'The equivalent of a magnitude 7.0': Slow-slip rupture spreads south along Hawke's Bay coast

The Hikurangi subduction zone: what is it and how can we all prepare? / East Coast LAB

A month-long "slow slip" event off Gisborne and Mahia is now the equivalent of a magnitude 7.0 earthquake and has spread to Hawke's Bay.

GNS Science Geophysicist Laura Wallace said this slow-slip event was now as large as the biggest previous slow-slip event GNS had recorded off Gisborne in March 2010.

The slow-slip quakes are now moving at a slower rate than during its first week, when scientist measured the largest amount of eastern movement, but continue to be steady.

"So far, the plate movement that has occurred in this event is equivalent to a magnitude 7.0 earthquake," Wallace said.

The estimated movement on the Hikurangi Subduction Zone represents up to 4 years' worth of tectonic motion between the Pacific Plate and the eastern North Island.

She said measuring stations in southern Hawke's Bay, including Cape Kidnappers and Pawanui, had picked up on the movement and were joining in on the slow-slip event.

"This suggests that the rupture of this slow-slip event is propagating south now, too.

"Propagation of slow-slip events from offshore Gisborne into the Hawke's Bay region was also observed in 2016."

A map of earthquakes of Mahia related to the slow slip event. Photo / Supplied

Displacement data from GeoNet GPS sites on the earth's surface reveal this slow-slip event has involved up to 20cm of movement on the Hikurangi plate boundary offshore of Gisborne, and "possibly even more offshore Hawke's Bay".

Increased seismic activity on the Mahia peninsular has died down, but Wallace said there might be a link between the slow-slip event and recent earthquakes in the southern part of Hawke's Bay and the Tararua District, such as Dannevirke and Porangahau.

"Most of these earthquakes have been too small for you to feel, with the exception of the Dannevirke quake, which had over 2700 felt reports."

Scientists at GNS had been predicting the event for a while. Slow-slip events happen every one to two years and larger events every four to six years.

Wallace said it was a reminder New Zealand is on a active tectonic plate boundary.

"Like always, we would like to remind you to help keep you and your family safe by having an earthquake plan.

"Know what you should do in the event of a large earthquake, be sure to drop, cover and hold.

"If you are near the coast and you feel a long or strong earthquake, get to higher ground once the shaking has stopped in case a tsunami was generated."