Following the devastating magnitude 6.3 earthquake in Christchurch, many will be assessing the risk of where they live. Over a long enough time frame pretty much anywhere in New Zealand, apart from Northland, is vulnerable to a large earthquake.
On the map above it is evident that Christchurch is among the lower ground-shaking hazard areas. These predictions were based on the distribution of active faults, how frequently faults have moved in the past, and the location of historic earthquakes. This illustrates that even lower-hazard areas are vulnerable – as previous ruptures on the Greendale Fault (the location of the 4 September 2010 magnitude 7.1 Darfield earthquake) do not seem to have occurred in the last 2,000 years.
Yet, hazard is not the same as risk. The highest-hazard area on this map is in the Southern Alps, where few people live. The highest-risk areas are where hazard intersects with population – our second and third largest cities: Christchurch and Wellington
There are three major known earthquake threat sources to Christchurch - and although all differ in their magnitude and distance from the city, all are expected to result in around the same amount of ground shaking. The rarest is from a major rupture of the Alpine Fault - around a magnitude 8 event. The return period is 1 in 100-300 years - the last was in 1717. Even though the fault is well over 100 kilometres away, the ground shaking from such a massive release of energy would still be considerable in Christchurch.
More frequent moderate-to-large earthquakes (6–7.5 magnitude) were expected from faults in the foothills of the mountains next to the Canterbury Plains, and in North Canterbury. Less frequent moderate earthquakes (5–6.5 magnitude) were expected from faults under the Canterbury Plains and Christchurch itself. The 4 September 2010 magnitude 7.1 Darfield earthquake and the 22 February 2011 magnitude 6.3 Lyttelton earthquake (thought by scientists to be an aftershock of the earlier earthquake) were in this later category. The strength and location of the Darfield earthquake surprised scientists and illustrates that our knowledge of the hazards we face is evolving.
One way that scientists use to estimate a locality’s future earthquake pattern is to look at its past. Since 1869, 11 strong earthquakes felt in Christchurch have resulted in Modified Mercalli scale ground shaking of 5 or more in the city:
• 1869, Christchurch, magnitude 5, ground shaking intensity 7–8
• 1870, Lake Ellesmere, magnitude 5.5, ground shaking intensity 6–7
• 1881, Castle Hill, magnitude 6.0, ground shaking intensity 5–6
• 1901, Cheviot, magnitude 6.9, ground shaking intensity 6
• 1922, Motunau, North Canterbury, magnitude 6.4, ground shaking intensity 6–7
• 1929, Buller, magnitude 7.8, ground shaking intensity 5–6
• 1994, Arthur’s Pass, magnitude 6.7, ground shaking intensity 3–6
• 2011, Lyttelton, magnitude 6.3, ground shaking intensity 8.
The 1869 and 1870 quakes were in the category of quakes beneath the Canterbury Plains and Christchurch itself. The largest of the above quakes in terms of ground shaking in the past century, prior to the 2011 Lyttelton quake, was the 1922 Motunau magnitude 6.4 earthquake in North Canterbury, which caused ground shaking of 6–7 in the city, as measured on the modified Mercalli scale. The 2010 Darfield magnitude 7.1 quake caused ground shaking of 5 in the city, while the magnitude 6.3 Lyttelton quake caused ground shaking of 8, because it was both closer to the city and shallower. Only the 1869 quake (believed to have been centred beneath New Brighton) caused ground shaking approaching this at 7–8.
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.