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What would a major Wellington earthquake be like?

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:

  • the size and depth of the earthquake - this determines the frequency and amplitude (strength) of the seismic waves as they travel through the earths crust.
  • the bedrock geology – for example, the soft and less consolidated sand and gravel sediments underlying much of the lower Hutt Valley will behave differently to the hard greywacke hills surrounding Wellington City.
  • the type of building you are in. Fortunately, New Zealand has a state-of-the-practice and strongly enforced building code that makes our structures well designed to cope with earthquake shaking.

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 a ‘seiche’, as was observed in the harbour after the 1855 Wairarapa earthquake. A seiche occurs when seismic waves passing through the water body 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, all of 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, so that 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 very good idea to have an emergency action plan.