Explanation of slow slip earthquakes associated with New Zealand's Hikurangi Subduction Zone. / GNS Science
The likeliest link to last night’s 6.3 earthquake wasn’t ex-tropical cyclone Gabrielle, as has already been suggested on social media - but another remarkable phenomenon many Kiwis won’t have heard of.
That’s what’s called “slow-slip” activity – and something recently responsible for gradually releasing the equivalent of a magnitude 7.0 earthquake on New Zealand’s largest fault zone.
The strong 7.38pm quake, which more than 60,000 people between Auckland and Christchurch reported feeling on GeoNet’s website, struck 50km northwest of Paraparaumu at a depth of 48km.
GNS seismic duty officer Jen Andrews said there had been eight aftershocks recorded overnight with the largest measuring 3.3.
“It’s not producing many aftershocks at the moment but we can probably expect quite a few more,” she said.
But due to the type and depth of the quake, most of the ongoing tremors linked to the initial 6.3 jolt weren’t expected to be felt.
While scientists were still assessing the mechanisms behind the quake, which took place on the vast tectonic plate boundary that is the Hikurangi Subduction Zone, its epicentre northwest of Wellington happened to be closely located to where a slow-slip event had been occurring since early last year, beneath the Manawatu and Horowhenua regions.
The Hikurangi Subduction Zone marks the margin where the the Pacific Plate dives - or subducts - westward beneath the North Island. Image / GeoNet
These mysterious events – essentially earthquakes in slow motion and observed only in the last two decades - can last from days to years, producing up to tens of centimetres of displacements along faults.
But 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, like GeoNet’s network of continuously-operating GNSS stations.
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.
Slow-slip events occur in an area where the Hikurangi Subduction Zone is transitioning from being "stuck" beneath the southern North Island, to an area where the subduction zone is "creeping" further north, around Gisborne and Hawkes Bay. Image / GeoNet
“We often see earthquakes like last night’s event during Manawatu slow slip events,” GNS geodetic scientist Dr Laura Wallace said.
One example was 2014′s 6.2 Eketahuna earthquake, which occurred during a previous large Manawatu slow slip event.
“When a slow slip event occurs, it is relieving a lot of pent-up plate boundary stress, but the slow slip can also increase stress on other nearby faults, and in some cases can trigger earthquakes like the one we experienced in Wellington last night,” Wallace said.
“Given that the earthquake was in the area of the ongoing Manawatu slow slip event, it was far more likely to have been triggered by the slow slip, and not by atmospheric pressure changes related to Cyclone Gabrielle.”
While there’d been some evidence to suggest a causal link between extremely low pressure associated with major storm systems and episodes of fault slip in the Earth’s crust, numbers remained small and not considered statistically significant.
As well, “earthquake weather” was generally considered a myth, given that, statistically, there’d been equal distribution of earthquakes occurring in all types of weather.
Last month, Wallace told the Herald that the Manawatu slow-slip event had picked up steam over the previous few weeks, causing three to four centimetres of motion on the plate boundary, some 30km to 40km beneath the surface.
And here’s our model of the slow slip events on @HikurangiSZ in the last month alone, based on @geonet GNSS network data. 3 SSEs going on at once in the last few weeks! pic.twitter.com/a7u2dpEvKI
It had kicked off in the same area as one that occurred shortly after the 7.8 Kaikoura Earthquake in November 2016, and was large enough to cause displace three continuously-operating GNSS sites by several centimetres.
