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4.2 earthquake linked to one of the slowest earthquakes in New Zealand on record

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Explanation of slow-slip earthquakes associated with the Hikurangi subduction zone in New Zealand. / GNS Science

It came, most likely caused a wave of tremors, and then disappeared again.

But one of New Zealand’s longest-running earthquakes in years has scientists scrutinizing a treasure trove of newly collected data.

The country’s most recent “slow slide” earthquake began off the coast of Biranjahau around May 23 and continued into the beginning of this month, likely causing a 4.2 quake in Hawkes Bay in the middle of the process.

These mysterious types of earthquakes, observed only in the past two decades, can last from days to years and produce up to tens of centimeters of displacement along faults.

But because they are too slow to be picked up by seismographs – or felt by humans – they can only be recorded using special GPS equipment that measures the slow motion of the Earth.

The latter event began in the same area as it occurred shortly after the 7.8 Kaikura earthquake in November 2016, and was large enough to dislodge three continuously operating GNSS sites by several centimeters.

“The recent slow slip event was similar to previous events we saw in that region, although it included slightly larger displacements at GeoNet GPS locations than those observed in 2006 and 2011, and was slightly smaller than the previous one in 2016,” said Dr. Laura Wallace from GNS Science.

The event also coincided with a series of small earthquakes around the area – the largest of which was a magnitude 4.2 tremor recorded in Waipukurau on May 30, which about 1,800 people reported feeling.

“We usually see swarms of earthquakes in that area during slow-slip events in Berangau,” she said.

“Although for this year’s slow slip event, there appeared to be slightly fewer earthquakes than previous slow slip events at Prangahau in 2006, 2011 and 2016.”

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

Scientists have set up nearly 50 other instruments across Hawke’s Bay, Tararua and Wiraraba to monitor “slow slip” events and related earthquakes. Photo / Science GNS علوم

Besides the 26 sensors recently installed offshore, nearly 50 other instruments across Hawke’s Bay, Tararua and Wairarapa were built by GNS and Victoria University scientists soon after the earthquake began, as part of a project supported by the Marsden Fund.

Slow-slide events are now known to be a relatively common feature of the Hikurangi subduction zone – a largely marine margin where the Pacific plate sinks – or dips – westward beneath the North Island.

Specifically, it tended to occur within regions where the subduction zone was going from being “stuck” below the southern North Island, to an area where the subduction zone was “creeping” north, around Gisborne and Hook Bay.

The slow slip event is located in a region where the Hikurangi subduction zone transitions from being “stuck” below the southern North Island, to a region where the subduction zone is “creeping” to the north, around Gisborne and Hawkes Bay. Image / GeoNet

The area – which represents one of New Zealand’s largest geological hazards – was also ideal for studying slow-slip earthquakes, because they occurred shallow enough to be imaged at high resolution using seismic techniques.

These instruments deployed both at sea and on land will provide a clearer picture of the energy created and released during events, said Dr. Emily Warren Smith, GNS Science Seismologist and co-leader of the new Marsden Project.

“Analysis of the small earthquakes that occurred before and during the Prangahau slow slip event is a great opportunity to test our ideas of how these slow slip events might occur,” she said.

“We’ve previously observed that the behavior of small earthquakes changes before and during slow earthquakes, but not in enough detail to fully understand the cause.

“The new data collected in this project, from additional tools, will greatly improve our understanding of how and why these slow earthquakes occur regularly, and what causes them to occur in the first place.”

Wallace said 26 marine sensors were to be collected during an expedition in November on the Niwa research vessel, Tangaroa.

While it remains unclear exactly why events tend to occur in five-year cycles, she said there are some theories in action.

“Possibly part of the answer is that the slow-slide region is steadily loaded by movement between tectonic plates, which is fairly constant,” she said.

“So there might be some sort of threshold – dependent on the strength of the fault – being reached every five years that causes it to happen regularly.”

The scientists were also exploring ideas developed by Warren Smith about the role of water accumulation in the fault zone, which may similarly affect timing.

In New Zealand, slow earthquakes tend to occur at shallower depths off Gisborne Bay and Hawke’s Bay, and at deeper levels observed off the Manawatu and Kapiti regions.

Source / GeoNet

In some events, large areas of land were observed moving eastward by as much as 4 cm over the course of days, weeks, or even months.

Because there is mounting evidence that such motions can shift pressure within the Earth’s crust and in very rare cases trigger large earthquakes, scientists have been monitoring slow earthquakes around the world closely.

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

They have preceded some of the most destructive earthquakes ever recorded – including the 9.1 Tohoku earthquake in 2011, the 8.1 Iquique earthquake in Chile in 2014, and the 7.2 earthquake off the coast of Mexico in 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.

However, because of their regular frequency in New Zealand, scientists now know that the events are part of normal behavior in our subduction zone – and their recording doesn’t mean a major breakup is down the road.

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