Slow-motion earthquakes unfolding deep beneath the North Island have released the equivalent energy of a magnitude 7.2 rupture – with intriguing potential links to last month’s big Hawke’s Bay shakes.

Scientists have also observed how ongoing “slow slip” earthquakes, occurring tens of kilometres below Kāpiti and Manawatū, have shifted our plate boundary by seven centimetres since just the start of 2023.

Some stations closest to this hidden activity have moved by as much as 1.5cm horizontally – and about a centimetre upwards – this year, showing how powerful these mysterious, silent earthquakes are.

While imperceptible to us, slow-slip events can be observed by GeoNet’s network of continuously-operating GNSS stations, capable of tracking millimetre-level changes in movement.

Compared with the violent, seconds-long earthquakes that Kiwis are more familiar with, these play out over days, weeks or months, along the vast Hikurangi Subduction Zone, where the Pacific Plate dives westward beneath the North Island.

They’re typically observed once a year off the East Coast – and at deeper depths, and around every five years, near Manawatū and Kāpiti.

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

One which kicked off below Manawatū about a year and a half ago was “still going strong”, GNS Science geophysicist Dr Laura Wallace said.

As much as 15cm to 20cm of movement had taken place along some portions of the plate boundary - equivalent to four or five years of normal plate motion.

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 “Based on the behaviour of previous slow slip events in this area, we expect this one to continue through much of this year,” she said.

“We have not seen a substantive slow slip event like this in the Kāpiti area since the one that was triggered in late 2016 by the Kaikōura earthquake.”

In just the past four months, those events had also squeezed out the energy of a magnitude 6.9 quake – while the plate boundary had moved an estimated seven centimetres.

The slow slip movement on the Hikurangi subduction plate boundary since January 2023 is colour-coded by centimetres. The white arrows show the horizontal displacement of GeoNet GNSS stations during the same time period, caused by the slow-motion earthquakes. Image / GeoNet

Over the events’ entirety, that equivalent energy release so far was comparable with that unleashed in a 7.2 earthquake – or roughly the scale of Canterbury’s Darfield Earthquake in 2010.

Most of the recent had occurred between 25km to 40km below us, yet it might have also contributed to some of 2023′s sharpest quakes so far.

Scientists saw it as a likely contributor to a 6.3 event that struck off Paraparaumu on February 15, but also to a spate of tremors in Pōrangahau last month.

“The Pōrangahau earthquake sequence appears to have occurred on - or very close to - the Hikurangi subduction plate boundary.”

Wallace explained that slow slip events could help relieve pent-up stress on the subduction plate boundary – but also increase stress on other nearby portions of the boundary while they were happening.

An earthquake sequence in Hawke's Bay began with a 5.9 event near Porangahau on April 26, as shown here. Photo / NIWA

“So yes, it is possible that the ongoing slow slip event could have played a part in triggering the ongoing Pōrangahau sequence.”

The active Manawatū slow slip had also moved to the east as far as Dannevirke - which happened to be just 40km from where the sequence kicked off with a 5.9 shake on April 26.

Because slow-slip quakes have been shown to precede some 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, as the vast majority of slow slip events in New Zealand - and worldwide - transpired without ensuing ruptures.

“Slow-slip events are a great reminder that New Zealanders live on a very active tectonic plate boundary.”