It is a wonderful, and funny place. New Zealand has more than its fair share of the world’s beauty. You want volcanoes, rivers, beaches, forests, mountains, liveable cities, it has it all. Its history is out of this world. Apart from a few bats and some dolphins, the first mammals to arrive were human. In the absence of mammalian competition, the land was ruled by birds, many of which gave up on flying. The mammals found a walking buffet. There is now a mammal eradication program; luckily humans are excluded from this (though possibly not the Australian rugby team). New Zealand also has a sense of humour: its national emblem is an overgrown, nocturnal mouse-like bird that can’t fly. This from the country that had the Moa, a 3.5 meter, 200-kg (luckily flightless) herbivorian bird, as well as the Haast eagle, the world’s largest eagle which made a living hunting these Moas. Says it all, really. Another example of New Zealand’s self-deprecation: foregoing the world’s largest eagle, the RNZAF is now the only air force in the world that has a flightless bird on its logo! But New Zealand is not perfect. It has its faults.

Geographistry

New Zealand consists of two main islands, imaginatively called the North Island and (hold it) the South Island, separated by the Cook Strait, narrow enough that you can see across from one to the other. As befitting its antipodal nature, the North Island looks like an upside-down version of the boot of southern Italy. And very much like southern Italy, it is a major volcanic risk area. Especially the Taupo Volcanic Zone around Lake Taupo is known for its many huge eruptions, an inverted -and enlarged- version of the Bay of Naples. Currently it is utterly quiescent, with no sign of inflation anywhere on land, but there is an inflating area off the coast which may be next in line, although not in the near future. About three quarters of the population of New Zealand live on the North Island, half of whom ended up in Auckland. Auckland is, in a typical oblivious New Zealand fashion, build inside an active monogenetic volcanic field. There will certainly be another eruption within the city limit, but the location can’t be predicted except that it won’t be one of the existing cones. The last one erupted inside the harbour. The Auckland eruptions are minor, though.

The South Island is larger and has the best mountains. A mountain range runs along the west coast: steep, high, and snowy, and called the Southern Alps. Here is the country’s highest mountain. Aoraki / Mt Cook (a certain uniformity in english naming is evident in New Zealand). The first part is the Maori name) at 3724 meter. The southern end has an unbelievable fiord system. The east central side of the South Island is a large plain where the main city of the south, Christchurch, is located.

Earthquakes

New Zealand suffers 15,000 detectable earthquakes per year. An earthquake strong enough to be felt happens about every other day; a magnitude-7 earthquake occurs on average once per decade. It is most definitely a shaky paradise. The largest earthquake on historical record (i.e. since 1840) was on 23 January 1855, the M8.2 Wairarapa earthquake. It hit the southern end of the North Island, affecting all areas around the Cook strait. The quake caused 15.5 meter of horizontal displacement – a world record, and a vertical slip of 2.5 meter. East of Wellington, 6 meter of uplift was reported. The uplift is still visible in the shoreline. A series of old shorelines suggests that at least three similar events have occurred before, around once every 2200 year. In 1855, a 145-km fault segment ruptured. The earthquake magnitude, and amount of slip, seem very large compared to the length of the fault. It has been suggested that the fault extended a long way down, perhaps linking up to the the subduction zone underneath, but there are other models. There are conflicting reports on the size of tsunami that followed the earthquake. Asymmetrical uplift in the Wellington Harbour caused local tsunamis there – and a 10m tsunami was reported out in the Cook Straight; likely the result of uplift on a submerged segment of the fault system. The number of casualties was small, aided by the fact that many of the houses in Wellington were made of wood, following a damaging earthquake a decade earlier.

The location of major earthquakes in New Zealand shows which regions are prone and which ones are avoided. The Cook Strait has seen quite a few. The southeast coastal region of the North Island is risky, as is the northern half of the South Island. Further south, things are quieter until you reach the southernmost tip. There is a significant gap in the earthquake distribution, and in fact this gap is one of the main areas at risk.

