Explainer: The science behind today’s Hawke’s Bay earthquakes

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Twin Hooke’s Bay tremors struck this morning in an area known to be seismically active – and may be linked to a deep-seated, slow-burning earthquake that has been unfolding under the North Island for months.

This afternoon scientists are assessing possible drivers behind the widely felt morning tremors in Perangahao, which began with a 24-kilometre-deep 5.9-magnitude quake that struck at 10.16am, and a 5.4-magnitude quake minutes later, at a slightly lower depth.

A third quake with a magnitude of 3.9 occurred at 10:21 a.m. and its epicenter was about 10 kilometers west of the coastal town.

GeoNet has recorded more than 20 other such aftershocks.

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“We certainly expect ongoing aftershocks, which could be of ‘felt’ magnitude, over the coming weeks to months,” said GNS Science seismologist Dr. Katie Jacobs.

“And we know that earthquakes tend to cluster in this region the most, which could also increase the potential for aftershocks.”

There was heavy shaking at Burangao this morning with M5.7 closely followed by M5.3. Our team is still looking into this, so volume/locations are currently preliminary. We will update the information as soon as possible. #eqnz pic.twitter.com/aZVxRxNSFu

— GeoNet (@geonet) April 25, 2023

While scientists have not yet established the earthquakes’ focal mechanism–or their type of fault–it is likely that they originated from the continued “subduction” of the Pacific plate under the Australian plate, west below the North Island.

This plate junction features the sprawling Hikurangi Subduction: potentially New Zealand’s largest earthquake and tsunami hazard.

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It remains unclear whether the main earthquake occurred along a main plate boundary fault – called the Hikurangi megathrust – or within the crust, above or below plate boundaries.

“It’s difficult to attribute specific drivers to an individual earthquake,” Jacobs said, “but we clearly know that this region is very active, as it is in a subduction zone.”

“And we know that this region, in particular, appears to be the epicenter of some of that activity.”

The Hikurangi subduction zone defines the margin where the Pacific Plate sinks – or subsides – westward under the North Island. Image / GeoNet

GNS geophysicist Dr. Laura Wallace said swarms of moderate to large earthquakes have been observed at Pōrangahau in the past: one in 2011 involved more than 30 in the space of a few months.

“But today’s two events were two of the biggest we’ve seen in the last 10 years or so.”

Professor John Townend, a geophysicist at the University of Victoria, said the area is known to be prone to various types of earthquakes.

“There are some that represent the friction of one plate against another — or the Australian plate placed on top of the Pacific plate and rubbing against it — and others that are more related to the curvature of the plate.”

The Hikurangi Subduction – which stretches from the upper East Cape to the upper eastern part of the South Island – is potentially New Zealand’s greatest source of earthquake and tsunami risk.

The area also happens to be a hotspot for mysterious slow-moving earthquakes called “slow slide events,” which can release pent-up energy over weeks, months, or years.

Only discovered in the last two decades, they can produce up to tens of centimeters of displacement along faults.

But because they are too slow to be picked up by seismometers – or felt by humans – they had to be monitored using sensitive GPS equipment that measures the slow motion of the Earth.

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In New Zealand, they tended to play within areas where the subduction zone was moving from being “stuck” down the southern North Island, to an area where the subduction zone was “creeping” to the north, around Gisborne and Hawke’s Bay.

New Zealand’s slow-gliding earthquakes occur in an area where the Hikurangi subduction zone goes from being “stuck” down the southern North Island, to an area where the subduction zone “creeps” north, around Gisborne and Hawke’s Bay. Image / GeoNet

In November 2016, the magnitude 5 earthquake that struck offshore Berangahao was likely caused by a slow slip event that itself caused the Kaikoura earthquake that month.

More recently, in 2021, a weeks-long episode off the coast of Pōrangahau — captured in detail by deployed GNS sensors — likely triggered a 4.2 event near Waipukurau.

Seismic activity of the North Island during the past two hours. Brown (🟠) is the strongest activity.

Data provided by gnsscience. pic.twitter.com/0wMBJN0GwY

– Niwa Weather (@NiwaWeather) April 25, 2023

Another slow-moving event — linked to a 6.3-magnitude earthquake in Kapiti in February — was still deepening under Manawatu and Horohenoa, Wallace said, and today’s tremors could have been related to the same process.

“Sometimes we can have an earthquake like this when we don’t have a slow slide, but I think there is a reasonably high probability that this is related to the ongoing Manawatu slide event.”

The earthquakes occurred when Jacobs, Wallace, and other scientists were investigating earthquakes in the region, and their relationship to slow slip processes and subduction dynamics.

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