Slow slip earthquake shifts entire Māhia Peninsula east

Geonet data shows sizable shifts east on the Hikurangi Plate boundary caused by "slow slip" events, including 2cm measured at the Māhia Peninsula. Graphic / Geonet - NZ Herald

Movements deep in the earth released energy comparable to a magnitude 6.6 earthquake shifted the entire Māhia Peninsula east - but nobody felt it.

Geonet says a “slow slip” earthquake took place 12 kilometres below Hawke’s Bay, with much of the movement occurring beneath Māhia Peninsula, shifting it two centimetres east.

The movement lasted three weeks, beginning around June 10 and slowing down at the end of the month, during which one to two years’ worth of tectonic plate motion took place.

“The slow slip earthquake generated five to eight centimetres of movement between the Australian and Pacific Plates, along the Hikurangi subduction zone,” the Geonet statement said.

“To put this in perspective, five to eight centimetres of movement represents one to two years’ worth of plate motion taking place in only three weeks.”

Geonet said a GNSS site at the Māhia Peninsula was displaced eastward by about 2cm during the event and half a centimetre of surface displacement was observed as far south as Cape Kidnappers.

Sam Taylor-Offord, of GNS Science, said the entire Māhia Peninsula and everywhere else affected by the “slow slip” are moving as one, but some regions move slightly more and slightly faster than others based on where they are in relation to the slipping.

“Because it’s a slow, continuous movement of the land it’s almost like the whole affected region is just gently sliding as one block over the fault, with a little bit of variation between slipping surface areas due to the variations of the fault movement,” Taylor-Offord said.

The Geonet statement said energy released by the movement on the plate boundary during the event was equivalent to a magnitude 6.6 earthquake.

“As this release of energy took place slowly, over a period of weeks rather than seconds, it did not cause shaking that is able to be detected by our seismographs.”

Geonet said they were able to observe the events unfold thanks to their continuously operating GNSS instruments tracking millimetre-level land movement daily.

“We often see slow slip events in the Māhia and Hawke’s Bay area, with the last event that we reported on at about the same time last year.”

Taylor-Offord said smooth, slow and deep-seated movement like this was unlikely to cause environmental effects on the surface.

“The slow slip isn’t going to cause breaks or tears at the surface,” he said.

He said the slow slip could, however, relieve stress on the fault and increase stress and strain wherever it causes a difference in the ground movement, potentially causing earthquakes.

“We often see earthquakes at the margins of slow-slip deformation as the ground breaks to accommodate the movement of the mobile land compared to that which was unaffected,” he said.

It is not the first time Geonet has detected movement on this scale in Hawke’s Bay. Hikurangi Margin slow slip events occur every one to two years at a shallow depth, meaning less than 10km, and last for 2-6 weeks according to Geonet.

James Pocock joined Hawke’s Bay Today in 2021 and writes breaking news and features, with a focus on environment, local government and post-cyclone issues in the region. He has a keen interest in finding the bigger picture in research and making it more accessible to audiences. He lives in Napier. james.pocock@nzme.co.nz