/ Maggie Koerth-Baker / 8:44 am Tue Feb 22, 2011

Why was the Christchurch Earthquake so destructive?

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Image: Simon Baker / Reuters

New Zealand is no stranger to the results of plate tectonics. The country sits almost directly on top of the boundary between two chunks of the Earth's crust, the Pacific Plate and the Australian Plate. The two plates grind against each other, creating tension spots where potential energy builds up and is released in the form of earthquakes—a lot like pushing on a stuck door until it finally flies open. New Zealanders feel as many as 200 earthquakes every year, but most are nothing more than a minor jiggle. And even big, throttling shakes, like the 7.1 magnitude quake that hit the country five months ago, can come and go without killing anyone.

Today's earthquake in Christchurch, New Zealand is different. Although it was relatively powerful—6.3 magnitude—it was still weaker than the quake last Fall. And yet, it's already the deadliest earthquake to hit New Zealand since 1931. What made this earthquake so dangerous? It's all about location, says New Scientist:

Last year's 7.1-magnitude earthquake was more than 10 times as strong as today's but caused no deaths, probably because it occurred at greater depth and further away from Christchurch: its epicentre was 70 kilometres west of the city. And the focus of September's quake was some 10 kilometres below ground - today's was half as deep.

Via Discover magazine's 80 Beats blog

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    1. Indy – Empathize with you greatly. In California for more than a decade but no major quakes in that time. Dreading the day my fam and I have to deal with it. Hope you stay safe and good luck down there .

    2. I sympathize about the aftershocks. I survived the Northridge quake in LA in a Mercalli-IX area, and I remember the aftershocks vividly.

      The worst part is that you couldn’t really tell, in the first couple of seconds, whether this was a typical aftershock, usually a full order of magnitude lower than the mainshock; or if it might be a repeat of the mainshock. The first second or two are pretty much identical either way.

      (Aftershocks as large as or larger than the mainshock are unusual in plate-margin quakes – but not impossible).

      Much was made in the immediate aftermath of Northridge about how cheerful and helpful and cooperative everyone was, working together to help each other cope with the disaster.

      But, I’ll tell you: by about three or four days later, when the initial novelty had worn off, and no one had had a decent nights’ sleep due to the hourly-or-so aftershocks, people were a LOT less cheerful. :-)

      It’s really odd to be stumbling around among several million people, every one of them cranky and tired and sleep-deprived.

      My advice is to be generous in giving each other the gift of slack; and remember that, like all things, this too shall pass.

  2. When talking to a friend who was a Geology prof he described the energy of an earthquake and its damage in terms of a nuclear explosion or chain of H-bombs. In fact this is how they seismo-triangulate underground nuclear tests. Deeper and farther away can really make a big difference especially in flexible soil or loose rock vs. bedrock which moves as one.
    Hey Dr Bob!

  3. The fact that this one happened at lunch time instead of 4am didn’t help with the increased human cost, either :(

  4. Having experienced both of these quakes, and all the afterschocks in between, the effect of distance from the city and depth is huge. This 6.3 was right under Lyttelton (port town just over the hills) and like the post said, was half as deep. It felt to me as if the shaking was stronger than the 7.1 last September. The fact that I was at work at the time, rather than in bed asleep as with the last one would also have affected my judgment of the size. I do know that the destruction to property caused at my workplace was far far worse than after the 7.1, so maybe the shaking was more intense.

  5. Having experienced both of these quakes, and all the afterschocks in between, the effect of distance from the city and depth is huge. This 6.3 was right under Lyttelton (port town just over the hills) and like the post said, was half as deep. It felt to me as if the shaking was stronger than the 7.1 last September. The fact that I was at work at the time, rather than in bed asleep as with the last one would also have affected my judgment of the size. I do know that the destruction to property caused at my workplace was far far worse than after the 7.1, so maybe the shaking was more intense.

  7. Take a look at the PAGER (Prompt Assessment of Global Earthquakes for Emergency Response) maps and for last September’s quake and the current quake and you can easily why the difference in overall damage.

    The Mercalli scale quantifies the violence of shaking at specific locations. Mercalli VIII and IX (or above) are the dangerous levels for well-built, seismically-resistant structures. (Vulnerable structures like unreinforced masonry – brick, stone, adobe – are also susceptible to Mercalli VII.)

