Faults of New Zealand

kiwi

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-Satellite-Map

New Zealand geographical features – http://www.freeworldmaps.net/oceania/new-zealand/map.html

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

The raised platforms at Turakirae head mark old shorelines (visible as pale bands). The first shoreline inland from the present day coast is where the shoreline was in 1855, before the area was uplifted by the Wairarapa earthquake. Source: GNS science; geonet.

The raised platforms at Turakirae head mark old shorelines (visible as pale bands). The first shoreline inland from the present day coast is where the shoreline was in 1855, before the area was uplifted by the Wairarapa earthquake. Source: GNS science; geonet.

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.

Large New Zealand earthquakes. Mind the gaps.

Large New Zealand earthquakes. Mind the gaps.

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

New_Zealand_plates

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

The Alpine Fault in its full glory.

The Alpine Fault in its full glory.

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

The Wellington fault in Upper Hutt. Source: Te Ara Encyclopedia of New Zealand

The Wellington fault in Upper Hutt. Source: Te Ara Encyclopedia of New Zealand

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.

North Island Fault System, and a stacked fault model

North Island Fault System, and a stacked fault model

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.

The failed faults of the November 2016 Kaikoura earthquake

The failed faults of the November 2016 Kaikoura earthquake

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 nerby 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.

The real source of many NZ earthquakes: the open-air fracking called rugby

The real source of many NZ earthquakes: the open-air fracking called rugby

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.

46 thoughts on “Faults of New Zealand

  1. Wonderful read Albert.

    I hadn’t even realized that the southern island had an opposing subduction zone. I would assume that the angle of subduction there is not conducive for volcanism?

    • There isn’t much subduction under the South Island. The northern subduction zone peters out, and the southern one only develops around the Fiord land. I think there are some (extinct) volcanoes just off the south coast. Of course Taupo can’t just be due to subduction. It is too overactive.

      • Yes, Taupo is as much a product of extension as it is subduction. Although, that extension is likely related to the subduction.

        • For them that don’t know… that extention is called a back arc basin. It happens a lot as the volcanism from the subduction zone initiates rifting along the volcanic arc.

    • And thank you for having a look at any faults that slipped through in the article!

      • It is the faults that didn’t slip that we are worried about! There is always another fault in New Zealand.

  2. Really interesting article that has popped up a few places recently – sharing it on here.

    http://www.nature.com/articles/ncomms13585

    Summarizing this in basic form, Cordon-Caulle had a shallow VEI-5 sized crust intrusion that occurred within a 1-month time span, which occurred immediately after the end of the eruption. That’s pretty incredible.

    Interestingly, what likely happened is that the upper conduit of the volcano became so blocked up during the eruption, that it could no longer emit eruptive products via the vent, and thus the eruption continued as a sub-crustal intrusion. That’s some extremely viscous magma to say the least. It will be interesting to see if further deep injections could trigger a future eruption of this subcrustal magma.

    In laymans terms, as the eruption progressed, the pressure pushing the magma from below gradually decreased, and fell below the pressure of friction and crust-strength pushing back on it. This caused the magma to form a pocket below the crust.

  3. Kiwis are the least of it BTW… enjoy the late great Douglas Adams giving his incomparable description of the mating behaviour of the kakapo: the world’s largest (and only flightless) parrot…

  4. I loved this article, maybe because I learned a lot, thanks to all that share their knowledge and passion.

  5. Thanks Albert and Mike for this great article. I was in New Zealand for 3 weeks in Dec of 2014. Absolutely fell in love with the country and swore one day I will have some form of living arrangement there possibly near Nelson. After reading this article I will make sure to be at least 30M above sea level just in case. We tried to understand the geology and plate intricacies while we were there and got about 25% actually right now that I read this article. This one is an absolute keeper for me thanks again

    • Well, that would get him above the Junge layer for sure. Great view with no aerosols to speak of… though it might be hard to breath. The Cell coverage would be phenomenal.

  6. Great post Albert. Always loved studying New Zealand’s fascinating (if rather scary) geological history.

    Held up by aksimet, our non-intelligent filter

    • Scary? You haven’t heard my mustard packet idea.

      Suppose that you are walking out of a fast food burger joint. You accidentally step on a mustard packet. As your weight gradually increases on the pack, the mustard inside becomes more and more pressurized… eventually, above the failure strength of the plastic packet. As it fails, the mustard sprays violently across and splatters whatever is in it’s way.

      I have a thought that New Zealand may experience similar mechanics with it’s subducting plates. Pre-existing melt becaomes hyper pressurized and bores up to the surface in a highly energetic dike, giving interesting eruptions like the 1886 Mount Tarawera event. There are also what appear to be voluminous events like large pāhoehoe floes from some of the mountains. I don’t think that you get flows that large unless it is very persistent, or quite fast. If it’s just long lived extrusion, the lava would turn into blocky ʻaʻā flows. Yet many of these flows appear to be pāhoehoe all the way to the tip.

      • Note: I have nothing to back this up. It’s just my personal “moon-bat” idea.

