Massive east coast seismic rupture heading south likely triggering more earthquakes

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".

"We've noticed in the last week that some of our southern Hawke's Bay GPS sites, such as at Cape Kidnappers and Pawanui, have been picking up movement and are joining in on this slow-slip event," she said.

This is similar to a slow-slip event from offshore Gisborne into the Hawkes Bay region observed in 2016.

A measurement site just north of Gisborne has seen 4cm of movement, while one "close to Gisborne" has moved to the east by 3cm.

This data can be used to understand the amount of movement on the Hikurangi Subduction Zone. So far, there has been 20cm of movement on the Hikurangi plate boundary offshore of Gisborne.

The movement represents up to four years' worth of tectonic motion.

The events are common but only tend to last for a few weeks.

It had earlier triggered a swarm of earthquakes off the Mahia Peninsula. But Wallace said that has "died down considerably".

She now believes there could be a connection between the slow-slip and several earthquakes which happened in the lower Hawke's Bay and Tararua districts this week.

On Wednesday, there was a 4.8 magnitude earthquake that rattled Dannevirke and surrounding regions, resulting in thousands of reports on the GeoNet site of people feeling the shake.

International seismologists have found the 2011 Japanese earthquake was preceded by a slow-slip earthquake that lasted a month, but the predictive value of the events remain unclear.

Scientists are studying the slow-slips off Gisborne through instruments on the seafloor off Poverty Bay to understand more about the movements and subduction zone processes.

This could help with earthquake forecasting in the future.