33,000 deaths, 27,000 injuries and $45 billion in property losses.

As things stand, Wang said, these monster events were inherently difficult — some might say impossible — to predict.

Yet there was the tantalizing possibility that, buried in years of seismic data, we might find telltale signals and patterns.

That’s exactly what Wang and a team of experts from GNS Science and Massey Universities, Otago, Victoria aim to do using the world’s first analysis of its kind.

With a mix of modeling techniques and machine learning methods, the team planned to first catalog slow-sliding earthquakes and seismic swarms in Hikurangi from geodetic and seismic data.

Then they would model the statistical relationships between each – and thus develop new ways to predict not just slow earthquakes, but large ones as well.

“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 would like to find out the patterns of occurrence of these and see if there are possible precursor signals in these patterns for large earthquakes along subduction zones.”

One of New Zealand’s leading experts on 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 around the world that slow slip events influence the likelihood of future earthquakes,” Wallace said.

“Due to the large datasets we now have access to, the time has come to tackle this important issue.

“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 allow us to use these observations to keep tabs on any changes. in the likelihood of future large earthquakes.”

The program is supported by a $1 million grant through the Department for Business, Innovation and Jobs’ Endeavor Fund.

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