The Alpine Fault

Seen from space, the Alpine Fault is a ruler-straight line along the western flank of New Zealand's South Island, separating the rainforested West Coast from the ice-capped Southern Alps. It is the boundary between the Australian and Pacific plates, and it is fast: the two sides slide past each other at nearly the rate of the San Andreas while the eastern side is shoved upward, lifting the mountains about as quickly as rain and ice wear them down. The fault has broken in a magnitude 8 earthquake roughly every three centuries, and the last one was around 1717. New Zealanders have a name for the next: AF8.

The fault runs about 600 kilometers from Milford Sound in Fiordland northeast to the Marlborough region, where it splays into the Wairau and other faults. It is a right-lateral strike-slip fault with a thrust component, moving about 27 to 38 millimeters a year sideways and up to about 10 millimeters a year vertically in its central section. That uplift has raised Aoraki / Mount Cook and its neighbors, the highest peaks in Australasia. Glaciers such as Franz Josef and Fox descend almost to the fault trace, to within a few hundred meters of sea level. A rupture would cut the few roads and passes that link West Coast towns such as Hokitika, Franz Josef, and Haast to the rest of the island, and would shake Queenstown, Christchurch, and Dunedin.

Geologist Harold Wellman recognized in the 1940s that rocks on either side had been offset by about 480 kilometers, among the first demonstrations of large horizontal movement on a fault. Tree rings and lake sediments date its last great rupture, about magnitude 8.1, to around 1717, and a record from Fiordland shows 27 ruptures in 8,000 years. A 2021 study led by Jamie Howarth of Victoria University of Wellington, with GNS Science researchers, put the chance of a major rupture in the next 50 years at about 75 percent, with an 82 percent likelihood that it reaches magnitude 8 or more. The AF8 program, a partnership of scientists, emergency managers, and South Island councils, has written a response plan for the scenario.

• The San Andreas Fault
  California's 1,200-kilometer seam between the Pacific and North American plates comparable transform fault
• Australia and New Zealand
  Two settler states, a continent and an archipelago, far from everywhere located in
• New Zealand
  Two islands of mountains, fjords, and Maori heritage located in
• Tasman Sea
  The stormy stretch of ocean dividing Australia and New Zealand parallels the coast of
• The Tectonic Plates
  The seven great slabs and dozens of smaller ones that make up Earth's moving shell plate boundary
• Sutherland Falls
  New Zealand's tallest waterfall, a three-tiered cascade from a glacial lake in Fiordland southern end near
• Aoraki / Mount Cook
  New Zealand's highest peak, the cloud piercer of the Southern Alps uplifted