4.2 shake linked to one of NZ's slowest-ever quakes

It came, likely triggered a flurry of tremors, and then was gone again.

But one of New Zealand's longest earthquakes in years now has scientists poring over a trove of freshly-collected data.

The country's most recent "slow-slip" earthquake began off the coast of Pōrangahau around May 23 and lasted until the start of this month, probably causing a 4.2 Central Hawke's Bay jolt in the process.

These mysterious types of quakes, observed only in the last two decades, can last from days to years and produce 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.

The latest event 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.

"The latest slow slip event was similar to previous ones we've seen in that area, although it involved slightly larger displacements at the GeoNet GPS sites than ones observed in 2006 and 2011, and was slightly smaller than the previous one in 2016," said Dr Laura Wallace of GNS Science.

The event also coincided with a series of small earthquakes around the region – the largest being a magnitude 4.2 shake recorded in Waipukurau on May 30, and which around 1800 people reported feeling.

"We typically see swarms of earthquakes in that area during the Pōrangahau slow slip events," she said.

"Although for this year's slow slip event, there seemed to be slightly fewer earthquakes compared to previous Pōrangahau slow slip events in 2006, 2011, and 2016."

As it happened, the relatively regular timing of these events meant scientists were able to deploy an array of sensors to capture one of the quakes for the first time.

Scientists have set up nearly 50 other instruments across Hawke's Bay, Tararua and Wairarapa to monitor "slow slip" events and related earthquakes. Photo / GNS Science

Along with 26 sensors that had recently been installed offshore, nearly 50 other instruments were set up across Hawke's Bay, Tararua and Wairarapa by GNS and Victoria University scientists just after the quake started, as part of a Marsden Fund-supported project.

Slow slip events were now known to be relatively common features of the Hikurangi Subduction Zone - a largely offshore margin where the Pacific Plate dives – or subducts – westward beneath the North Island.

Specifically, they tended to happen within areas where the subduction zone was transitioning from being "stuck" beneath the southern North Island, to an area where the subduction zone was "creeping" further north, around Gisborne and Hawke's Bay.

The slow-slip event is located 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

The zone – presenting one of New Zealand's largest geological hazards – was also ideal for studying slow-slip quakes, because they occurred shallow enough to be imaged at high resolution using seismic techniques.

GNS Science seismologist Dr Emily Warren-Smith, a co-leader of the new Marsden project, said those instruments deployed at sea and on land would provide a clearer picture of energy was built up and released during the events.

"Analysing the small earthquakes occurring both before and during a Pōrangahau slow slip event is a fantastic opportunity to test our ideas around how these slow slip events happen," she said.

"We've observed previously that the behaviour of small earthquakes changes both prior to and during slow slip earthquakes, but not in enough detail to fully understand why.

"New data collected in this project, from additional instruments, will greatly improve our understanding of how and why these slow earthquakes occur so regularly, and what causes them to happen in the first place."

Wallace said the 26 offshore sensors were due to be collected during a November expedition on Niwa's research vessel, Tangaroa.

While it was still unclear precisely why the events tended to happen in five-year cycles, she said there were some working theories.

"Probably part of the answer is that the slow slip area is being loaded steadily by motion between the tectonic plates, which is pretty constant," she said.

"So there may be some kind of threshold - dependent on the strength of the fault - that is reached every five years that causes them to occur so regularly."

Scientists were also exploring ideas that Warren-Smith had developed around the role of build-up of water in the fault zone, which might similarly influence the timing.

In New Zealand, slow slip quakes tended to occur at shallow depths off Gisborne and Hawke's Bay, and at deeper levels observed off the Manawatu and Kapiti regions.

Source / GeoNet

In some events, large areas of land had been observed to move eastward by up to 4cm over days, weeks, or even months.

As there's been increasing evidence to suggest such movements can shift stress within the Earth's crust and in very rare cases trigger large earthquakes, scientists have been watching slow earthquakes around the world all the more closely.

Wallace believed that solving the mystery of slow-slip events would help us to better understand the potential of the Hikurangi subduction zone to produce major earthquakes.

They've have preceded some of the most devastating quakes recorded - including the 9.1 Tohuku earthquake in 2011, the 8.1 Iquique earthquake in Chile in 2014, and a 7.2 shake off the coast of Mexico the same year.

Last month, researchers reported how the slowest earthquake ever recorded - lasting 32 years - eventually led to the catastrophic 1861 Sumatra earthquake in Indonesia.

Yet, because of their regular frequency in New Zealand, scientists now know the events to be part of normal behaviour in our subduction zone – and recording one didn't mean a major rupture was on the way.