The Alpine Fault runs roughly 600 kilometres along the western side of New Zealand's South Island, tracing the base of the Southern Alps. It marks the boundary where the Pacific and Australian plates grind past and into each other, uplifting the mountains at a remarkable rate.
It is an oblique right-lateral strike-slip fault: most motion is horizontal at about 3 centimetres per year, but a compressional component drives the rapid rise of the Southern Alps. Trenching and geological records reveal one of the most regular earthquake cycles known on Earth.
The fault has produced great earthquakes of around magnitude 8 roughly every 300 years, the last in 1717, making it statistically well overdue for another. Scientists estimate a high probability of a major Alpine Fault rupture within the coming decades, which would severely affect the South Island.