  7. Because of the recent activity in the earthquakes (although small in magnitude) near Teide Tenerife and in the ocean near Volcano Enmeido just wonder if these could be connected in anway and does it have any meaning .

    ww.ign.es/ign/resources/volcanologia/tproximos/canarias.html

  8. Slightly OT but there’s definite incandescence at Aso tonight. I don’t recall seeing that before:

    Oh and I’d love to know what they’re using for a webcam there; it’s stonking good! The night images are almost as clear and colorful as the day ones!

    • Amazing. Hats off to JMA for not only this webcam, but it looks like they’ve installed equal quality cameras at most of their sites. And with the slide bar at the bottom you can do a time lapse. And they put a small map next to each display for all the cameras.

    • Indeed.

      Thursday
      24.11.2016 20:12:22 64.000 -19.701 7.0 km 0.6 99.0 1.9 km WNW of Hekla

      These small quakes have become almost a common occurrence now. Obviously there’s growing unrest in the area but, as always, Lady Hekla loves to keep us on our toes. 😛

    • My wife flew to and from Christchurch the other day – turboprop; low-level. She said the area in and around Kaikoura had got most of the media attention – but the countryside inland away from the sea was even worse; massive landslides everywhere. ‘Completely munted’ was her very kiwi description of it!

  9. The South African site for a new nucelar power station is deemed at risk of tsunamis (pretty rare though in this part of the world). Fukushima comes to mind: designed to survive the biggest tsunami they expected. Just not the one they got. Of course you do want water coolant near your nuclear power. It is trade-off: close enough to water to benefit, far enough not to get wet.

    http://www.voanews.com/a/south-afdrica-s-proposed-nuclear-power-plant-unsafe-report-says/3611738.html

    • I was totally astounded when you dropped that tidbit of knowledge about what they had based the seawall on. I had assumed (my bad) that they had done a statistical (mis)analysis of the tsunami threat and had only designed it for 85% of the actual threat. My assumption was based on the fact that Dr Demming had brought the idea to their auto manufacturing sector and that it had been used in other areas. That it was instead based on the largest one seen in the pacific basin and designed to handle that, just blew me away.

      But, in keeping with Taleb’s Black Swan definition, analyzing it out of the way after the fact is one of the criteria. (“If only we had known about ‘yada yada yada’…”

      Note:
      1) Poorly underestimated the likelihood of it’s occurrence to the point that it is assumed to be impossible
      2) Profound in it’s impact.
      3) Analyzed away after the fact.

      And yes, I am aware of the IYI article by Taleb. I don’t claim to know the subject well. I’m just some dude siting in Florida pondering a glass of iced tea.

      • Side Note: “Mathematically, the Aristotelian focuses on a variable, say X ; the
        practitioner on a function of that variable F(X). The dynamics of F(X) can be markedly
        different from those of X, particularly when F(X) is nonlinear. We may never understand X,
        or be marred with perceptual errors, but we can control F(X). The fool thinks the Black
        Swan problem resides in better prediction of X, rather than mitigation by controlling F(X).”

        Anti-fragile Glossary. (Taleb)

        How it applies here. Sure predicting the next eruption is what a lot of people focus on (X), but the most survivable solution {mitigation}, is to not be there when it happens (F(x)).

        BTW, I’m currently reading “Antifragile: Things That Gain From Disorder” in bits and spurts. It seems to be an excellent follow-up to his “Black Swan” book.

        A nice addition to this batch, is “Outliers” by Malcom Gladwell. In it, Gladwell describes the 10,000 hour rule. (That mastery of a skill takes about ten thousand hours of correct practice). An interesting observation, is that many insurance agencies show a premium drop for drivers at the age of 25. (the price goes down). What else happens at 25? Well, that’s about how long a newly licensed teen driver needs before he accumulates ten thousand hours of experience behind the wheel. I think it supports his idea quite well.

      • My my estimates… the nearest volcanic feature to me is a Jurassic era volcanic field buried under 14 km of sediment. By the time it will reactivate during the next part of the Wilson Cycle, I will have become part of the sediment of the Mississippi Embayment and will not care.

  10. And Kilauea is rumbling. Four earthquakes around the caldera of M2.5-M2.9 in the past two hours. Not too shallow though.

    • Best to put the image here for people to see actually. So that slow-slip event is now getting a lot more extensive; slow-slip has been seen in all these places before – but never anything like this simultaneously. See the big black area where the greatest slip has occurred? Note it has a pretty sharp southern boundary below which nothing has moved at all. And *that* is the big problem; that southern area is unlikely to slow-slip; if it does move it will probably all move at once – and in a big way 🙁

  11. BTW just watched :” The Norse: an Arctic Mystery.” on Amazon prime
    has a goodly bit on the Isle of Lewis and the Scot/Viking/Native American
    Trading connection… CBC production..
    Enjoyed the field archeologists in this story-done a bit of that…
    The Vikings got around BTW Dr. Sutherland, the Archeologist in charge of this
    project.. Got screwed over…
    https://en.wikipedia.org/wiki/Patricia_Sutherland

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