Why does New Zealand have so many earthquakes and volcanoes?

A 6.3 earthquake has just struck the city of Christchurch, New Zealand, killing dozens and leaving dozens more buried in rubble with rescue workers trying to dig them out. On the TV this morning, the mayor of Christchurch told his story: Having just left a series of meetings, he was sitting on a balcony outside the city offices in a tall building with his executive assistant planning their next activities when the quake struck. They tried to re-enter the building but were repeatedly thrown back away from the entrance way. When the powerful earthquake stopped, he picked himself up off the floor of the balcony and gazed across his city to see absolute devastation including numerous collapsed buildings and destroyed infrastructure.

You hear things like this from New Zealand now and then, including just a short time ago when the same region was struck buy a somewhat larger quake, but one apparently located farther from the center of population. Also, you hear about a number of volcanoes in the southern Pacific island nation.

Here’s New Zealand:


The most recent earthquake there, as I write this, was actually tomorrow, because New Zealand is on the other side of the international dateline so it occurred officially at 1:21 AM on the 23rd local time, but that was the most recent of several aftershocks. According to the GeoNet archives New Zealand has experienced 296 earthquakes of 3.0 magnitude or greater over the last 30 days. During the same period, an equivalent size area that includes one of the more earthquake prone states in the US, California, had 22 magnitude 3.0 or higher events (Data from the USGS).

So, yes, it is true that New Zealand has a lot of earthquakes, and one could even say that tectonic activity in New Zealand is ten times more frequent than it is in California.

Earthquakes tend to be concentrated at the edges of tectonic plates, as these areas build up energy as plates move in relation to each other. Earthquakes represent the release of that energy. Both New Zealand and California sit on the edge of the Pacific Plate, but in very different places and on very different types of plate boundaries. California borders the northeastern edge where the Pacific plate, rotating counter-clockwise grinds against the plate that makes up that part of the continental US. This is pretty energetic and can create substantial movements of earth, very large and complex fault systems, and plenty of earthquakes. But that is nothing compared to what happens to New Zealand as well as Japan and several other places.

There are areas where one plate is moving towards another, or two plates move towards each other. When that occurs there are two major effects that may occur: One of the plates dives under the other (subduction) and the other buckles, forming mountains. Look at this blow-up of a map from Wikipedia showing the relative movements along boundaries of several tectonic plates:


I’ve circled three things on this map, in green. On the upper right is the California coast. Note that the relative movement of the plates, indicated by the arrow, is as described, with one plate grinding past the other. To the lower right I’ve circled the western coast of South America. Here the Nasca plate is pushing into South America. Thus, the Andes, a major mountain range, rises as the South American plate buckles. To the lower left, also circled in green, is New Zealand. Note here that the pacific plate is moving northwest, and the Australian plate is moving (actually, spreading) southeast, the two plates slamming into each other. Also note, if you look really close, that New Zealand is not restricted to either plate, but rather, straddles the two.

Australia and North America are continents because they are huge land masses sitting on top of plates. Hawaii is not on a continent. That island state consists of a series of volcanoes coming up from the middle of a plate, but not on a continental mass. The Hawaiian Islands are true oceanic islands. But what is New Zealand? Is it a bit of continent isolated from other larger masses, as is the case with Madagascar? Is it an oceanic island?

Well, both. New Zealand might be thought of as a very large oceanic island, but it really isn’t. It has a long (half a billion years) geological history consisting of volcanism, uplift, subduction (the opposite of uplift) followed by the formation of sediments, more uplift, and so on. But this complexity can be characterized in a rather simple way that probably describes the geology of New Zealand quite accurately: Modern New Zealand is the froth that forms on the surface of the earth over a subduction zone, where one continental plate is diving under another. New Zealand, Japan, and many other areas that form the so-called “Ring of Fire” making up the boundary of the Pacific plate consist of a combination of volcanic material and sediments related to the instability of subduction zones.

