What we learn about New Zealand’s largest quake maker

Scientists are about to get an unprecedented look at mysterious earthquakes that could help predict the next massive rupture in our most dangerous fault zone.

An international team of researchers is deploying 50 instruments just off the east coast of the North Island, where they will spend two years on the seabed collecting data on minute movements.

The project will make New Zealand’s high-risk Hikurangi Subduction Zone (HSZ) one of the world’s best-studied sites for mysterious slow-slip earthquakes.

Only recently discovered, these events – essentially slow-motion tremors – have been linked to some of the planet’s most devastating natural disasters.

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

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

Our own subduction zone – where the Pacific Plate dips or subducts westward beneath the North Island – is thought to be similarly capable of generating “megathrust” earthquakes of up to 9.0 magnitude, posing a tsunami risk for represents parts of our coast.

Headline forecasts in a report commissioned by EQC estimated the impact of a worst-case scenario of an event occurring every 500 years at 33,000 deaths, 27,000 injuries and $45 billion in property damage.

Recent research has found a 26 percent chance of a magnitude 8.0 or larger event occurring beneath the lower North Island within the next 50 years – and scientists increasingly believe that understanding slow-slip tremors is key to predicting larger ones is quake.

Along the subduction zone, these silent events have been shown to slowly unfold at shallow depths off the coast of Gisborne and Hawke’s Bay and at deeper levels off the Manawatū and Kapiti regions, releasing pent-up energy equivalent to a magnitude 7.0 earthquake.

Scientists have located them in certain areas where the zone shifted from being “stuck” under the southern North Island to an area where it “crawls” further north – around Gisborne and Hawke’s Bay.

But much remains unknown.

The geodetic scientist Dr. Laura Wallace of GNS Science said the new program – which also involves researchers from the University of Tokyo, Kyoto University and Tōhoku University in Japan, and Columbia University and the University of Rhode Island in the US – would help some test possible explanations.

These include the areas where they regularly reach some sort of sill, having been subjected to constant stresses from plate movement or primed by an accumulation of water around the fault zone.

“For example, to test the fluid motion hypothesis, we will have seismometers that can help us understand the properties of earthquakes that occur during slow-slip events,” Wallace told the Herald today.

While significant events have been recorded in roughly five-year cycles — the most recent being a pair of sequences last year — Wallace said moderately large events tended to occur near Gisborne at least every two years.

“So I’m hoping that in those two years we’ll see at least one major event and maybe two or three smaller ones.”

Of the 50 instruments deployed on board Niwas RV Tangaroa, most are equipped with seabed pressure sensors capable of detecting vertical movements on the seabed to within one centimetre.

Ultimately, the team believes their wider array will help detect shifts as small as half a centimeter – offering our sharpest view yet.

“Another really exciting thing is that we’ll also have some oceanographic sensors that will be able to measure currents and changes in oceanographic fluctuations off Gisborne,” Wallace said.

“This will really help us lower the noise level and see the much finer up or down motion of the seafloor in slow motion during earthquakes.”

What the team found at the HSZ — where more than $70 million in funding has flowed into a range of scientific endeavors over the past decade — could also have crucial implications elsewhere in the world.

“I suspect that the kind of things we’re seeing off Gisborne are actually a lot more common in other subduction zones than we realize,” she said.

“We also know that there are several subduction zones around the world with fairly similar properties to this one – so we could expect similar behaviors to occur there.”

It was in part its accessibility – similar stretches of subduction zones lay up to 150 km offshore in most other parts of the world – that made the HSZ a global observatory for these events.

“And this is really the first experiment of this kind.”

A separate program Wallace is collaborating on is tapping into treasure troves of seismic data to uncover the relationship between slow-slip events and earthquake swarms in hopes of uncovering telltale signals.

“One big piece of the puzzle”

Another study has turned to another intriguing indicator to estimate the size of the next HSZ cataclysm: tiny creatures that lived tens of millions of years ago.

dr Victoria University’s Carolyn Boulton led a team of earthquake scientists studying a rocky cliff on the Hungaroa Fault, located at the edges of the zone.

Layers of limestone, mudstone and siltstone on the bluff near Tora, some 35 km south-east of Martinborough, provided a practical clue to what was happening in the offshore subduction zone.

Rocks similar to those on the cliff were deposited on the sea floor between 35 and 65 million years ago, but their location makes them difficult to study.

Instead, scientists could look at the rocks on land to tell them more about what’s happening beneath the sea.

“The rocks all contain calcite from ancient unicellular marine organisms, mostly foraminifera like plankton,” Boulton said.

“We found that calcite from these tiny organisms can affect movement in the subduction zone.

“Imagine these tiny, long-dead organisms being able to affect the mechanical interaction of two huge tectonic plates.”

If the calcite in the rocks could dissolve – like sugar in tea – the fault could be weak and slip easily without an earthquake.

However, if the calcite couldn’t dissipate, the fault could close up instead – storing energy that could be released in a large tremor.

“Calcite dissolves more quickly when it’s being used heavily and when temperatures are cooler,” she said.

“It dissolves more easily at low temperatures – let’s say room temperature. But it gets harder to dissipate as the temperature rises — say, deeper in the earth.

“In the subduction zone, the temperature rises more slowly than on land – only by about 10 degrees per kilometer.

“So the bug is very sensitive to what calcite, these shells of ancient dead sea organisms, is doing.

“The amount and behavior of calcite from these organisms is a big piece of the puzzle of how big the next earthquake might be.”

Boulton said observations at Tora showed that the shallow part of the subduction zone can accommodate plate motion by slow slipping — or fast slipping in large and damaging tremors.

“What we really want to know is: Are there slow-slip events out there that we haven’t detected

“Are the rocks moving without an earthquake, or are they really locked in? That will help us tell what might happen in the next earthquake.”

Meanwhile, in a separate EQC-funded study, scientists are setting up seismometers at 19 sites in Southland to fill in a potentially large blind spot in New Zealand’s earthquake risk.

“Historically, we think Southland is an area of ​​low seismicity, but the distance between Geonet sensors is over 100 km, so many of the smaller earthquakes go unrecorded,” said the team leader, Dr. Jack Williams from Otago University.

“These new sensors will be approximately 30km apart, giving us a much better idea of ​​active fault lines in Southland.”

While past seismic activity has often been judged by surface indicators, at Southland these indicators may not tell the whole story.

“We can see a lot of prehistoric earthquake scars in the Central Otago landscape because it’s dry, but the weather in Southland is very wet and so the evidence of past earthquakes is being removed from the landscape much faster,” he said.

“These regions are similar in that we assume active faults are hidden beneath these levels, but we can’t necessarily identify them just by looking at the landscape.

“The Canterbury landscape hasn’t given us many clues as to where the active faults were, but uplifted land can be seen throughout this region today.

“Of course, the surface doesn’t always tell you what’s going on below.”

The new projects come as New Zealand’s vibration hazard from future earthquakes has been updated in a freshly updated model with implications for building design.