Silent earthquake linked to series of jolts

Shaking: A cluster of quakes which were centred around Porangahau yesterday were believed to be linked to the silent earthquake which was off the East Coast of the North Island.

A silent earthquake off the North Island's East Coast has moved parts of the shoreline eastwards by up to 3 centimetres.

Geonet scientist Laura Wallace has linked this slow-moving seismic activity to the spate of small earthquakes centred around Porangahau.

Yesterday a magnitude 5.6 earthquake rattled the region at 1.19pm 70 kilometres south-east of Porangahau.

"The swarm of quakes were in the same area as the slow-slip event detected offshore along the East Coast. Often these events will increase stress in some areas and decrease the stress in others resulting in a cluster of shakes," Ms Wallace said.

Jen Ormand who lives in the Wallingford Homestead along Porangahau Rd said she was sitting in her office when she noticed the house started to shake.

"I watched the pens fall off the edge of my desk but I live in a big, old, one-storey house where I can just run outside if I need to."

Porangahau local Selina Wakefield was more concerned and ran out of the house when the shaking started.

"When the last one hit I thought 'that's not stopping'. So I got up and got my son and ran outside."

Ms Wakefield said she could see the blinds swaying and felt the house "rocking and moving".

Geonet's Ms Wallace said it was not unusual for a slow-slip event to coincide with a proper earthquake like what happened yesterday.

"Swarms of small quakes have occurred multiple times offshore in this area. It happened back in 2006 and 2011 but we are unsure about the one in 2001 as we only installed the GPS instruments in 2002 but it would seem likely."

What was unusual about this silent quake was that the slip appeared to happen simultaneously from Hawke's Bay up to the East Cape.

"Normally these events happen in patches but this time it happened the whole way along the plate boundary, all at once."

Ms Wallace said this simultaneous movement was most likely caused by the magnitude 7.8 Kaikoura Earthquake which struck last Monday.

"It is possible that passing seismic waves from the magnitude 7.8 earthquake caused stress changes that triggered the slow-slip event," she said.

Four GPS stations, from Gisborne down to Cape Kidnappers and which can measure ground movement of a few millimetres, were moved up to 3cm.

GeoNet has been monitoring this relatively newly discovered phenomenon of silent earthquakes, or slow-slip events, which aren't big enough to be picked up by normal seismographs, where the Pacific and Australian plates meet.

However Ms Wallace said they can have the effect of magnitude 6-plus earthquakes over weeks or months with no detectable shaking.

"It is a matter of centimetres moving over months compared to metres of movement in four seconds, which is what happens when an earthquake hit," she said.

Slow-slip events were discovered in North America a few decades ago and only picked up in New Zealand when GPS stations were installed around the North Island in the 2000s.

"The precise linkage between slow-slip events and standard earthquakes is not well understood. This is still an area of active research," Ms Wallace said.

Meanwhile, GeoNet drone footage has shown the more dramatic surface impact of the Kekerengu Fault, one of several faults that ruptured during the Kaikoura shake.

The surface rupture, which moved 10 metres horizontally and 2m vertically, runs 30km through rolling farmland in southern Marlborough.