Last year's 7.8 Kaikoura earthquake produced what one Kiwi scientist says is perhaps the world's clearest-yet case of rippling waves from a big shake setting off a far-away, deep-seated, slow-burning quake.
What are called "slow-slip" events are very similar to earthquakes, because they involve more-rapid-than-normal movement between two pieces of the Earth's crust along a fault.
But unlike earthquakes, where the movement occurs in seconds, shifts in these "silent earthquakes" can take weeks or months to occur.
In the weeks and months following the November earthquake, GeoNet's national network of GPS instruments detected slow-slip events on the plate boundary of the Hikurangi subduction zone plate, where Pacific Plate dives beneath the eastern North Island.
The slow-slip occurred at less than 15km deep below the surface and spanned an area of more than 15,000sq km off the Hawke's Bay and Gisborne coasts - a distance comparable to the land area of New Caledonia.
It was the largest and most widespread episode of slow-slip observed in New Zealand since scientists first recognised this phenomenon under the seafloor east of Gisborne in 2002.
There was also a slow-slip event triggered on the subduction zone at a 25km to 40km depth beneath the Kapiti coast region, and up to half a metre of post-earthquake slip under Marlborough.
READ MORE: 'Stuck' fault zone could unleash big quake
The Kapiti slow-slip event and Marlborough afterslip still continue today, although they have slowed substantially over the past several months.
"This is probably the clearest example worldwide of large-scale slow slip being triggered over long distances by seismic waves from a large earthquake," said GNS Science geophysicist Dr Laura Wallace, the lead author of a new study published today in the journal Nature Geoscience.
"One of the most intriguing factors is that the quake triggered slow-slip off the Gisborne coast, up to 600km away from its epicentre in North Canterbury."
Seismologist Dr Yoshihiro Kaneko, of GNS Science, said the slow-slip was triggered by stress changes in the Earth's crust caused by passing seismic waves from the Kaikoura quake.
The triggering effect was likely accentuated by an offshore "sedimentary wedge" - a mass of sedimentary rock piled up at the edge of the subduction zone boundary under the seafloor off the North Island's east coast.
This layer of more compliant rock is particularly susceptible to trapping seismic energy, which in turn, promotes fault slip at the base of the sedimentary wedge where the slow-slip events occur.
READ MORE: Silent quakes help faults let off steam
Kaneko said the study also suggested that the northward-travelling rupture during the Kaikoura quake directed strong pulses of seismic energy towards the North Island, and this also influenced the long-distance triggering of the slow-slip events beneath.
The east coast slow-slip event was also responsible for sparking a series of small to moderate earthquakes off the east coast last November.
The largest earthquake in this sequence was a magnitude 6.0 quake off the southern Hawke's Bay coast at Porangahau on November 22.
Although scientists are still in the early stages of trying to understand the relationships between slow-slip events and earthquakes, Wallace said these exciting observations highlighted additional linkages between slow slip events and other earthquake processes.
The scientists are planning to produce separate research papers on the post-earthquake slip on the subduction zone beneath the Kapiti and Marlborough regions, once those events finish.