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What happened..

Written By: - Date published: 2:49 pm, September 4th, 2010 - 14 comments
Categories: science - Tags: ,

There is an interesting* post at Highly Allochthonous looking at the likely reasons and type of the earthquake (hattip: NickS).

It appears to be quite a shallow rupture, on the Canterbury Plains close to Christchurch, and the focal mechanism indicates largely strike-slip motion.

New Zealand is one of the oddest places geologically (Japan is similar) in the world because of our position between two  plates and because we are at the twist where the plates change how they interact with each other.

If you look at chart on the right and think of it like a weather map. South of the south island the Australian seafloor plate is sub-ducting (or going under) the Pacific plate. Whereas in the north island the Pacific plate is going down under the Australian plate. The southern alps are going up mainly because they’re at the twist pressing upwards.

The subduction zone around Marlborough has been rapidly (in geological terms) been moving southwards for some time. This causes faulting all the way down towards Christchurch as the block structures adjust to ongoing pressure.

The remainder of the HA post is relatively readable

This tectonic evolution is ongoing, and since the end of the subduction zone is now actually to the south of the southernmost and youngest of the Marlborough faults. Some of the plate boundary deformation is probably therefore being shunted into the region around Christchurch, where it needs to be accommodated by dextral strike-slip faulting. Eventually, over geological time, this deformation will lead to the formation of a new, more southerly strand of the Marlborough Fault system. It also means that earthquakes of this type of size are unlikely to be a one-off event in this area. Unsurprisingly, then, seismic surveys have identified a number of active faults beneath the recent sedimentary cover on the Canterbury plains (although they were identified in the linked study as reverse faults accommodating compression, strike-slip deformation is very difficult to identify if you only have a 2 dimensional cross section to work with).

Whilst this map of large historic earthquakes in New Zealand shows that earthquakes of this magnitude can occur pretty much anywhere in New Zealand, seismicity in this particular area has some particular hazards; it is close to a heavily populated region (Christchurch) built mainly on unconsolidated Quaternary sediments, which will intensify the potential shaking and damage to unreinforced buildings. Fortunately, whilst this earthquake appears to have caused a fair amount of damage, from the early reports casualties seem to be light.

That last paragraph indicates why the action of the earthquake seems to more violent at ground level than the amount of energy expended. Local seismographs would over report the energy because of this effect. Those further away would get a better picture of what happened. This is probably why the magnitude dropped from the initially reported 7.4 to 7.1

* at least to me

14 comments on “What happened.. ”

  1. Lanthanide 1

    What does the red/white circle mean? I noticed one of the others on the site was purely perpendicular for the cross, whereas the cross in this one has been skewed/rolled to the left.

    • lprent 1.1

      From memory it is some kind of classification of how the quake formed. In this case I’d guess something like a strike-slip with deformation

  2. ferret 2

    According to the news, Key overflew Chch with Parker to see the damage.

    If it had been Norman Kirk, he would have walked around, talked to survivors, assessed their situations, and tried to encourage them and lift their morale.

    Why is Key afraid of face-to-face meetings with earthquake survivors ?

    It may be the prospect of lynching by followers of SCF.

    • Vicky32 2.1

      He’s simply doing the Presidential American thing – drop in from above (or overlook), state what it is or is not to be done – and off again,

    • joe bloggs 2.2

      newsflash dickhead … JK was on the ground in Christchurch within 12 hours of the quake, and did in fact walk around, talk to survivors, assess their situations, encourage them and lift their morale. Not only JK but several of his ministers as well.

      It took Phil Goff another 24 hours before he was on the scene. Afraid of face-to-face meetings with survivors obviously.

      As for Jim Anderton – there’s no sign of him anywhere – still hiding under his bed no doubt

  3. grumpy 3

    There seems to be a lot happenning below the surface as far as ground water is concerned. People I know at dunsandel have mud coming up in their well and recently driven wells, previously with a water level 60m below the surface are now artesianing.

    We will hear more about this in the near future.

    • Jenny 3.1

      For some people without reticulated water, able to locate a bore head in their suburb, the high natural artesian pressure has proved to be fortunate.

      capcha – “pool”

    • lprent 3.2

      There was a 3-4 metre horizontal movement according to the reports and some vertical shifting. Plus there would have been a hell of shaking of unconsolidated sediments. There will be a lot of change in ground water

  4. Jenny 4

    The hardship visited on the people of Christchurch and surrounding areas is about to be worsened by a predicted onset of foul weather.

    The MetService is currently issuing a weather warning for the Canterbury Plains.

    click here

    …Northwesterlies reaching 65 km/h in exposed places, with gusts of 130 km/h –
    between 9am Sunday and 6am Monday

    The Met say it is possible that the winds and heavy rain predicted may cause further damage to already compromised structures.

    There may also be a danger from unsecured loose materials being caught in the predicted high winds.

    In addition, severe gale force northwesterlies are forecast for the east of the South Island from inland Southland to Marlborough, and also for Wellington, Wairarapa and Hawkes Bay. These winds have the potential to bring down trees and powerlines, and make driving hazardous. They are also likely to be dangerous to structures already weakened by the recent earthquake.

    capcha – “dislike”

  5. Kevyn 5

    The geology directly under Christchurch is sufficiently well known for Yetton, et al, to accurately determine (in 1998) the types and general severity of damage across greater Christchurch from quakes delivering MM6-8 ground motions. “Probability and Consequences of the Next Alpine Fault Earthquake – Specific Report to Christchurch City Council”, but it doesn’t seem to be on the CCC website any more.

    It seems emergency services and most infrastructure providers have read this report and subsequent studies. The notable exception being no apparent plans in place to re-route buses to an alternative hub outside the four avenues.

    Any idea what effect this quake is likely t have had on the probability and/or severity forecasts for the next Alpine Fault earthquake? I suspect, from recent papers on super-sheer quakes and the coincidence of Great Wairarapa fault movements with the largest paleoseismic events on the Alpine fault, that yesterday’s quake is just a dress rehearsal for the big one that the city can expect to experience some time thiscentury.

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