Faults

Fault-wise, New Zealand is playing a hard-fought game of rugby with its neighbours. The neighbours are, on one side, the Pacific plate, and on the opposing side, the Australian plate. New Zealand is the battle zone between them, and it is feeling the pressure. Nothing here is simple. In the north, a quick attack has given Australia the upper hand, rolling over the Pacific plate which has decided to roll over and subduct. In the south, the Pacific defence has stood up to the onslaught and obtained the upper hand. Pacifica has gone on the attack and here the Australian plate is subducting. So there are two opposite subduction zones, in the northern half and the southernmost tip. In addition to this complexity, the Australian plate moves north at a high speed, near 10 cm per year (which for a continent is Bolt-speed). So you have both the plates moving towards each other, with conflicting subduction direction, and high sideways slip motion of around 3 cm per year. Not all of the large earthquakes above are close to areas of subduction.

The making of New Zealand

In a subduction zone, the subducting plate forms a deep trench, and the overriding plate is pushed up. The island thus forms on the upper plate. As such, New Zealand is in two minds. The conflicting subductions mean that the North Island is on the Australian plate, but most of the South Island is on the Pacific plate. The dividing line between two plates is a sharp fault line, following the west side of the South Island, but than crossing the island, and lying off-shore to the east of the North Island.

The Alpine Fault

On the South Island, the plate separation is called the Alpine Fault, and until recently this was considered the most dangerous fault on New Zealand. The Alpine Fault has formed the Southern Alps, New Zealand’s highest mountain range. The range has a sharp edge on its western front, as is clearly seen in satellite images. This edge is the fault. Compression is uplifting the mountains at a rate of 2 mm per year, and this has been going on for the past 10 million year. A quick calculation shows that this makes the current height 20 km. In reality, of course, even New Zealand has to obey the laws of physics, and mountains this high aren’t stable. In fact the highest mountain is not even 4 km. What happened? Well, New Zealand suffers from bad erosion. The Alps attract rain – lots of it, and as a result much of the uplifted mountains has found its way back to the ocean, courtesy of the rain-filled rivers. It would drive any self-respecting plate to despair.

The slip motion along the Alpine fault is stop-start: it occurs in shocks. Because of the length of the fault, these shocks can be huge. The fault is is capable of M8 earthquakes, on average once every 300 year. It is currently locked, and there hasn’t been a large event here since about 1717. And its 300 year anniversary will be in 2017.. Earthquake do not occur to a strict schedule and this does not mean one will happen shortly. But the risks are increasing and a major earthquake on the New Zealand west coast is more likely than not within the next 50 year.

The broken country

In the region where the fault moves from the west coast to the east, the alpine fault breaks up in a series of many faults, some parallel, some not, continuing into North Island. The North Island has the North Island Fault System (NIFS for short), with many parallel faults, closely spaced. Together they have pushed up a series of narrow mountain ranges. Each one of these faults can rupture, and each rupture transfers stress to the next fault. The strength may vary from fault to fault. The Ruahine fault, for instance, appears to rupture every 500 year with slips of several meter. The Wellington fault in particular deserves monitoring. This fault begins in the sea 20 km south of Wellington and extends northward through Wellington itself and up along the island parallel to the coast. Movement is mainly along the fault: there is also some vertical slip but it is 10 times less than the horizontal motion. No historical earthquake is known. The northern end, crossing Wellington, shows evidence for prehistoric events with 4 meter shifts (M7), most recently 300-450 year ago. Further north, events with similar slip have occured every 500-1000 year. The Wellington fault is one of the more dangerous fault lines, because of the location which crosses towns and cities, and because it is someway into its recurrence time. But it is only one of the faults along this mountain range. The series of parallel faults has been compared to an onion skin, and each fault can fail unexpectedly. New Zealand is geologically speaking an eye-watering place.

The South Island has a similar series of parallel faults stretching from near the end of the Southern Alps across the island, and along the coast towards the Cook Strait. If anything, this system is even more fragmented and complex than the northern counterpart. It is called the Marlborough Fault System; it has been the location of many historical and very recent earthquakes.

