The Hikurangi Subduction Zone is the 600-kilometre-long tectonic boundary where the Pacific Plate dives underneath the Australian Plate off the east coast of the North Island. It is capable of producing the single largest earthquakes New Zealand can generate — a full-margin rupture could reach magnitude 9 — and because the fault is offshore, such an event would send a major tsunami into every east-coast community from Gisborne to Wellington within minutes of the shaking.
What a subduction zone earthquake is
At a subduction zone, two plates are not sliding past each other (like the Alpine Fault) — one is being forced underneath the other. The locked interface between them accumulates enormous amounts of strain, and when it slips it can do so along hundreds of kilometres at once, raising or lowering the sea floor by several metres. The largest earthquakes ever recorded — 1960 Chile (M9.5), 1964 Alaska (M9.2), 2004 Sumatra (M9.1), 2011 Tōhoku (M9.1) — were all subduction-zone events, and all produced devastating tsunamis. The Hikurangi margin has the same geological setting.
Evidence of past ruptures
Palaeotsunami research — coring coastal marshes for layers of sand deposited by past tsunamis — has identified at least 10 probable subduction-zone ruptures along the Hikurangi margin in the last 7,000 years, with an average interval of 400–900 years. The most recent full-margin event is thought to have occurred around 1460 CE; that date is consistent with Māori oral tradition of coastal destruction on the East Cape. Partial ruptures (covering only portions of the margin) appear to occur more frequently.
The slow-slip phenomenon
The northern section of the Hikurangi margin — off Gisborne and East Cape — exhibits regular slow-slip events, where strain is released as the two plates creep past each other over weeks rather than snapping in seconds. These have been recorded every 1–2 years since continuous GPS monitoring began in the early 2000s. Slow slip is probably reducing the seismic hazard in the northern section, but it is not well understood how it interacts with the southern (and more strongly locked) section of the fault.
What a rupture would do
A full-margin M8.5–9.0 rupture would produce: severe shaking (MM8–9) from East Cape to Wellington lasting two to four minutes; a tsunami with leading wave heights of 10+ metres on the East Cape, reaching the coast within 7–15 minutes of the shaking starting (depending on location); major inundation of Gisborne, Napier, Wairoa, and every east-coast township; damage to Wellington city from long-period shaking even though the tsunami itself may be smaller there. The 'long or strong, get gone' rule applies: any severe or extended shaking on the east coast means you should get to high ground immediately, without waiting for an official warning.
What to prepare if you live on the east coast
The tsunami will arrive before official warnings. So: know your safe zone — the nearest point at 35 metres above sea level or 1 km inland, whichever you reach first. Walk that route in daylight so you know where it goes. Keep a grab-bag by the door. Have an agreed family meeting point at the evacuation zone. If you are camping or visiting the coast, scan for evacuation route signs when you arrive. The East Coast LAB (Life at the Boundary) programme publishes detailed preparation guidance for each east-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.