When is the Wellington Earthquake going to hit?

It is inevitable and now merely a waiting game.

Christchurch is a pre-warning to the wider Wellington region that something is stirring, just waiting, tick tock tick....and although most people like to think optimistically (and that is a good thing) sometimes we have to sit down and look at the raw facts. This information is straight from a local website. It has more detailed information if you wish to access this at the link below

Wellington's Shaky Foundations

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!

How often do earthquakes occur?

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.

How much do our fault lines move?

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.

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. We have also invented effective technologies for increasing building safety.

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.

A deeper understanding of plate tectonics

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.

Here is some more information from a recent news article:

Yesterday's deadly Christchurch earthquake could be a sign of further devastation to come, with one seismologist warning that it had increased the likelihood of a large earthquake hitting Wellington.

Seismologists expected an aftershock as big as yesterday's 6.3 magnitude quake would follow the 7.1 magnitude quake that struck Canterbury on September 4, but thought it would strike much sooner.

Seismologist Kevin McCue, director of the private Australian Seismological Centre, said geological activity was impossible to predict, "but it tends to occur in clusters". "If you have one (quake) it ups the hazard."

The Canberra-based seismologist, also an adjunct professor at Central Queensland University, said the quake would have had some impact on the boundary between the Australian and Pacific tectonic plates.

The fault runs up the South Island's West Coast, across the Cook Strait, under Wellington and up the east coast of the North Island.

"This quake has the potential to load up the plate boundary, increasing the likelihood of a quake at Wellington," he said. "Wellington has always been considered much more at risk (than Christchurch) because it straddles the plate boundary.

"New Zealand has been relatively quiet since the 1930s - maybe we are about to catch up."

So what are you waiting for? Print your survival check list and go through the steps. Each week start buying extra tinned food, non-perishable food etc, water and all the necessities. Store these items in an easily accessible secured storage box. You can buy those clear plastic boxes from the warehouse. Whats your plan of survival? Where will your family be? How will you get to them etc etc. Nows the time to think 'better safe than sorry'