Jumping Earthquakes – One Fault at a Time

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Jumping Earthquakes – One Fault at a Time

Published in Nature Geosciences on 1 October 2018, new research by a team of scientists from the Victoria University of Wellington and the Earth Observatory of Singapore (EOS) has revealed how understanding the events leading up to the 2016 Kaikoura earthquake may lead to a different approach to forecasting earthquakes.

The Kaikoura earthquake, which measured Mw 7.8 in magnitude, had struck the South Island of New Zealand in 2016. It resulted in a rupture that stretched over 200 kilometres (km), ripping through 21 faults – a world record for the most number of faults observed to rupture in a single earthquake event.

It is now rightly regarded as the most complex earthquake ever to be studied, and has resulted in scientists rethinking several assumptions about the causes, processes, and effects of earthquakes.

Dr James Moore, a Research Fellow at the Earth Observatory of Singapore (Source: Antoinette Jade/Earth Observatory of Singapore)

According to co-author Dr James D P Moore, a Research Fellow at EOS, “It has been commonly thought that the best way to assess the likely times of future earthquakes is to analyse the earthquake histories of individual faults.

“Here, the earthquake history of Kaikoura is incorporated into the modelling software and used to predict possible future earthquakes on each fault. This method assumes that each fault has its own in-built metronome or driving mechanism, giving rise to semi-regular earthquakes on the fault.” 

However, there are a number of issues with this method. Firstly, it is impractical to characterise every fault – there are just too many and many are not visible at the surface.

But a more fundamental issue with this method was revealed by the analysis done in the team’s work. It showed that instead of each fault having its own characteristic clock with earthquakes occurring at semi-regular intervals, in some cases the earthquakes that happen on faults are triggered by earthquakes on faults elsewhere. 

To come to this conclusion, the team looked at the slow movements of the landscape in the two decades prior to the 2016 Kaikoura earthquake, measured very precisely with satellite mapping of ground motions. 

“We found that the measured ground motions were caused by slippage only on the single major fault separating the two tectonic plates that lie under this part of New Zealand.  This large fault, called the megathrust, underlies much of North Island and the Northern portion of South Island, and only reaches the surface offshore,” explained Dr Moore.

The megathrust moves freely at depths of 30 km or more, but at shallower depths it is locked in place. This combination of steady movement and locked sections slowly forces the southern North Island and northern South Island to bend elastically, and this movement puts extreme stress on the landscape. Eventually the land can’t take the buildup of stress anymore and it fails – in this case, it caused the 2016 Kaikoura quake. 

The Kaikoura earthquake initiated a complex pattern of fault movements, essentially shattering the landscape and causing a cascade of earthquakes on twenty or more faults. The data we studied shows a strong link between the pattern of shattering and locking of the underlying megathrust prior to the earthquake and the movement of the earthquake itself.

The damage caused by the Kaikoura 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.

Lead author Associate Professor Simon Lamb of Victoria University of Wellington said, “These findings may be significant for the way we predict future earthquakes, and 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.”

(Source of thumbnail image: Gareth Kiernan/www.infometrics.co.nz)

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