The direction of the last major earthquake on the Alpine fault will help New Zealand prepare for the inevitable next one

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

With a 75% chance of occurring within the next 50 years, there is legitimate interest in the scale, extent, and intensity of ground shaking and its impact on landscapes, infrastructure, and buildings.

An important, and so far unanswered, question is in which direction the fault ruptures.

Our new research reveals for the first time that the Alpine Fault ruptured from south to north during the 1717 earthquake of magnitude 8 or greater.

We developed a method to determine rupture direction based on the Kekerengu fault after the 2016 Kaikoura earthquake. However, our method can be applied worldwide to realistic earthquake scenarios and thus contribute to improving societal preparedness.

With the Alpine fault earthquake, no direction is good news for the South Island’s west coast. However, a north-to-south rupture would send excess seismic energy into the relatively sparsely populated regions of Fiordland and the Tasman Sea.

Meanwhile, rupture from south to north is predicted to cause stronger shaking in populated areas of Canterbury, Marlborough, the Tasman and the northern West Coast.

During the Kaikoura earthquake, Wellingtonians experienced the impact of rupture direction on shaking strength. The rupture from south to north meant that more seismic energy was concentrated towards the capital than, say, Christchurch.

Therefore, while rupture direction has been observed to make a large difference in modern earthquakes, it is not something that geologists have been able to directly determine for past earthquakes.

traces of rock surface

Kaikoura earthquakes are well recorded by seismographs. It is known to have started in the south near Waiau and traveled north for two minutes into Cook Strait.

Traces of scratching on the fault plane were observed. Like rough sandpaper on wood, these scratches, or “slickenlines,” record the movement of rock surfaces as they slide past each other during an earthquake. Some of these markings are curved, which allows our method to tell the direction in which the earthquake rupture progressed.

By using a computer model to simulate how an earthquake unfolds moment by moment, they were able to reproduce the curved Slickenlines observed in the field and associate them with the direction of rupture. . This provided the necessary framework to investigate past earthquake rupture directions on the Alpine fault.

No major surface rupture has occurred on the Alpine Fault since 1717. During the field survey, we visited three sites along the fault, investigated natural outcrops, and carefully exposed the fault plane using hand tools. We found 146 slickenlines, 30 of which were curved.

The curved shape of the Slickenline from the most recent earthquake on the Alpine Fault indicated that it moved from south to north. They also found evidence of rupture in the opposite direction, suggesting that earthquakes can initiate in both the north and south.

At one outcrop, we found evidence of a slickenline from multiple earthquakes. This is a rare and interesting finding that suggests that a longer history of fracture direction development may be possible.

The technique we applied is a new observation method on faults to determine past rupture directions. Although its full potential has not yet been tested, it is already applicable to disorders all over the world.

Our study shows that the last alpine fault rupture was from the south and that both directions are possible. This new information about past earthquakes helps the scientific community create realistic scenarios for the next big earthquake.

We now need to prepare for a scenario in which the next major Alpine fault earthquake causes very strong to severe shaking in the northern West Coast, Tasman, Marlborough and Canterbury regions. I got the evidence.

This article is republished from The Conversation under a Creative Commons license. Read the original article.