The accelerations of the quake

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1. Colonial Viper 1

   24 February 2011 at 9:58 am

   Wow…being just 5-10km further out meant the G-load dropped in half or a quarter. Pi R squared and all that.
   • lprent 1.1

     24 February 2011 at 10:06 am

     Yeah. You can clearly see the strike of the fault that cut loose as well running from Lyttleton to Pages Road. The velocity change falls off even more strongly when you look at it along that line.
2. Zorr 2

   24 February 2011 at 10:12 am

   haha… would almost ask if you know a friend of mine who was posting these figures immediately after the quake…

   But yeah, 2G is insane…
   • lprent 2.1

     24 February 2011 at 10:23 am

     Unlikely. I gave up wanting to work in science during the undergraduate degree, went into management (then did a postgrad management degree and promptly went into programming).

     But I’m still fascinated by science and use it throughout my current profession (and the same with the management skills as well).

     At these accelerations I’m more astonished that there are high-rise buildings left in the CBD, and even more astonished that there are buildings still standing in Lyttleton.
     • Marty G 2.1.1

       24 February 2011 at 10:57 am

       you can see why people aren’t joking about being thrown off their feet. the ground suddenly moves sideways at 22m/s/s for a few milli-seconds and your body doesn’t, well your feet get dragged with the ground far more than your centre of mass, and over you go. literally like a rug being pulled out from under you. and the samething happening to all those buildings.
       • Lanthanide 2.1.1.1

         24 February 2011 at 1:54 pm

         This is why, as soon as you feel a quake that seems ‘strong’, start moving to get to safety. It is not uncommon for there to be foreshocks – on Sept 4 there was a 5.8M 8 seconds before the 7.1 hit. I woke up and got under the doorframe of my bedroom about 2 seconds before the major shaking started.

         For aftershocks it’s a bit less of an issue (as they’ll generally be weaker) and you’ll be much more prepared to judge how severe a particular one is. But if a quake hits out of the blue, get for cover no matter how ‘weak’ it might feel at first.
     • Lanthanide 2.1.2

       24 February 2011 at 1:51 pm

       My dad says the shaking seemed more vertical which I think wouldn’t be quite as destructive to buildings as horizontal ones.
       • lprent 2.1.2.1

         24 February 2011 at 2:39 pm

         It’d be worse. For a starter vertical shaking is a lot rarer because you almost have to be on top of the earthquake strike to get it – exactl like christchurch was. So buildings are not as designed for it. Secondly vertical motion is far more like to cause sediment compaction which does nasty things to foundations and liquidification.
     • Kevyn Miller 2.1.3

       24 February 2011 at 4:02 pm

       Lprent – google “earthquake resonant frequencies” and Mexico City arthquak to find some good xplanations of how geology and duration of shaking affect building damage. Also some good info on Christchurch’s 700m deep aluvial base alter the shaockwave spectra.

       Colonial Viper is spot with r squared. Energy pr sq m should drop 75% every time the distance from epicentre doubls. Not sur tho if energy being squar of velocity has to be factored in to work out th structural loads but that seems to be whats needed to arriv at th recorded delta v’s from the 6.3 and 7.1 since that is a 16fold diffrence in nergy rlease at th picentre and th difference in distance to the picntr is fourfold so the enrgy reaching the CBD should have dropped by a factor of 16. Back of nvelope calculation says one more squaring produces the rcorded delta v differences.

       It’s also important that the lyttleton quake propogated towards the city instead of away from it as the Grendale quake did.

       Again, apaolgies for the randomly functioning e, speakers will be scrwed to shelves in futur.
3. Inventory2 3

   24 February 2011 at 11:05 am

   I was down in Christchurch when the swarm of aftershocks hit on Boxing Day, and it was an experience I’m not keen to repeat any time soon. At that time, it was reported that monitoring points in town reported the mid-morning ‘quake as stronger than that of September 4th; it was much closer to town. That Tuesday’s quake has been so devastating is no surprise given its proximity to the CBD and it’s shallow epicentre. Thanks for sharing the scientific stuff Lprent.
   • lprent 3.1

     24 February 2011 at 11:12 am

     I was in ChCh for boxing day as well. Where I was, I’d have guessed the main quake that day had an acceleration of 5% of a gravity 10% at the outside. I have felt stronger quakes at my parents in Rotorua or when I was stationed in Waiouru – so I wasn’t too worried (it did freak Lyn out though)
4. M 4

   24 February 2011 at 11:34 am

   Impressive image underpinning why things were so bad.

