On 14 November 2016, six years after the start of the earthquake sequence that devastated the city of Christchurch, New Zealand was again hit by a severe quake with a moment magnitude of 7.8. It was the country’s strongest tremor since the Wairarapa earthquake back in 1855 (magnitude 8.2–8.3). The area hardest hit was between Christchurch and Wellington, in the northeastern part of New Zealand’s South Island. In spite of the magnitude of the quake, just two lives were lost. The low population density in the epicentral region provides one explanation for this. Furthermore, New Zealand is known for its experience with earthquake-resistant structures, and has a modern building code that has been in force for many years.
According to the New Zealand Institute of Geological and Nuclear Sciences (GNS Science), the quake did not take place along a single fault. Instead, at least nine separate faults ruptured, some of which, like the Waipapa Bay fault, were previously unknown. The greatest displacement occurred along the Kekerengu fault, which was already known to geologists. The picture of one house that sat right on the fault (page 48, top) shows just how powerful the earth movement was, and how much it altered the landscape. The fault line now visible on the surface (page 48, bottom) is reminiscent of Hadrian’s Wall, close to the border between Scotland and England, which once marked the northern limit of the Roman Empire.
The fault ruptured north of the epicentre, generating the strongest ground shaking in the village of Ward, with peak ground acceleration (PGA) of 1.3 times the acceleration due to Earth’s gravity, “g”. PGAs exceeded 0.2 g in parts of the capital, Wellington, and in Lower Hutt, while values were lower in Christchurch. In Wellington, the peak acceleration was similar to that generated by the Seddon earthquake on 21 July 2013 (magnitude 6.5), although this time an unusual damage pattern was observed: whereas low-rise buildings sustained only slight damage, mid-rise buildings suffered severe damage.
The main reason for this phenomenon is related to the long-period ground motion components. On this occasion, they were significantly higher than would normally be expected for such a PGA value. This is understandable to a degree if one considers the magnitude of the quake and the northerly directivity of the radiation of energy. Nevertheless, the key question is why buildings of a particular size (ten to fifteen floors) proved so vulnerable to the tremors.
One positive aspect is that unreinforced masonry buildings that had been retrofitted for seismic events performed well in the quakes. Given the high insurance density and the proximity of towns and cities such as Blenheim, Wellington and Christchurch, all of which are found in potential damage zones, the insured losses – while significant – are much lower than those reported during the 2010–2011 Christchurch earthquake sequence.
Based on the present level of knowledge, it is impossible to say whether the likelihood of another large earthquake in the Wellington area has risen or fallen. It is perhaps worth noting that several major earthquakes have struck New Zealand since the 2009 Dusky Sound earthquake (magnitude 7.8) in Fiordland. This was also the case in the period between 1848 and 1942, when the country was hit by a number of earthquakes with magnitudes greater than 7.0. Despite this, it is impossible to forecast whether the next few decades will see greater seismic activity than in the period mentioned above. However, experience in New Zealand demonstrates that awareness of seismic risk, combined with the right provisions, is the only way to more effectively manage the risks posed by earthquakes.