The second "slow-motion" earthquake to unfold beneath New Zealand within weeks appears to have driven a cluster of tremors north of Gisborne – including a 4.2 shake.
What are called slow-slip earthquakes – a phenomenon 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.
One that played out over two weeks in late May, off the coast of Pōrangahau, was observed to cause a flurry of small earthquakes around Central Hawke's Bay.
Now, GeoNet scientists have just reported a new event that appeared to have begun near Gisborne on June 14 – and already caused several centimetres of movement within the plate boundary.
Dr Laura Wallace, of GNS Science, said a GPS station set up at Gisborne had also recorded about 2cm of eastward displacement.
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.
While they occurred around every five years at Pōrangahau, further up the coast at Gisborne, they played out as frequently as every one to two years.
But larger events like this one occurred around each four to six years – and the last slow quake of comparable size was recorded over April and June in 2019.
"Even though these two East Coast slow-slip events are a couple of hundred kilometres apart, we often see such events clustering in time," Wallace said.
"For example, a series of slow-slip events like this occurred in 2011 and 2016."
During the 2019 event near Gisborne, scientists recorded many small-to-moderate earthquakes in the area, including a 5.1 quake near Mahia Peninsula.
"We expect to see a similar pattern of small earthquakes near Gisborne for this current slow-slip event, in line with what has been observed during previous events."
Over the past two weeks, scientists have observed a cluster of about 60 small earthquakes north of Gisborne - coincidental to the timing of the slow-slip event.
Five of these have measured greater than magnitude 3.0 – and the largest was a magnitude 4.2 on June 16 that was only reported as "felt" on GeoNet by 123 people.
"Most of these events have been unnoticeable," Wallace said.
"We're keeping an eye on this data to better understand the possible relationship between the recent earthquake cluster and the slow-slip event."
She noted that slow-slip events in New Zealand were typically associated with increased numbers of small earthquakes.
"We located 50 per cent more earthquakes in the Pōrangahau region in the last month than we would normally locate."
Currently, there were a range of seismic and geodetic instruments deployed along the East Coast in place to capture data from slow-slip events.
"Insights from this data will help scientists better understand what drives slow-slip activity and could aid in future monitoring to forecast large earthquakes."
The Hikurangi Subduction 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.
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 Manawatū and Kāpiti regions.
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.
"Slow slip events are a great reminder that we live on a very active tectonic plate boundary here in New Zealand," Wallace said.