   With the liquefaction, seemingly unending quakes, for the moment at least, and perhaps the near-impossibility of rebuilding in these post peak times Christchurch may become the earthquake Chernobyl of the South Pacific.

   Might be time for most to get out of Dodge.
   • Lanthanide 4.1

     24 February 2011 at 1:55 pm

     I’m in Oamaru at the moment, planning to stay here until Sunday but might delay it a day or two past that. Depends what my work is like.
   • DS 4.2

     24 February 2011 at 11:57 pm

     Yeah I got the hell out of dodge. Blenheim doesn’t shake.

     Currently conflicted. I want to be back there. But work’s in the middle of the CBD (it was not fun) and I don’t know if the building will survive. And there’s no running water at home. So. Yeah. I just don’t know. About anything.
5. lprent 5

   24 February 2011 at 11:34 am

   

   Dust clouds over Christchurch immediately after the quake. Amazing photo – click it to see full size.

   hat-tip: John Darroch
   • pollywog 5.1

     24 February 2011 at 12:29 pm

     O for OSSUM
     • A Nonny Moose 5.1.1

       24 February 2011 at 5:43 pm

       It’s not Ossum when you live through it.
6. Tanz 6

   24 February 2011 at 1:50 pm

   John Key is doing a marvellous job though. He is in there, at the grass-roots level, dong all he can to help, as is Phil Goff. Good on both of them, NZ is still a fantastic country, when it really comes together in a time of crisis as this. Of course, many people are doing a great job, Kiwis a very caring bunch, overall. Lots of people going out of their way to do all they can, whatever they can. Offers of accommodation, donations, hands-on-help, etc. Also, neat to see the offers of International help flowing in.

   Christchuch will be rebuiilt, it’s a national treasure, always will be.!
   • Zaphod Beeblebrox 6.1

     24 February 2011 at 2:23 pm

     So very true.

     When we do re-build, however, my hope is that we take our time and make good decisions. Otherwise we just end up paying over and over again for our mistakes.
     • Tanz 6.1.1

       24 February 2011 at 2:38 pm

       Agreed. One of the beauties of Christchurch is/was its historical flavour. Hopefully some of this can be retained, somehow. At the end of the day though, buildings are just buildings. bricks and mortar, it’s the people who cannot be replaced, ever. Why has this happened to ChCh yet again? Shows how fragile life can be. the savargey/severity of nature.
7. tsmithfield 7

   24 February 2011 at 2:03 pm

   “With the liquefaction, seemingly unending quakes, for the moment at least, and perhaps the near-impossibility of rebuilding in these post peak times Christchurch may become the earthquake Chernobyl of the South Pacific.”

   I certainly think we need to think about the foundation structures of housing in certain areas, and where large buildings such as those in the centre of the city are located. It seems to me that it is liquifaction that has taken down and destabalised some of the newer buildings such as the CTV building, Grand Chancelor etc.

   The North West of Christchurch has been largely unaffected by either of the quakes. I live in Redwood and still haven’t even had a crack in my brickwork!! My business is in Sockburn, and that hasn’t suffered damage either, as is the case with most businesses in that area. So, I think it is a good idea to migrate all our business in that direction. A possibility would be the land from the old Wigram airbase.
   • Lanthanide 7.1

     24 February 2011 at 2:14 pm

     Ngai Tahu owns the Wigram airbase land.

     But yeah, the western suburbs have gotten off well from both quakes, very little liquefaction around Spreydon and Riccarton (where me & family live and work) in general. The house I’m in hasn’t had any visible damage from either quake, although the front house on the property has.
     • ianmac 7.1.1

       24 February 2011 at 2:51 pm

       Relatives in Bryndwr and Redwood seem to be unscathed, but other relatives in a two-storied triple bricked older house in the centre of Papanui is finally destroyed after Sep damage. A line of shops at the top end of Papanui road is roped off after at least frontage collapse.

