Stuck Plate Boundary

This diagram illustrates the impact on GPS velocities in the North Island due to "locking" or "coupling" of the subduction interface fault between major earthquakes.

This diagram illustrates the impact on GPS velocities in the North Island due to "locking" or "coupling" of the subduction interface fault between major earthquakes.

Since the early 1990's, GNS Science has been measuring a widespread array of Global Positioning System (GPS) sites in order to monitor their locations with great precision. Over time these recordings have shown that the surface of the landscape is being deformed by tectonic movements as the Australian and Pacific plates slowly converge.

These GPS measurements indicate that a large segment of the crust of the Australian Plate in the Lower North Island is stuck to the underlying slab of Pacific Plate, and is being dragged along “backwards” to the west. Different possible reasons for this have been proposed by scientists, but it is believed to be caused mainly by friction on the interface between the two plates, which prevents them from sliding past each other. Eventually, this friction causes a large amount of stress to build-up along the fault and within the surrounding crust. Once the accumulated stresses are large enough to overcome the strength of the subduction zone fault, they will cause the Australian and Pacific Plates to slip rapidly past each other in a large earthquake on the subduction zone fault.

In the coloured image, the red colour indicates the highest degree of coupling (sticking) between the subducting and overriding plates in the Lower North Island. This segment of stuck plate boundary is about 70 km wide and 140 km long.

Diagram showing the locked plate interface under the North Island

If the coupled segment of the subduction zone fault beneath the lower North Island ruptured it could produce an earthquake of magnitude 8 or above. It is even possible for larger sections (eg the length of the North Island) to rupture occasionally in a single massive earthquake. Examples of recent large subduction fault earthquakes (or megathrusts) are those that occurred in Japan in 2011 and Indonesia in 2004. As well as prolonged violent shaking, they were responsible for large devastating tsunamis. Ongoing monitoring of deformation is one of the most important ways to assess potential for major earthquakes on the subduction zone beneath the North Island

Find out about the slow slip events that occur on the downslope edge of the stuck portion of the plate boundary.

Read more about New Zealand's Alpine Fault – the non subducting portion of our plate boundary that runs the length of the South Island.