Quake triggers 'silent' tremor

INSIDIOUS: A slow-slip "silent" quake under the North Island's East Coast has been triggered by last Monday's massive Kaikoura earthquake. PHOTO FILE

The Kaikoura earthquake may have triggered a "silent earthquake" off the North Island's east coast which moved the parts of shoreline up to 3cm east.

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

However, they can have the effect of magnitude-6+ quakes over weeks or months with no detectable shaking, GeoNet's Laura Wallace says in a blog.

When the 7.8 Kaikoura quake struck last Monday, four GPS stations, from Gisborne down to Cape Kidnappers and which can measure ground movement of a few millimetres, were moved up to 3cm.

The movement was not necessarily abnormal as such movement had been seen before and after big quakes.

"This slow-slip event is particularly interesting as it appears to involve slip along the plate boundary from Hawke's Bay up to East Cape at the same time," Dr Wallace said.

Normally slow-slip events are separated in time or happen one after the other.

"It is possible that passing seismic waves from the M7.8 earthquake caused stress changes that triggered the slow-slip event," 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," Dr Wallace said.

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

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