Shaky Wellington: A City Built on five Faultlines
The 1855 earthquake was the most dramatic tremor of the earth ever recorded in New Zealand but not the first and not the last one, especially not in Wellington. The magnitude was 8.2 on the Richter scale, and it rocked the southern part of the North Island. It was caused by a movement of a fault in Palliser Bay, east of the capital and the North Island’s southernmost point.
The city is prone to earthquakes because it rests on the point where two tectonic plates meet. Several kilometres beneath Wellington the light and thick Australian plate rides over the heavier, but thinner Pacific plate. These plate movements have resulted in five major fault-lines running either through or very close to Wellington City. When one of these faults shifts suddenly an earthquake occurs.
Thanks to the frequent earthquakes Wellington has become one of the world's leading centres for the study and research of earthquake activity and for the development of seismic strengthening techniques in buildings. An impressive example of the latter – the shock absorbers – can be seen in the Quake Brake exhibition at the entrance of Te Papa Tongarewa, the National Museum.
The European settlers experienced their first major earthquake in Wellington in 1848 when a 7.1 magnitude shock claimed three lives. In 1855 – when 80 per cent of all chimneys collapsed - only one person died in Wellington when a two-storey hotel collapsed, and three people in the Wairarapa. The coastline was raised by up to 1.5 metres. Part of this land – which should have become a shipping basin – is now home of a famous cricket ground, named the Basin Reserve. (There, BTW, is NZ’s only cricket museum.)
One of the early safety measures was to replace brick buildings by timber constructions. However, whereas most residential homes were built of timber most commercial buildings were reconstructed in brick because of the fire risk of a wooden construction.
On the website of the Greater Wellington Regional Council I read that today a large, shallow earthquake magnitude about 7.4 along the Wellington fault during the day could cause 500 deaths and 4000 injuries. 100,000 buildings would be damaged in some way.
In the whole of NZ we have about 150 significant earthquakes every year, and 10,000 to 15,000 in total, many of them are not felt and are only measured by seismographs. Records suggest that NZ has to expect several magnitude 6 earthquakes every year, one magnitude 7 every ten years, and one magnitude 8 every century. But remember: Earthquakes are not evenly spaced!
In 1855 the Wairarapa Fault ruptured; it has a recurrence interval of 1150 to 1200 years. The Ohariu Fault ruptured about 1100–1200 years ago, and has a recurrence interval of 1500 to 5000 years. The Wairau Fault last ruptured more than 800 years ago and has a recurrence interval of 1000 to 2300 years. Shepherds Gully Fault last ruptured about 1200 years ago and has a recurrence interval of 2500 to 5000 years. The Wellington Fault last ruptured between 300 and 500 years ago producing an earthquake of about magnitude 7.6. This fault produces a large earthquake about every 500 to 700 years. So this fault has the highest probability of rupturing next in the Wellington region. And with five faultlines Wellington’s destiny is to shake more or less lightly every some days.
More information on the following websites: