Megathrust behind 'shattering' Kaikōura earthquake - expert

  • 02/10/2018
A crack in the road from the 2016 quake.
A crack in the road from the 2016 quake. Photo credit: Getty

New research into the events leading up to the 2016 Kaikōura earthquake suggests the Hikurangi subduction zone or a megathrust fault is behind it.

The megathrust fault is a subduction zone, known as the Hikurangi, where one tectonic plate dives under another. In New Zealand the Pacific plate plunges beneath the Australian plate, causing incredible stress where the pair meet.

The research conducted by Victoria University geophysicist associate professor Simon Lamb found most earthquakes that occurred on a fault line were triggered by earthquakes on faults elsewhere.

"This method assumes that each fault has its own in-built pacemaker or mechanism, giving rise to semi-regular earthquakes on the fault," Prof Lamb said.

He said there are a number of issues with this method, such as the vast number of faults, some of which are not visible at the surface.

His team looked at the slow movements of the landscape in the two decades leading up to the 2016 Kaikōura quake and measured with precise satellite mapping of ground motions.

"We found that the measured ground motions were caused by slippage only on the single major fault line separating the two tectonic plates that lie under New Zealand. This large fault, called megathrust, underlies much of New Zealand and only reaches the surface offshore."

The megathrust moves freely at depths of 30 kilometres but shows no movement at levels closer to the surface.

This creates an elastic-like movement, causing extreme stress on the landscape and in this case causing the November 2016 quake.

"The Kaikōura earthquake initiated a complex pattern of fault movement, essentially shattering the landscape, and causing a cascade of earthquakes on 20 or more faults," Prof Lamb said.

"The data we studied show a strong link between the pattern of shattering and locking of the underlying megathrust prior to the earthquake and the movement during the earthquake itself.

"The damage caused by the Kaikōura earthquake runs parallel to this locking of the megathrust, but cuts across many of the big surface faults in the area, indicating a strong link to the movement of the megathrust rather than any of the individual faults."

Prof Lamb said the findings could now be used to help predict future earthquakes.

"While we may not be able to predict the movement of individual faults, we can track the underlying cause of an earthquake and give an indication of where future shaking might occur by understanding and modelling the megathrust."