The Alpine Fault is the tectonic boundary between the Pacific and Australian plates that runs for 850 km up the western edge of the South Island, from Fiordland to just past Marlborough. It is one of the most hazardous faults on Earth — not because it ruptures frequently, but because when it does rupture, it does so on an enormous scale. The last full rupture was in 1717, which means the fault is now 309 years into a cycle that has historically averaged 300 years.
What makes the Alpine Fault so dangerous
The Alpine Fault is a transform fault — the two plates slide past each other at about 27 millimetres per year, which over 300 years accumulates about eight metres of strain. When that strain releases, it does so in a single massive rupture that propagates along hundreds of kilometres of fault at once. GNS Science's research programme AF8 (Alpine Fault magnitude 8) has modelled a full rupture scenario and concluded that the expected magnitude is between 8.1 and 8.3, comparable to the 1906 San Francisco earthquake or the 1855 Wairarapa earthquake — both of which devastated the regions they struck. The difference is that the Alpine Fault sits directly under the Southern Alps and its rupture would affect the entire central and northern South Island.
What happened in 1717, and before that
The 1717 event is the most recent known rupture. It was reconstructed by geologists studying disturbed layers in lake sediments and coastal terraces — the fault hasn't ruptured during recorded European settlement, so everything we know about it comes from the geological record. That record also shows 27 previous ruptures going back about 8,000 years, with an average interval of about 300 years between events, but with real variability — some cycles have been as short as 200 years, others as long as 450. The pattern is consistent enough that GNS Science rates the 50-year probability of rupture at around 75%.
What a full rupture would actually look like
The AF8 scenario models a rupture beginning near Milford Sound and propagating north-east for around 90 seconds, reaching speeds of several kilometres per second. Shaking would last for about two minutes in the worst-affected areas. The expected consequences include: widespread landsliding across the Southern Alps blocking every major state highway on the West Coast; tsunamis of several metres in Fiordland sounds; severe shaking reaching MM9 intensity in towns like Franz Josef, Hokitika, Greymouth and Westport; moderate shaking (MM7–8) across Canterbury, Otago and Marlborough; and the cutting of essentially all road and rail links between the West Coast and the rest of the South Island for weeks to months.
Why the probability is higher than a simple Poisson calculation suggests
A naive calculation based on Poisson statistics would say that if the average interval is 300 years, the probability of rupture in any given 50-year window is about 15%. GNS Science's 75% figure is much higher because the Alpine Fault doesn't rupture randomly — it follows a roughly periodic pattern driven by strain accumulation. The longer it has been since the last rupture, the more strain has built up, and the more likely rupture becomes. With 309 years already elapsed, the fault is past its average interval and the conditional probability climbs rapidly. This is why the common shorthand — 'the Alpine Fault is overdue' — is technically correct.
What to do now to prepare
Every West Coast and central South Island household should have a week's worth of food and water — not the standard three days — because road access will take longer to restore. Have a portable radio, battery torches, a first aid kit, and your prescription medicines duplicated. Make sure your house is bolted to its foundations and that heavy furniture is secured to walls. Know your workplace and school emergency plans. Identify safe spots outdoors away from buildings, power lines and steep slopes. The AF8 programme publishes preparation guidance specific to each West Coast community.
Other New Zealand geohazards
This page is part of the NZ Geohazards dashboard — a live tracker of the six major geological hazards New Zealand faces, with probabilities and elapsed time updated continuously.