But it is a little more complicated than that. New Zeland started out as a fragment of the ancient southern continent, Gondwana, and made up part of the land mass that is now Australia. Much of the rock that makes up New Zealand is from the edge of that ancient continent, and uplift and subduction occurred several times because of the relative movement of the plates. After about 100 million years ago, New Zealand started to move increasingly far from major continents, with a full scale separation from Gonwanaland happening about 85 million years ago. During this time, what was to become New Zealand separated from what is now Australia. After about 50 million years of this, New Zealand sat low in the ocean and, mainly underwater, accumulated extensive marine sediments. Then, starting about 24 million years ago, the Australian plate (still spreading) and the Pacific plate started to interact more energetically, both moving towards each other and rinding past each other, causing the formation of the massive mountain ridges seen today in New Zealand, as well as a great deal of volcanic activity.

The following drawing represents a schematic east-west cross section through New Zealand:


The green curve on the right is the sea floor of the Pacific Plate diving beneath the part of the Australian plate that carries much of New Zealand. New Zealand consists in part of fragments of continent (in green) carried away from Australia millions of years ago. As subduction happens, interesting things happen that cause the formation of lava (in red) along the margins of the plates, which moves upwards pushing through and expanding the cracks that inevitably form when continents are mushing together, often forming pools of lava much nearer the surface as well. These pools … those at the subduction juncture itself and those formed secondarily higher up … are the source of lava for numerous volcanoes.

The plate on top of this unholy geological intercourse is forced upwards, forming mountains, and the whole process is highly energetic and causes frequent earthquakes, which can be quite severe.

New Zealand has so many earthquakes and volcanoes because it is in the wrong place (at the juncture of two tectonic plates) at the wrong time (while one plate is diving beneath the other).

For the most recent news on the earthquake in New Zealand, click here.

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30 thoughts on “Why does New Zealand have so many earthquakes and volcanoes?

  1. Don’t let that liberal reality fool you. I’m sure Pat Robertson will set us straight on why these earthquakes happened. It’s because of the gays, or something else that gawd is mad at the Kiwis for.


  2. The plates move up, the plates move down, you can’t explain that.

    Clearly, earthquakes require the existence of a higher being as explanation, so if New Zealand has a lot of them, they obviously pray to the wrong god. Let’s check…55.6 % Christian? Thus proven as a false belief. Q.E.D.

    See? Common sense works much better than all that fancy-pants factual stuff.

  3. Great explanation of the science behind why we here in New Zealand experience so many earthquakes. The facts and evidence from the natural enviroment on why the earth behaves as it does (eg earthquakes, volcanoes and other geological and meteorological activity) and the animals (of which we are selves are) and plants are the way they are, is far more interesting and wholely believable; than saying it is a punishment from some mythological being that does not exist. Though in saying that, and despite the fact I am an atheist myself, if believing in a mythical being or beings brings people comfort and hope in a time of disaster such as the aftermath of an earthquake, than they should be free to have the belief and no one should tell them it is wrong. Cayhina232000

  4. As explained on CNN International this morning, Christ Church is especially vulnerable to quake damage because it is built on top of mostly sand.

  5. Not to worry Phillip V – We’re 55.6% Christian only until the Census on Tuesday after next. It’s being predicted that the number ticking no religion is going to rise and the number ticking Christian is going to fall. New Zealand is a pretty secular country, so far no politician here has mentioned God or God’s will or any of that nonsense in their responses to the earthquake. It would be nice to that relected in the Census figures and the local skeptics have been pusing the ‘if you don’t beleive, don’t tick the box’ message.

  6. quibbilista: Well, that explains it, the flying spaghetti monster is upset that he is being spurned!

    Aside from shallowness of this quake -and unconsilidated sediments, are their any other explanations for the massive damage? Is it a thrust fault? I wooda thunk that New Zealand being a rich enlightened country, and being in such a geologically active area would have very good construction standards.

  7. Very interesting post, you just taught me quite a lot about a country I’ve lived in all my life! (Well, except those few days I spent in Australia, but since we used to be part of Australia that barely counts…).

    And yes, we are thankfully not a particularly religious country. I too am an atheist. As is my husband, and a large number of my friends. This is a wonderfully free country to live in. Even more wonderful when we’re not going through hell though 🙁

  8. I wooda thunk that New Zealand being a rich enlightened country, and being in such a geologically active area would have very good construction standards.

