A slow slip event that unfolded off the East Coast over April this year triggered a swarm of quakes that rattled Mahia. Image / GNS Science
A slow slip event that unfolded off the East Coast over April this year triggered a swarm of quakes that rattled Mahia. Image / GNS Science
Unusual slow-burning quakes that silently unfold deep beneath New Zealand could allow scientists to better understand the violent, quick-fire shakes that Kiwis know much better.
Tracking what are called slow-slip earthquakes – known to shift faults over days or months without any perceptible shifts in the earth - has onlybeen made possible with recent advances in GPS technology.
These events play out when faults grind incredibly slowly against each other, like an earthquake in slow motion.
Over the course of weeks, one might release the same amount of energy as a typical big quake like the 7.1 event that shook Canterbury in 2010.
Because they occur deep in the earth and release energy so slowly, they're marked by very little deformation at the surface, even though they might affect an area of thousands of square kilometres.
In the two decades since their discovery, they've only been observed in a handful of places, including Japan, Mexico, the north-west coast of the US – and here.
Scientists have watched them play out every one to two years near Gisborne, at a relatively shallow depth beneath the seabed, and usually driving a spate of localised quake activity.
The most recent one, which was considered among the biggest on record and lasted for weeks, triggered a swarm of East Coast quakes - among them a magnitude 5.1 jolt that struck near Mahia in May.
'A clearer picture'
In a study just published in major journal Nature, a team of US scientists have discovered intriguing similarities between slow-slip earthquakes and normal ones – with potentially big implications.
The Hikurangi Subduction Zone is a major offshore fault where the Pacific Plate dives - or subducts - westward beneath the North Island. Image / GNS Science
Caltech geoscientist Professor Jean-Philippe Avouac said there had been much uncertainty surrounding slow-slip events.
"You can't study them using traditional seismological techniques because the signal they create is too faint and gets lost in the noise from human activities as well as from natural geological processes like ocean waves, rivers, and winds."
But his team has managed to get a clearer picture of slow-slip events along Washington state's Cascadia Subduction Zone - where the North American tectonic plate slides southwest over the Pacific Ocean plate - using a network of 352 GPS stations.
By analysing a decade of data, they were able to catalogue more than 40 slow-slip events of varied sizes, and then characterise their features more precisely.
One key finding was that slow-slip events obey the same "scaling" laws as regular earthquakes.
This graphic shows displacement of the GeoNet GPS sites during a slow slip quake recorded around the East Coast earlier this year. Source / GNS Science
These laws described the "moment" of a slip event on a fault - which quantified the elastic energy released by slip on a fault - as a function of the duration of slip.
In practical terms, that meant that a big slip across a broad area yielded a long-lasting earthquake.
It had long been known that the moment of an earthquake was proportional to the amount of time the earthquake lasted.
In 2007, a team from the University of Tokyo and Stanford suggested that slow-slip events appeared to be different, with the moment seemingly directly proportional to time.
But now, the US scientists believed magnitudes of slow-slip events also were proportional to their duration, just like regular earthquakes.
This cross-section shows slow-slip zones at the boundary between the Australian and Pacific Plates. Slow slips in the south (Kapiti and Manawatu) happen deeper than those in the north (Hawke's Bay and Gisborne). Image / GNS Science
Since these events behaved similarly to regular earthquakes, studying them could shed light on their more destructive cousins, Avouac said.
While a traditional magnitude 7.0 earthquake might only occur along a fault every couple of hundred years, a slow-slip event of that magnitude can reoccur along the same fault every year or two.
"If we study a fault for a dozen years, we might see 10 of these events," he said.
"That lets us test models of the seismic cycle, learning how different segments of a fault interact with one another.
"It gives us a clearer picture of how energy builds up and is released with time along a major fault."
Such information could offer more insight into earthquake mechanics and the physics governing their timing and magnitude.
GNS Science geoscientist Dr Laura Wallace said as the study was focused on one location, the model would need to be applied to other areas to fully test it.
"If correct, it really does highlight a greater similarity between earthquakes and slow slip events than what we previously thought," she said.
"And that opens the door to using slow-slip events themselves to understand earthquake rupture processes."
NZ's slow slip quakes
The new findings come as Wallace and an international team of scientists aboard Niwa research vessel Tangaroa set out for the North Island's East Coast to collect instruments placed along New Zealand's own offshore plate boundary.
Some of the seismometers that are being hauled from the East Coast seafloor over coming weeks. Photo / GNS Science
For a year along the Hikurangi Subduction Zone - where the Pacific plate dives under the Australian plate to create our largest, most active and potentially most threatening fault – the instruments have been continuously tracking movement in the seafloor.
Some of them have recorded pressure at the seafloor due to the overlying ocean, and were sensitive to even just a centimetre of movement that came during quakes or slow-slip events.
"Lots of small earthquakes also occurred during the 2019 slow-slip event, so the data from the seismometers will be very exciting," said Victoria University PhD student Katherine Woods, who will be working with the data for her PhD studies.
Scientists onboard will also be using high-tech equipment to measure the position of seafloor transponders – also at a centimetre level - to track the horizontal movement of the seafloor over time, due to tectonic activity along the subduction zone.
Although there have been cases of slow slip events preceding big quakes – like Tohoku in Japan in 2011 – such instances were rare, so scientists say they shouldn't be seen as warning signals.
