Slow-moving earthquakes occur deep in the North Island, below Kapiti and Manawati

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Slow-moving earthquakes unfolding deep in the North Island released energy equivalent to a magnitude 7.2 rupture – with intriguing potential links to last month’s large Hawke’s Bay tremors.

Scientists have also noted how persistent “slow sliding” earthquakes, occurring tens of kilometers below Kapiti and Manawati, have shifted our plate boundaries by seven centimeters since the start of 2023.

Some of the stations closest to this hidden activity have also moved 1.5 centimeters horizontally – and about a centimeter upward – showing just how powerful these mysterious and silent earthquakes are.

While imperceptible to us, slow slip events can be observed through the GeoNet network of continuously operating GNSS stations, which are able to track millimeter-level changes in motion.

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Compared to the violent, seconds-long earthquakes that Kiwis are more familiar with, these work over days, weeks, or months, along the vast Hikurangi subduction zone, where the Pacific Plate plunges westward under the North Island.

They are usually observed annually off the east coast, and at deeper depths, about every five years, near Manawati and Kapiti.

Slow slide events occur in an area where the Hikurangi subduction zone is moving from being ‘stuck’ down the southern North Island, to an area where the subduction zone is ‘creeping’ to the north, around Gisborne and Hawke’s Bay. Image / GeoNet

One that started under Manawatu about a year and a half ago was “still going strong,” said Dr. Laura Wallace, a scientific geophysicist with GNS.

Movement of 15 cm to 20 cm has occurred along some parts of the plate boundaries – the equivalent of four or five years of normal plate movement.

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In just the past four months, these events have also compressed the energy of a magnitude 6.9 earthquake – while moving plate boundaries by about seven centimeters.

“Based on the behavior of previous slow slide events in this region, we expect this slide to continue through most of this year,” she said.

“We haven’t seen a substantial slow-slip event like this in the Kapiti region since the event triggered in late 2016 by the Kaikoura earthquake.”

Slow sliding motion at the Hikurangi subduction plate boundary since January 2023 has been coded in centimeters. The white arrows show the horizontal displacement of the GeoNet GNSS stations over the same time period, due to slow-moving earthquakes. Image / GeoNet

Over the entire events, the equivalent energy released was comparable to what would be released in a 7.2 earthquake – or roughly on the scale of the Darfield earthquake in Canterbury in 2010.

Most of the recent activity occurred between 25 and 40 kilometers below us, yet it may also have contributed to some of the most severe earthquakes of 2023 so far.

Scientists saw it as likely contributing to the 6.3 event that hit Paraparaumu on February 15, but also to a series of tremors in Biranghao last month.

“The Piranghao earthquake sequence appears to have occurred at — or very close to — the Hikurangi subduction plate boundary.”

Wallace showed that slow slip events can help relieve pent-up pressure at subduction plate boundaries — but also increase pressure on other nearby parts of the boundary as they occur.

An earthquake sequence in Hawke’s Bay began with a magnitude 5.9 event near Burungau on April 26, shown here. photo / niwa

“So yes, it is possible that a continuous slow sliding event may have played a role in triggering the continuous Pōrangahau sequence.”

Manawatu’s active slow slide has also moved to the east as far as Dannevirke – which happens to be just 40 kilometers from where the sequence began with a 5.9 quake on April 26.

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Because slow-sliding earthquakes have been shown to precede some subduction zone disasters — including Japan’s devastating magnitude 9.1 event in 2011 — researchers think they could be key to predicting major disasters.

But their occurrence doesn’t necessarily mean the next big hikurangi tremor was on its way, as the vast majority of slow slide events in New Zealand – and around the world – have appeared without cracks.

“The slow sliding events are a great reminder that New Zealanders live at the boundary of a very active tectonic plate.”

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