    For new construction, this is almost certainly true. Older construction, not necessarily. As the affected area includes some historic buildings (including the cathedral, which BBC reports has lost its spire) in the Christchurch city center, I would not be surprised to hear that many of these older buildings wouldn’t meet modern construction codes.

    I am extrapolating here from what I know about seismic codes in the Western US. I have relatives in Seattle, which is susceptible to similar earthquakes (the Juan de Fuca plate is diving under the North American plate along the Cascadian coast, hence the earthquakes and volcanoes in the region)–older parts of the city were built before the extent of vulnerability to earthquakes was known. In San Francisco, many important buildings have been retrofit to meet current earthquake codes (among them Brooks Hall in the Civic Center, which ironically was the venue for meetings of the world’s foremost association of earthquake experts, the American Geophysical Union, in the years prior to its temporary closure for such a retrofit). In both places, newer buildings have to comply with the code, but older buildings might not.

  9. Omega Centauri: Most of the buldings in Christchurch were damaged in Setpember’s 7.1 earthquake. Pretty much everything stayed standing during that one and there was no loss of life, but I think the buildings were just too weak to survive another large earthquake so close to the surface. Especially as repair work was still underway.

  10. Omega and Eric,

    Yes, we’re generally pretty proud of our earthquake-proofing standards. There was quite a lot of chatter after the last Chch quake about them, and how many lives they had probably saved. Interestingly, I hear the Chch cathedral had just received a couple of million dollars-worth of earthquake reinforcing, which may go some way to explaining why it was only the thin, flimsy bit at the top that fell off.

    This quake is thought to have caused damage mainly because it was so shallow, with a secondary emphasis on liquefaction. The occasional GNS scientists who are making statements on local TV are also pointing out that soil-surface acceleration in this quake was approximately twice as high as in the 7.1 quake in September. The non-zero death toll in this one has been largely put down to the fact that people were at work in the central city (where the older buildings are) for this one, in contrast to the September quake which struck in the small hours of the morning while everyone was out their (comparatively) light, wooden homes in the suburbs.

  11. As others have said, the building standards in NZ are very high with specific earthquake-related elements. We typically get about 20 or so of M=5 per year, but most often well away from the larger centres. I may be repeating, but there are several things to remember:

    – Christchurch (abbreviated Chch by locals) has had several quakes over the past 6 months, including 7.1 (Sep 4) and 5.1 (Boxing Day, close to city and shallow) among others. If you include all those > M=3, it has had literally hundreds in the last 6 months.

    – Most of the heavy brick’n’mortar buildings are in the older parts of town, including the centre.

    – The recent event is much closer and shallower. The Sep 4th event was a good 30-40 km outside the centre; this is only 10 km from the centre and shallower, centred in/near Lyttelton, the port village south of the town. I’m not a geologist, but you’d think that all other thing being equal the force would dissipate with the inverse square of the distance from the focal point, so the location & distance would matter.

    – Liquefaction plays a role. I getting the impression the liquefaction occurred in areas it didn’t before (?).

    – Importantly, this event occurred at a bad time, in the lunch hour with people out and about. The Sep 4th event occurred in the wee hours of the morning, when all but those living in inner city apartments would be in the suburbs at home.

    Eric, the older buildings would have been strengthened (or should have) – it’s a requirement. Some older buildings (from some time ago) were pulled down as the cost of strengthening didn’t figure well with the cost of replacing -at least this is my understanding. A problem here will simply be that this latest event will no doubt have conflicted with repair work still being underway.

    Greg, You should be able to locate an USGS Pacific map featuring the “ring of fire” as it’s nicknamed showing the earthquakes. It says a lot in an image. (Haven’t time to track it down, sorry.)

  12. Well, the “ring of fire” is more or less the edge of the huge Pacific plate (made up of smaller plates), plus/minus, as shown in the diagram above.

