While Christchurch is considered to have a lower earthquake hazard than Wellington, the risk it faces from medium-sized quakes is similar – which, prior to the recent quakes, would have surprised many people. This is due in part to the underlying geology of unconsolidated silts and sands that Christchurch is built upon. While hazard maps of areas prone to liquefaction exist, they largely join the dots of information gained from drill cores, while underlying geology in these types of deposits can vary from metre to metre. The predicted areas of liquefaction in existing hazard maps proved to be poor predictors of liquefaction in the September 2010 earthquake. As the water table is quite near the surface, many of these sands are water-saturated and when shaken turn to a jelly-like consistency. Not only does this lead to water and silt bursting through the surface, the ground shaking intensity is much worse than for areas built on bedrock, and the ground can also subside unevenly – with subsequent building collapse. This underlying geology is what brings Christchurch’s earthquake risk close to that of Wellington’s – for medium-size ground shaking events that is. Wellington’s risk from a very large catastrophic earthquake is higher than Christchurch’s.
Wellington is built on a faultline. You only have to fly into the city to experience the city’s earthquake history, as the airport is built on land upraised in the Haowhenua earthquake, which probably occurred in the 15th century. New Zealand’s largest ever recorded earthquake (magnitude 8.2) occurred on the Wairarapa Fault in 1855. The main threats to Wellington are from earthquakes on the Wellington Fault or the Wairarapa Fault, along with faults under Cook Strait which would likely cause a tsunami. Reclaimed areas such as the CBD, Miramar and Petone are at high risk from liquefaction, higher ground shaking intensity and tsunami, while areas with steep slopes risk landslides.
A large, shallow daytime earthquake of around magnitude 7.4 along the Wellington fault would probably result in around 500 deaths, 4,000 injuries, and perhaps 1,800 people trapped and over 100,000 buildings damaged. The return period is 1 in 700 years, and for a larger event (magnitude 8.2) on the Wairarapa Fault, 1 in 1,000 years. Yet, due to the multitude of faults, the return period for a very strong quake causing extreme ground shaking in Wellington is just 150 years.
Our recent earthquake history has not been an accurate gauge for our risk and hazard, and in that respect we have been lucky. Up until 22 February 2011 there had not been a large on-land earthquake close enough to a major city to cause very strong ground shaking since Napier in 1931. It is unlikely that it will be another 80 years before another large quake occurs close to one of our cities or towns. So get ready.