However, the largest fault of all, the interface between the Pacific and Australian Plates, underlies the whole region.
The two dimensional map shows the line of the boundary between the plates east of the North Island. In three dimensions, it is a sloping boundary (known as a subduction zone), with the Pacific Plate dipping under the Australian Plate. Plate collision is occurring at an oblique angle rather than head on, which is why there is such a large component of strike slip (sideways) motion in the North Island Fault System.
The hidden, subsurface plate boundary has been mapped over the years using evidence from thousands of small or medium sized earthquakes generated on or nearby to it. Seismometers are used to locate these earthquakes, and the seismic waves give information about the geological structures and rock types that make up the two interacting plates. Under Wellington the boundary dips gently down to the North-West at an angle of about 9 degrees, and is about 25 kilometers deep under the city.
Scientists also carry out GPS campaigns to make repeated measurements at a large number of locations when they want more detailed coverage.
If it ruptured it would 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. For more information about the locked plates under the North Island, check out our website here.
In order to improve our knowledge of the plate boundary, a major GNS Science co-ordinated project is being carried out next week. This involves a 90 km seismic survey crossing the lower North Island from one side to the other. Instead of listening out for natural earthquakes, the survey will use explosives, detonated down boreholes, to produce the seismic waves. Hundreds of geophones, spaced 100 metres apart, will pick up reflected sound waves to map the plate interface, faults and other features in the crust. Scientists from GNS Science, Victoria University, Tokyo and California are collaborating in this project.
For some more background to this project, have a look at our media release, or listen to Tim Stern of Victoria University in this radio interview.