    Thanks for the insights on the ChCh NZ building situation. One way to put this in perspective is to ask: What would happen in LA or San Francisco if there were five or six 5+ quakes in a few months, with one being a 7.1 right on the edge of town. Lots of earthquake proofing in those two cities, but there would be a lot of damage anyway.

  13. Grant, it is also interesting to see how emergency systems and communications systems fail (or not). Based on what I see in your blog, similar problems are occurring as happened in New York on 9/11 with cell phones and such.

  14. re: the ring of fire, what I meant is that their (USGS) maps of all the (larger) quakes mapped onto the Pacific region is quite impressive. You’d have to see to get what I mean. It’s a case of the data presented one way telling it’s own story.

    Re: cell phones. I don’t know what happened in NYC, but a problem for us is that if the power goes out in a quake (as it does in large ones) the cell towers switch over to batteries, which only last for so long. The telecomm people then try to bring in generators to cover them until the power is back up. Heavy usage draws on the batteries, and phone calls (as opposed to texts) tend to occupy the capacity. (Not an expert, just what I’ve picked up; it’s the second time around on this for Chch.)

    About reading about the earthquake at sciblogs, you’re most welcome to read my post on it, but I think realistically you’d do better to look at my colleagues efforts. Mine is a stream of snippets and links as the events unfolded and somewhat aimed at locals who know the places. It has a lot more material, but it’s also a *lot* more chaotic! (Christchurch is my hometown, btw; I now live in Dunedin.)

  15. Нow to search people under the rubble of concrete when will be discharged battery on theirs mobile phones?
    The only effective way of detecting people under the rubble – it’s infrared camera of a military helicopter.

    Quality infrared cameras mounted also on fighters, but the possibility of detection of small objects is limited by the speed of aircraft.

    Ð?nly amphibious military hovercraft can travel on the destroyed roads and deliver food and medicines and to evacuate large groups of people.

    Рike riddle: that from this list will be used in New Zealand?

  16. As the geologists investigate the earthquake details are emerging for why it has been so devastating. The Building Code in New Zealand is for high quality surviable construction

    two articles of interest are

    “…But he said the earthquake hit central Christchurch with more than twice the intensity of the larger magnitude 7.1 but deeper-seated quake of September 4.

    He said Tuesday’s quake was of a force statistically unlikely to occur more than once in 1000 years and produced ground acceleration 1.5 to 1.8 times greater than modern buildings were designed to withstand.”

    and discussion of seismic lensing off the basalt of the Port Hills here:


  17. As the geologists investigate the earthquake details are emerging for why it has been so devastating. The Building Code in New Zealand is for high quality surviable construction

    two articles of interest are

    “…But he said the earthquake hit central Christchurch with more than twice the intensity of the larger magnitude 7.1 but deeper-seated quake of September 4.

    He said Tuesday’s quake was of a force statistically unlikely to occur more than once in 1000 years and produced ground acceleration 1.5 to 1.8 times greater than modern buildings were designed to withstand.”

    and discussion of seismic lensing off the basalt of the Port Hills here:


  18. Hello from the States. Watching a broadcast about the evolutionary origins of the Kiwi and geological history of NZ brought me to this site, where I was pleasantly surprised to read that yours is a region populated by HSs that are overwhelmingly not infected by those I regard as the “willfully ignorant” (free educational opps here are overwhelmingly numerous yet most citizens are far too intellectually lazy to embrace, take advantage of or sustain). Now, I find myself wondering, how difficult it is to establish NZ residency? Best regards! DD

  19. It’s so evident with volcanoes and earthquakes that we are just inhabiting the surface of so much planetary greatness happening underneath. I don’t care how large an island seems to be when you are standing in the middle of it..ex. (Hawaii) It is still an Island !! I often wonder over the millenia how many Islands that at one time were huge and habitable are now hidden under the waves somewhere. With that many earthquakes happening in Christchurch, I would be relocating…Fast !! Wouldn’t you??

  20. Sorry but people are so stupid when it comes to religion its just what you believe and if you have faith. I don’t believe in god but there is a hire power but it not from this earth. If you think we are the only one in this big space of the galaxy or universe your just stupid and you have a shallow mind.

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