    As you can see on the PAGER maps and charts, many more people were exposed to dangerous levels of shaking in the current quake, even though it had a smaller magnitude and its shaking affected a smaller area than the September quake.

    Location, location, location.

  8. From the footage we’re getting in the UK it looks like most modern buildings constructed from steel frames survived extremely well, but as always it is the older, masonry buildings that failed catastrophically.

    Sympathies to everyone trying to get their lives back together for the second time in less than a year.

  9. Here’s a map which includes the ocean topography. Lots of little shocks just beforehand. If I remember correctly, the east side of the South Island is slipping past the west side of the North Island.

    How much of Chch is on unconsolidated sediments (like those of Northridge (?), CA) so damage might be worse? Also, any idea why some buildings look fine and others are rubble (photo bias, sediments, building standards)?

    Christchurchquakemap, New Zealand)

    1. “Also, any idea why some buildings look fine and others are rubble (photo bias, sediments, building standards)?”

      Building standards in NZ are very high with regards to earthquake proofing. In last September’s earthquake many modern houses suffered cracks but did not collapse; this contributed to low injury levels.

      However, my partner works in the construction industry and over the past 25 years has often had to mediate between architects and engineers over design issues, with the architects pushing the engineers to reduce structural strength features that interfere with aesthetics. He tells me that many commercial buildings in Auckland NZ (where we live) would not survive a major earthquake.

      Fortunately Auckland is not on a faultline – but then again, until last September’s earthquake, no one knew Christchurch was on one either.

      1. I was never fond of the glass towers.

        My first day and hour of teaching up the hill at varsity was swinging in an 8 storey building because of the earthquake to the east of us, http://www.geonet.org.nz/earthquake/historic-earthquakes/top-nz/quake-10.html

        The images of the CTV building are hard to view. The blanket declaration that no one is alive there was callous in the least. Glad to hear today that others continue to search for survivors (and at least one was found).

  12. Actually, it’s all about structural integrity and second-store storefronts. The torsional stability of a three-wall ground floor with a front plate glass and/or metal rollup door is about the same as a house of cards. Positioning a heavy masonry four-wall above, is just about the best way to kill people on the street.

    We lived in Canoga Park and got out just before the ‘Big One’. All those ground floor apartments with big sliding glass doors were pancaked by upper floors, and that lack of structural integrity, including the ground floor apartment we had been living in. Same goes for apartments with garages fronting on alleys, you’ll lose the car in the garage, and anything in the room above the garage as the apartment pancakes down, why, kinda like a house of cards!

  13. There’s a brief technical report here
    http://www.geonet.org.nz/news/feb-2011-christchurch-badly-damaged-by-magnitude-6-3-earthquake.html
    Note it includes recored shaking of up to 220%g. As this guy put it – hanging the building sideways and holding it by the base (except the data indicated around 100%g at the time).

    Christchurch has seen a lot of liquefaction (we learned that word last September) and I gathered from last time the way the waves bounce around in the sandy ground mean a very uneven shake depending where you are.

    The ground and the building will have been unevenly undermined in September (and since) too.

    Self-promo: We’re working hard on from-the-horses mouth coverage at scoop.co.nz/

    Fortunately Auckland is not on a faultline

    It is on a volcanic field though.

  14. One minor point, the September quake didn’t happen in ‘Fall’. It happened in Spring and secondly New Zealand doesn’t have Fall it has Autumn.

  15. Lived in christchurch (briefly) in the hostel just off the main square…

    also live just north of baja, CA. Last week we had a 5.5 during a meeting. Dude didn’t skip a beat.

    Now, I realize the scale is logarithmic. But the range from ‘can’t be bothered to miss a slide’ to ‘ZOMG panic’ is STILL more than 10.

    So I don’t think maggie’s original question–why is this a problem?–has been answered.

    Christchurch doesn’t have tall buildings, though I suppose it has older buildings it holds on to to seem more English. I’m sorry, but when a small earthquake kills people–that’s a human disaster, not a natural one.

    But NZ…I genuinely do think they expect quakes and are prepared. So what gives?

    1. Earthquakes have lateral movement, up and down movement and a wave frequency. Maybe other characteristics as well. Every quake is different. If your buildings are structured to withstand lateral stresses, for example, they can still be flattened by a quake with a strong up/down movement. You can also potentially get a resonance effect based on frequency. And periodically, there’s a quake with characteristics that nobody ever observed before. It’s hard to build for every eventuality when you don’t know everything about the phenomenon.

