A massive lump of softer material beneath Gisborne – packing nearly 40 times the volume of Mt Everest - may explain why the region sways like jelly during big earthquakes.
While scientists knew that softer rock lay below the East Coast, it was only by analysing seismic waves fired out from the 7.8 Kaikōura earthquake in 2016 that they could see the enormous deposit's true size.
The wedge-shaped chunk was estimated to be between 5km and 12km thick, and around 300km long by 150km wide - making it probably the largest such body beneath the North Island.
More importantly, it offered a clearer picture of why big quakes around Gisborne can cause slow, swaying movements that last for several minutes.
During the Kaikoura quake, seismic instruments around the region showed how these motions carried on for around eight minutes.
"It's a bit like a bowl of jelly," explained GNS Science seismologist Dr Yoshi Kaneko, who has just reported the new findings in the Journal of Geophysical Research – Solid Earth.
"When you jiggle the bowl a bit, the jelly moves even more. In this case, the stiffer 'bowl' is the Pacific Plate subducting under the North Island and the sediment wedge is above the plate."
While people in the area might not have noticed this swaying amid the quake, because they'd been rattled by the shorter, more violent shaking that preceded it, it stood out to Kaneko and colleagues.
"We were curious about the unusual behaviour of seismic waves from the Kaikōura earthquake in this region and that prompted us to try and identify the cause."
As "long-period" seismic waves travelled north through the Earth, they slowed down dramatically and started reverberating off Hawke's Bay north of Napier.
"Based on the way the seismic waves behaved, we were able to model the body of softer rock and sediment."
This meant the region was susceptible to long, slow swaying during large earthquakes – perhaps even from earthquakes as far away as the Kermadec region.
He added that the slow swaying effect only becomes pronounced in large earthquakes – those above magnitude 7.
Although dramatic, the large body was typical of subduction zones like that where the Pacific Plate dived westward beneath the North Island.
With the increasing use of networks of seismic instruments on the seafloor in many countries, scientists anticipate they will gradually find out more about the properties of these bodies.
And at the East Coast, scientists have been gaining incredible new insights into earthquake behaviour, thanks to a major international research programme focused there over recent years.
In particular, the region has become a laboratory to study slow slip events - silent, slow-burning earthquakes that can displace faults over days or months, and which may help forecast future shakes.
Scientists also believe the East Coast fault zone may have the potential to unleash "mega-thrust" earthquakes larger than magnitude 8, such as those which created tsunamis that devastated Indonesia in 2004 and Japan in 2011.