       Be interesting to know about Marshland through to Papanui as this is peat country and has always been unstable. Chimneys would cause the house to sink down to make floors slope towards the chimney. You would think that peat ground would be worse or would it absorb the shock?
       • Kevyn Miller 7.1.1.1

         24 February 2011 at 4:19 pm

         In Mairehau and just done a marble test and the floor is mostly flat but one cornr does slope down away from the chimney. However the expressway has definitely sunk and/or culverts hav pumped upwards. Same for the manholes in Philpotts Rd which are back to being as bad as before last weeks hotmix patchup.
8. todd 8

   24 February 2011 at 8:08 pm

   Check out googles earthquake map:

   http://www.christchurchquakemap.co.nz/

   The Christchurch Quake Map allows you to play time-lapse visualisations of the 4,870 quakes and aftershocks that have struck in the Christchurch area since September 2010. It is also possible to view visualisations of the last seven days of quakes or the 47 that have hit today. The Google Maps plot earthquake data from GeoNet.
9. Rich 9

   28 February 2011 at 11:32 am

   I’m told by a seismologist friend that those accelerations were a 10,000 year event.
   • lprent 9.1

     28 February 2011 at 11:47 am

     You do get those types of accelerations in quakes if they are close enough to the surface. Usually however the measuring systems are not in place. But around a urban area they are there.

     But what is unlucky is that this quakes strike line was directly below the city so the relatively small amount of energy released in the quake had a massively disproportionate effect in terms of damage.

     I can’t recall offhand where there has been a quake where the strike line was directly below such a built up area. Most quakes I’m aware of have been further away from cities and would require far higher magnitudes (typically close to magnitude 8 or above) to cause similar damage on a earthquake resistant city.

     That localalization is why your friend would have said it was such an infrequent event.
   • Colonial Viper 9.2

     28 February 2011 at 11:56 am

     I’m told by a seismologist friend that those accelerations were a 10,000 year event.

     Estimates like this are complete academic nonsense. Meaningless. No one has seismographic/accelerometer data for the Canterbury region covering 10,000 years. And to say it is a one in 10,000 year event, you need to have collected data over several ten thousand year spans. Its a statement based on theory, not based on empirical knowledge. A vast difference.

     A simple test is: would your friend professionally recommend that there is now no need to rebuild Christchurch to withstand a similar shock. Since it is likely that many centuries or thousands of years will pass before it happens again.

     I am betting no, and if that is the case, he doesn’t believe in his own estimates.
     • Rich 9.2.1

       1 March 2011 at 9:48 pm

       I think his work is based around looking at rock formations and the effect recent and historical earthquakes have had on them.

       Anyhow, I bow to your superior knowledge, as commenters on websites are always right…
10. Bored 10

    28 February 2011 at 11:55 am

    I did geology at Uni so many years ago that I am now so uncurrent, and a layman at best. A question for anybody qualified to answer: does the removal of the aquifers to a fraction of their former mass have any effect on Canterbury earthquake events? Is anything known about the dynamics of alluvial deposits – aquifers – earthquakes?
    • lprent 10.1

      28 February 2011 at 12:09 pm

      At a guess, at the 5km down there could have been an effect from water as it may have been cool enough to not immediately vaporize and bind into the rocks. That is also ignoring the pressure effects which would have been considerable. The probability of having anything like liquid water at 5km is really really low.

      But leaving all that aside, since the Canterbury aquifers are relatively close to the surface, it’d have taken considerable periods of time (ie at least centuries) for changes in the near surface aquifers to affect down where this earthquake took place. Those aquifers are depleted but not empty.

      I’d think it was highly unlikely that depletion of near surface aquifers had any effect.
      • Bored 10.1.1

        28 February 2011 at 1:17 pm

        Thanks Iprent, seems a pretty good answer to me. I also wondered about intrusion into the aquifers of polluted water, but thats another question altogether which i am sure the health authorities will already have got onto.
    • Colonial Viper 10.2

      28 February 2011 at 1:20 pm

      Emptying aquifers means a reduced mass under Christchurch? So less mass for seismic forces to move? Which means higher accelerations?
      • lprent 10.2.1

        28 February 2011 at 1:35 pm

        Not a significant effect. The aquifers are shallow and sedimentary. They consist of a little bit of spare space in porous rock in the first few hundred meters of the surface (at the best – in fact most would be within 50m-100m of the surface). They aren’t an artesian body of water pooled on top of bedrock, they are effectively sub artesian rivers. As a mass they are a small fraction of the mass of sediment in the top few percent of a rock column 5km deep. It wouldn’t have much effect when you start thinking about mess effects.

        In any case most of the area of the strike is covered by bloody dense basalt from the Banks Peninsula volcanoes that as far as I’m aware doesn’t have much in the way of sedimentary structure for aquifers.

Links to post

1. The Standard: posted on 23 February 2011 at 8:51 pm
2. Christchurch power chart « The Standard: posted on 28 February 2011 at 11:33 am

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