    2. Yeah, um, what Antonius says but also: Japan expects and is prepared for earthquakes, as is San Fran. But when shit happens, shit happens. It’s not like an event like that can be made 100% safe to anyone.
      I read a report that compares the stresses applied as being equivalent to gravity going sideways. So – everything in your room being flung against the far wall with you underneath. People will get hurt, prepared or not.

      After last years events, the populous and civil defense know exactly what to do – and have done and are doing it. With all sympathy to the devastated individuals, it still could have been even worse had the building standards not been as high as they were.

      Buildings built 100+ years ago, 99+ years before Christchurch was considered a quake zone – have not fared well, that’s true.

      What gives? The foundations, that’s what.

      The issue with the Richter scale is valid – which is why folk who care nominate a replacement measure (Mercalli) that apparently takes into account real intensity, not just magnitude. Last years 7.1 (Richter) was bigger to the Haiti one on paper, but had no casualties. Now a 6.3 (Richter) at a much closer depth, distance and speed has a *much* different effect.

      1. Despite the fact that news reports may get it wrong, the Richter scale is not actually used anymore (at least, not any of the main earthquake reporting centers) for earthquakes larger than ~3.5. Any figure you see for a notable earthquake is a moment-magnitude scale (MMS) figure. It’s calibrated to give a similar number as Richter, but is more versatile and useful globally (Richter was designed for SoCal earthquakes, specifically, and below ~7.0).

        This is different from the Modified Mercalli Intensity scale (MMI) you mention. Both are in current use. Mercalli scale does what you suggest, it rates the intensity for a given location.

        But you can’t assign a single MMI number to an earthquake, because the rating depends on the location. You’d give different numbers for different affected regions – it’s higher close to the epicenter, and lower as you get further away (with many variations possible depending on the geology, naturally). Thus you see the “Shake Maps”, with color-coded areas showing where the most shaking occurred, based off of the MMI for each area.

        I have an MS in geology, and I went to grad school in Southern California and took courses specifically on seismology and earthquake physics :)

      2. IIRC, there was a large earthquake in New Zealand while Captain Cook was yet there – thus, Europeans (British) have known that New Zealand is an earthquake zone since shortly after its discovery to them.

        Forewarned is forearmed: but against such a foe, it is often not enough.

        I hope the New Zealanders recover quickly: I am sorry for their losses.

  16. The issue with the Richter scale is valid – which is why folk who care nominate a replacement measure (Mercalli) that apparently takes into account real intensity, not just magnitude.

    But even the Mercalli won’t answer the most critical questions – namely, “How much damage, how many casualties?” – because intense shaking that occurs in empty quarters or lightly populated areas with only a few lightly-built, quake-resistant structures will still cause less far damage and fewer casualties than identical (or even less-intense) shaking that strikes densely-populated areas packed with vulnerable buildings.

    Both the September Mag. 7.0 and the current Mag. 6.3 produced maximum Mercalli Intensities of IX – and the 7.0 produced those intensities over a much larger area.

    But the Mercalli VIII-IX areas -the ones where the bulk of the damage will occur – from the 6.3 affected far more people and many more vulnerable buildings, because it struck directly under the city center, where, not only is the population density much greater, but there are many more vulnerable buildings, especially late 19th/early 20th C. multistory unreinforced masonry
    buildings.

    Look at the PAGER population exposure charts linked above. The estimated Mercalli VIII-IX exposure in the 7.0 was only about 22k people, while the estimated VIII-IX exposure in the 6.3 was about 317k people.

    Combine that with more vulnerable buildings in the city center, and it’s easy to see why the smaller quake did more damage and resulted in more casualties.

    (And that, by the way, is the whole purpose of the PAGER estimates – to try to get an early, reliable estimate of just how serious a quake is in terms of damage and casualties, rather than relying on numbers that can only tell you how much total energy was released or how intense the shaking was.)

  17. A lot has to do with the ground. If there is liquifaction, it makes a big difference to trees, telephone poles, building pilings, because they are suddenly in nothing. The roads in parts of Shirley where I’ve been are like a bumpy beach. Our neighbourhood by luck is on a solid patch. Let me emphasize luck. By the way, here is the web site ( Chris Crowe at Trimble here) most people in Christchurch look to to track the aftershocks which include all scales: http://quake.crowe.co.nz/

    Ian Wells

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