Wellington is sitting on the relatively light continental crust of the Australian Tectonic Plate which is riding over the dense oceanic crust of the Pacific Plate. The main boundary between the two plates (the subduction interface) slopes westward down beneath the North Island and is about 25-30 km below Wellington City. At Wellington the two plates are moving against each other at a rate of about 3.5 cm per year. This slow collision puts immense pressure on the crust and has broken it up into several large pieces, separated along fault lines – including the Wellington and Wairarapa faults, and the subduction interface. When the strain between these blocks of crust overcomes the resistance that locks them together, they move relative to each other and we experience the jarring, shaking jolt of a large earthquake!
The last time the Wellington Fault ruptured through the Wellington region, causing a major earthquake, was around 200 - 450 years ago. Geoscientists estimate the Wellington Fault will cause a major earthquake every 500-1000 years. However other faults around the Wellington region are also active and capable of generating a major earthquakes, for example the Ohariu Fault and the Wairarapa Fault which last ruptured in 1855 causing a great earthquake that severely affected Wellington. The frequency of large earthquakes affecting the Wellington Region is therefore much higher, with an average return time of about 150 years for a very strong or extreme ground shaking quake.
Major faults in the Wellington Region move mainly sideways, with some up or down displacement as well. Scientists describe them as ‘oblique dextral strike-slip’ faults, Which means that if you look across the fault the ground on the far side will move to the right, as well as a bit up or down!
In New Zealand's biggest historic 'quake in 1855, The Wairarapa Fault moved about 15 metres sideways and about 6 metres vertically! As a result of this great earthquake, the whole Wellington region was severely shaken, uplifted and tilted to the west. In fact, the land and sea floor near the harbour rose up about 1 - 1.5 metres.
When the Wellington Fault next ruptures it is expected to cause about 4 – 6 m of dextral strike-slip, as well as a variable but lesser amount of vertical displacement – some areas will experience uplift, but others, like the Hutt Valley, may subside (sink) by about a metre.Rupture of the Wellington Fault, (as well as the many other active faults in the Wellington region, including the Wairarapa Fault, the Ohariu Fault and the subduction interface) would cause a variety of major earthquake hazards.
The most severe and damaging effect will be the strong ground shaking. The impacts of the ground shaking will vary around the region depending on:
There will be other damaging effects of a large Wellington earthquake: Many slips will occur throughout the region, especially if the hill slopes are already saturated by recent rainfall. In flat areas underlain by unconsolidated sediments the ground can liquefy, tilting buildings and causing buried pipelines and other structures to float to the surface!
Many of the coastal areas of the lower North Island will be at risk of a tsunami, caused by the Wellington fault displacing the seafloor or triggering a submarine slump. Within Wellington harbour and on rivers and lakes in the region there may also be ‘seiche’, as was observed in Harbour after the 1855 Wairarapa earthquake. A seiche occurs when seismic waves passing through the waterbody set up standing waves that can then inundate the surrounding shorelines.
The Wellington Fault passes under significant infrastructure such as the ferry terminal, motorway, railway, and several bridges along the Hutt Valley, which could be put out of action when the fault next ruptures. Transport routes throughout the region may also be affected by landslides and liquefaction, this means people could be stuck at work or at school or somewhere in between. Water supplies, electricity and phone lines may also be disrupted so it is a good idea to have an emergency action plan! Here is another good place to start.
New Zealand is located astride the boundary between two of Earth’s great tectonic plates. Here the Pacific and Australian plates bash into each other, causing earthquakes and volcanism that we experience as part of everyday life.
The Earth's crust is not all one thickness. Underneath the ocean, the crust is thin compared to the crust underneath land (usually called "oceanic" and "continental" crust). When oceanic crust meets continental crust, the oceanic crust is forced underneath (is subducted) into the mantle.
Subduction occurs in two places in New Zealand. Underneath the North Island the Pacific plate is subducted westward underneath the Australian plate, and underneath Fiordland the Australian plate is subducted eastward beneath the Pacific plate.
Along the west coast of the South Island and linking the two subduction zones, the Australian and Pacific Plates slide past each other along the Alpine Fault, this is called a transform plate boundary. This 3D animation shows earthquake locations below New Zealand. (6 mb mpeg).
Go to the VISIBLE EARTH catalog from NASA to learn more about this image.
