Wellington's Shaky Foundations

North Island fault lines

North Island fault lines

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 or subduction zone fault) 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 an average 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 that extend above 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! Find out more about the locking of the subduction interface under the Lower North Island here.