A massive seismic event underway off the east coast is spreading south and is likely the force behind several large earthquakes which shook New Zealand this week.
Earlier this month, GNS Science confirmed one of the largest "slow-slips" ever observed in New Zealand is currently underway off the coast of Gisborne in the Hikurangi subduction zone.
A slow-slip is essentially a slow, silent earthquake undetectable by humans and the seismograph network because it is the movement of faults over weeks or months. That compares to typical quakes which happen over minutes or seconds.
The event is now just as large as a Gisborne slow-slip from March 2010 - the equivalent to a magnitude 7 earthquake.
It began registering in late March with a large amount of eastward movement recorded during its first week.
While the slip has continued at a slower rate, albeit a steady one, GNS Science Geophysicist Laura Wallace has confirmed the rupture of the slow-slip is "propagating south".