Current earthquake series

On 4 Sept 2010, a major earthquake occured west of Christchurch. It had a magnitude of 7.1. Called the Darfield (or Canterbury) earthquake, it involved the failure of segments of as many as 7 separate faults. There was considerable damage, as might be expected of a quake this size. But the worst effect came from an aftershock. On 22 Feb 2011, a nearby fault, stressed by the Darfield quake, failed in an M6.3 earthquake. It was shallow, only 5km, and located very close to Christchurch. This became among the most damaging earthquakes to hit a major city; Christchurch still hasn’t fully recovered. The ground acceleration was far higher than had been expected and exceeded the maximum loads assuming in the local building codes. We now know that the recurrence rate of moderate (damaging) earthquakes at Christchurch is about twice per century. One wonders whether the city is build in the right place.

On 21 July and 16 August 2013, a double quake took place at the northeastern tip of South Island, of M6.5 and M6.6, on adjacent faults. There was some damage, but overall New Zealand coped well with this event. And on 14 November 2016, a M7.8 earthquake occurred midway between the earlier events. This Kaikoura earthquake was a complex event, rupturing 6 separate faults. Whether it was related to the earlier earthquakes is hard to tell. The location is suggestive, but this is an area with many earthquakes and it may have been a coincidence. Kaikoura affected a 200-km length of faults, traveling northward at 2 km/sec. The largest slip may have been 10 meter, although mostly rather less, and uplift reached 2 meter in one place. We now know that large earthquakes in the New Zealand fault systems can rupture multiple faults. One of the ruptured faults was not known prior to the earthquake, but given the complexity of the area this is not a surprise.

In addition to the expected aftershock activity taking place as we write this, up and down the fault systems, a large ‘slow-slip’ event (where the plates slide past each other continuously without producing strong earthquakes for a period of weeks) has apparently been triggered along the subduction zone much further North – off the East coast of the central North Island. An M5.2 on 22 Nov was part of this.

The subduction zone under the southern third of the North Island – stretching from Cape Turnagain down under Wellington and into the Cook Strait – is currently ‘locked’ and has been for some considerable time. GPS measurements carried out since the 1990’s show that this section of the New Zealand crust is being dragged to the west, carried by friction with the subducting Pacific plate below. The largest westward movement is near Wellington, where the subduction zone is about 20 km deep. When it eventually gives way it will give rise to possibly the most dangerous of all the quakes New Zealand faces; a subduction zone quake with a magnitude estimated by GNS at or in excess of 8.5, greater than the Alpine Fault and with the large tsunami frequently associated with large subduction/megathrust quakes. It could indeed be New Zealand’s version of the Tohoku earthquake of 2011. Whether the current activity has brought that quake closer is something that remains to be seen.

Risks

New Zealand is not at risk of earthquakes : it is at certainty. The two conflicting subduction zones and plethora of connecting faults provide many points of predictable failure; predictable in the sense of knowing it will occur while being somewhat vague on the date. The largest risks are in areas with few historical quakes but evidence for frequent and large past events predating the historical era. A 2013 study (prior to the 2013 double quakes) listed the main population centres at risk, in order, as Wellington, Christchurch, Dunedin, and Auckland. The northern subduction zone has not had large earthquakes and the chance of major quakes from this zone is unknown. The risk for Wellington is dominated by the Wellington fault but other faults contribute. In other places the risk comes from distributed sources, rather than one main nearby fault. The biggest risk for damaging earthquakes is along the Southern Alps, including the fault belt crossing South Island north of Christchurch. But it is hard to find any area of New Zealand which is safe. It is a wonderful, and funny, but scary country.

This post drew heavily on material published GNS, and I can strongly recommend their website. Start at
www.gns.cri.nz/gns/Home/Learning/Science-Topics/Earthquakes/New-Zealand-Earthquakes . Mike Ross wrote part of the text and provided valuable comments on the rest.