Digging into the past of sleeping giant faults

The Nevis Fault is a sleeping giant fault, one that awakens only every 10,000 years or so. This week, a team of geologists use paleoseismic trenching to answer questions about this fault and to figure out the pattern of past earthquakes.

Aotearoa New Zealand is in a collision zone. It is the place where the mighty Australian and Pacific tectonic plates meet. The main fault line runs the length of Te Wai Pounamu and off the east coast of Te Ika a Māui, but the pressure of these two plates meeting causes cracks and fissures across Aotearoa.

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These interconnected fault lines rupture at different times, releasing some of the pressure of this massive collision. Some of them rupture relatively frequently. We know that part of the main fault line, the Alpine Fault, is one of these, producing large earthquakes about every 300 years. Others are low recurrence earthquake fault lines, with a larger time interval of more than ten thousand years between ruptures.

The Nevis Valley fault is one of these low recurrence ones. Found in the grassy high country of Otago it runs the length of the Nevis Valley, up to the Kawerau Gorge. Past studies have indicated that it ruptures every 10 to 12,000 years.

This was determined by a method called paleoseismic trenching - digging a deep trench perpendicular to the fault line and looking at the sedimentary layers for evidence of pre-historic earthquakes. This was first done on this fault in the 1980s, but recent advances in technology and dating techniques warrant a second look, says Professor Mark Stirling of the University of Otago.

Stirling is co-lead of an Earthquake Commission-funded project to investigate the seismic risk in low seismicity areas in Southland and Otago.

Dr. Jack Williams, a postdoctoral researcher at the University of Otago, is also working on this project, and he is part of the team who are carefully cleaning and marking the trench wall. Once the wall is tidied up after the digger's impact, a grid is set up, and the visible sedimentary layers are marked.

https://players.brightcove.net/6093072280001/default_default/index.html?videoId=6307389939112

This is the beginning of the tricky interpretation step…

Go to this episode on rnz.co.nz for more details