Clues left by the last major earthquake on the Alpine Fault reveal its direction – and this will help New Zealand prepare for the inevitable next rupture

One of the most anticipated earthquakes in the world is the next major surface rupture of the Alpine Fault in the South Island of New Zealand.

With a 75% chance of this occurring within the next 50 years, there is justifiable interest in the potential magnitude, extent and intensity of ground shaking and its impacts on landscapes, infrastructure and buildings.

The key question – which has not yet been answered – is which direction the fault rupture will take.

Our new research reveals for the first time that the Alpine Fault ruptured from south to north in the large magnitude 8+ earthquake of 1717.

We developed our technique to determine rupture direction based on the Kikerengu fault after the 2016 Kaikoura earthquake. But our method is universally applicable for use in real-life earthquake scenarios and can therefore contribute to improving community preparedness.

In the Alpine earthquake, no trend is good news for the South Island's west coast. But a north-south rupture would send excess seismic energy into the relatively uninhabited Fiordland and Tasman Sea.

On the other hand, a south-to-north rupture is expected to cause high-intensity shaking in populated areas of Canterbury, Marlborough, Tasman and the northern West Coast.

Simulation of earthquake shaking from south to north along an Alpine fault. Credit: Brendon Bradley, University of Canterbury.

In the Kaikoura earthquake, Wellington residents witnessed the effect of rupture direction on the intensity of shaking. Ripping from south to north meant that more seismic energy was focused towards the capital than, say, Christchurch.

So, while the direction of rupture has been observed to make a big difference in modern earthquakes, geologists have not been able to determine it directly for past earthquakes.

Marks on the rock face

The Kaikoura earthquake is well documented by seismographs. We know that it started near Waiau in the south and traveled north into Cook Strait over the course of two minutes.

We noticed marks that had been scratched at the level of the crack. Like coarse sandpaper on wood, these scratches, or “slip lines,” record the movement when rock faces slide past each other during an earthquake. Some of these marks were curved, and our method could tell us the direction in which the seismic rupture was moving.

Slickenlines from the Kekerengu fault, taken days after the 2016 Kaikoura earthquake. Kate Clark, CC BY-SA

Using computer models to simulate how an earthquake would occur moment by moment, we were able to replicate the curved lines observed in the field and correlate them to the direction of the rupture. This gave us the framework we needed to investigate the rupture direction of past earthquakes on the Alpine Fault.

The Alpine Fault has not experienced any major surface rupture since 1717. During fieldwork, we visited three sites along the fault and examined natural outcrops, carefully exposing the fault plane using hand tools. We found 146 curved lines, 30 of which were curved.

Geologist Tim Little measures squiggly lines on an Alpine fault. Nick Barth, CC BY-SA

The curved geometry of the zigzags from the recent earthquake on the Alpine Fault indicated that they moved from south to north. We also found evidence of rupture in the opposite direction, suggesting that earthquakes could start to the north and south.

In one outcrop, we found evidence of zigzags resulting from multiple earthquakes – a rare and exciting finding that suggests it may be possible to develop a longer history of rupture direction.

The technique we have applied is a new observational method to determine past rupture trends. Its full potential has yet to be tested, but it is already applicable to faults around the world.

Our research shows that the last rifting of the Alps was from the south, and that both directions are possible. New information about past earthquakes like this one helps the scientific community produce realistic scenarios for the next big earthquake.

We now have direct evidence from the fault itself that we need to prepare for a scenario of very strong to severe tremors in the North West Coast, Tasman, Marlborough and Canterbury areas in the next major Alpine Fault earthquake.

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