New Zealand’s largest fault zone just released the equivalent energy of a magnitude 7.0 earthquake, through slow-burning “silent” events linked to recent swarms above Gisborne.
Scientists have been watching several slow slip earthquakes – silent but powerful events that roll over days, weeks or months, rather than seconds – unfold along the Hikurangi Subduction Zone.
Imperceptible to us, these slow-motion quakes can be observed by GeoNet’s network of continuously-operating GNSS stations, capable of tracking millimetre-level changes in movement.
They’re relatively-infrequent occurrences along the subduction zone - where the Pacific Plate dives westward beneath the North Island - and usually play out at shallow depths off the East Coast, and at deeper levels near the Manawatū and Kāpiti regions.
GNS Science geophysicist Dr Laura Wallace said one event had been happening beneath Manawatu since early 2022, but had “picked up a bit of steam” over the past few weeks.
In just the last month, it had caused three to four centimetres of motion on the plate boundary, some 30km to 40km beneath the surface.
Events in the region, which tended to occur twice a decade and last one to two years, had previously driven up to 30cm of movement.
“We expect this one to continue well into 2023,” Wallace said.
“It’s not quite a super-big one yet, and will probably be similar to ones we recorded in 2010 and 2014.”
Since the start of the New Year, a separate event just off the coast of Tologa Bay has generated enough movement to shift GNSS stations about 1cm to 2cm eastward, and was likely linked to two small recent earthquake swarms.
One involved more than a dozen quakes between magnitude 1 and 2.5 near Ruatōria, and the other, centred around Tokomaru Bay, numbered 25 quakes – some as strong as 3.5.
“When slow-slip earthquakes occur we often see an increase in the number of small earthquakes in the same region,” Wallace said.
“This is because the slow slip event produces changes in stress in the earth’s crust, causing some nearby faults to rupture in small earthquakes.”
Another event, off the coast of southern Hawke’s Bay, began on January 7.
Altogether, Wallace said the amount of slow slip movement around the boundary during the last month roughly equated to the energy release of a 7.0 tremor.
“If each of these East Coast patches of slow slip went individually, you’d still be looking at something like a 6.8,” she said.
“So this shows us that these slow slip events can accommodate and release a lot of pent-up energy from the tectonic plates in a slow way, rather than in earthquakes, which is a good thing.”
The events come several months after a large slow slip event near Hawke’s Bay last year, on a portion of the fault sandwiched between where the latest events were occurring.
Recent programmes that have deployed dozens of seafloor sensors along the subduction zone, and drawing around $70m in funding, have turned New Zealand into something of a global slow-slip event observatory.
“In October last year, together with American and Japanese scientists, GNS Scientists deployed our largest-ever array of over 50 seafloor sensors offshore Gisborne,” Wallace said.
Onshore, there was a large temporary network of seismometers deployed around southern Hawke’s Bay region to detect quakes during slow slip events, as part of a new Marsden Fund-supported project.
“So, this latest sequence of slow slip events is well-timed for us to learn far more than we ever have about New Zealand’s slow-motion earthquakes.”
Scientists have been tackling the puzzle from several directions, through studies focused on seafloor measurements, mathematical modelling and tell-tale traces of ancient earthquakes gleaned from tiny ocean creatures that lived thousands of years ago.
They’re particularly interested in answering why slow-slip quakes tend to happen in regular cycles – something that could be due to the fault zone reaching some form of threshold after being constantly loaded with stress by plate motion, or being primed by a build-up of water underground.
The Hikurangi Subduction Zone is thought capable of generating monster “megathrust” quakes, with the most recent research indicating a 26 per cent chance of an event 8.0 or larger striking beneath the lower North Island within the next 50 years.
The potential impacts of such a disaster were troubling: one EQC-commissioned report estimated a one-in-500-year event could cause tens of thousands of fatalities and injuries, along with tens of billions of dollars of property loss.
Because slow-slip quakes have been shown to precede subduction zone cataclysms – including Japan’s devastating 9.1 event in 2011 – researchers believe they could be key to forecasting major disasters.
But their occurrence didn’t necessarily mean Hikurangi’s next big shake was on its way, with the vast majority of events transpiring without ensuing ruptures.