By Sam Bain
New Zealand has a lot of earthquakes. Many of you will have heard of the recent deadly Christchurch earthquake sequence. Some of you may know about the relatively large earthquake sequence that occurred near Seddon, a small town to the south of Wellington, New Zealand’s capital, in 2013. Perhaps those strange types out there with a passion for seismology might have noticed the relatively common occurrence of medium to large earthquakes near New Zealand on the USGS earthquake map. They might not realise that in a typical month New Zealand experiences approx. 3000 earthquakes (I am cheating a bit here as around one third of these are too small to be felt). On average, New Zealanders felt 414 earthquakes larger than magnitude 4 each year from 1960 to 2011.
If we look through the relatively short written history of New Zealand there have been many large and often deadly earthquakes. A couple of good lists of these earthquakes can be found on Wikipedia New Zealand earthquakes and List of earthquakes in New Zealand. It is worth looking at some of these to get an idea of their effects and the locations relative to population centers. Europeans arrived in New Zealand around 1800, whereas the Maori settlement began circa 1250AD. We know that there have been significant earthquakes during this period of settlement, however there is less information on these events as the Maori people did not have a written language. Some stories of devastating loss of entire communities through earthquake subsidence, landslide, and tsunami have handed down through the generations (see historic earthquakes). It is interesting to note that early Maori settlements were mostly low-lying coastal sites but in the 15th century they moved to hilltops and inland, coinciding with evidence for widespread tsunami inundation of coastal settlements.
One of the first significant earthquakes that occurred during European settlement was the 1848 Marlborough earthquake. This M7.2 rattled most of central New Zealand and damaged almost all brick and stone buildings in the region. This was closely followed by the 1855 Wairarapa earthquake. This had a magnitude of 8.2 and occurred below the early settlements near what would become Wellington. Fatalities were low as there were few people about. Few buildings collapsed as most had been rebuilt of wood following the earlier Marlborough earthquake. However, the earthquake raised much of the coast along Wellington by several meters and generated a localised 10m high tsunami. The newly exposed land was rapidly built on and now constitutes most of the Wellington CBD and the main road and rail route north from the city!
Three relatively large events – the 1869 Christchurch earthquake (M5.7), the 1888 North Canterbury earthquake (M7), and the 1901 Cheviot earthquake (M6.8) – repeatedly damaged brick and masonry buildings in the Christchurch area and caused liquefaction. Unfortunately, these results have been repeated on a much larger scale during the recent Canterbury earthquakes. Three large earthquakes (M7.1, M6.3, M6.0) occurred along with thousands of aftershocks from late 2010 through to 2012. Most of these events occurred directly beneath the city of Christchurch, and at relatively shallow depths (<10km). The M6.3 on the 22 of February 2011 resulted in widespread damage to buildings, liquefaction, and loss of utilities. One hundred and eighty five people died during the earthquake, with many dying in the collapse of two large buildings. The aftershock sequence has died away now, though we are still occasionally rattled by shallow M4 events.
The 1931 Hawkes Bay or Napier earthquake (7.8M) is the most devastating in terms of lives lost, with 256 killed. This earthquake was generated by movement on a thrust fault in an accretionary wedge to the east of Napier. In layman’s terms, the Pacific Plate is sliding under the North Island of NZ and in the process crinkling the land above into a series of stacked folds. The earthquake caused the coast to rise by up to 2 metres, destroyed most buildings in Napier, and started several large fires. The widespread collapse of buildings during this earthquake led to the development of stronger building codes in New Zealand that required new buildings to be heavily reinforced. The New Zealand building code has developed from this point with a strong emphasis towards buildings that should survive large ground movements. Innovative solutions to these problems have been developed, including the use ground isolating shock absorbers under large buildings such as Te Papa, the national museum.
Why does New Zealand get so many earthquakes? Well, I touched on this in my last blog. New Zealand sits on top of a plate boundary. Here the Pacific and Indo-Australian plates collide. To the north the Pacific Plate is forced under the Indo-Australian plate, whereas in the south the situation is reversed with the Indo-Australian plate being driven under the Pacific Plate. In these two areas there are low angle (shallowly dipping) faults from the thrusting of one plate under the other. These can rupture and cause the seabed to spring upwards, sometimes generating tsunamis. There are also earthquakes at depth within the subducting slab (caused by stretching and metamorphism) which can be quite powerful. The transition between the two subduction zones, which happens to be directly below Wellington and to the north and west of Christchurch, is an area of complex tectonics. This area is dominated by faults where the two plates mostly slide past each other; however there is still some compression and subduction. Earthquakes here can occur as strike-slip on near vertical faults (such as the Alpine Fault), on thrust faults, and sometimes even on normal faults in pull-apart basins along major faults.
All these faults in New Zealand definitely have a plus side – see above. We would not have our spectacular mountains without uplift along faults driven by the ongoing continental collision. Unfortunately, with this scenery comes ever-present risk, but with increasing awareness of the hazard we can at least try minimise the danger through preparedness and precautions (such as knowing the best actions to take in an earthquake and taking care in how we make our buildings). I will finish up with an interesting earthquake image from our very own Leapfrog Geo. The rendered image below shows the earthquake sequence below Christchurch from the first earthquake in September 2010. There have been a few shakes!