Two more earthquakes shake Christchurch

A post by Chris RowanJust as it seemed that seismic activity was finally dying down in Christchurch, the city has been shaken by two more earthquakes. The USGS currently has the first shock pegged as a magnitude 5.8, and the second as a magnitude 5.9; the NZ Herald reports that the effects in Christchurch itself include loss of power, liquefaction and flooding, and rockfalls, but only minor injuries so far.

The first shock occurred about 2pm local time. The rupture was shallow (about 5 km deep) and located offshore, about 25 km miles east of Christchurch. The focal mechanism suggests westward thrusting on a north-south oriented fault. This earthquake was quickly followed by a magnitude 5.3 event. The second M 5.9 hit just under two-and-a-half hours later, 5 miles closer to the shore. It was also a relatively shallow rupture, and the focal mechanism also indicates thrusting, this time in a more northwest direction, mixed in with a bit of dextral strike-slip.

USGS focal mechanisms for the December 23rd M 5.8 earthquakes near Christchurch, with the focal mechanisms for the September 2010 Darfield and February 2011 Port Hills earthquakes, and their approximate ruptures, plotted for comparison.

(a primer on interpreting focal mechanisms)

It’s quite early, and the focal mechanism on the first shock in particular is a little poorly constrained by the look of things, but a few things stand out.

  • The area that these earthquakes occurred in is along the trend of the fault – the Port Hills Fault, that ruptured in Feburary’s magnitude 6.3 earthquake. It’s likely that the Port Hills earthquake would have caused the stress in the crust in this region offshore to have increased slightly.
  • Looking at the focal mechanism for February’s quake, it also indicates north-west directed thrusting with some dextral strike-slip.
  • So whilst based on the first, less well-constrained, focal mechanism, I thought that this new sequence was due to motion on an entirely new fault, it is possible this is just stress being released on an eastward extension of the Port Hills fault that ruptured in February, or possibly a parallel strand of the same fault system.

It will probably become clearer as more data is collected and analysed in the next few hours and days. In the meantime, the NZ Herald has a rolling updates page on the situation in Christchurch. My guess would be that the smaller size of these earthquakes compared to the Darfield and Port Hills events, and the fact that they were further away than February’s shock, meant that the shaking from these earthquakes was much less likely to cause catastrophic damage on its own. However, the cumulative effect on already damaged buildings may be an issue, and liquefaction, and the flooding and subsidence that are associated with it, could greatly increase the long-term impact.

Categories: earthquakes, focal mechanisms, geohazards
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Comments (5)

  1. Duncan White says:

    Hi Chris,
    Your articles are always well received by the Christchurch community & we like to see your thoughts on how the situation is developing in our small part of the world.
    The shaking intensity in today’s aftershocks appeared significantly less violent than the M6.3 Port Hills & M7.1 Darfield event due to the greater distance from the epicentre to Christchurch & also the fact that I was slightly further away this time around. I hope this doesn’t trigger some of the larger faults in the Pegasus Bay region to rupture, although the migration of the entire Darfield aftershock sequence seems to be gradually moving further east away from Christchurch so further significant damage should be less likely.
    People seem incredibly weary today, it’s obviously been a tough 15 months for us Cantabrians. I hope my friends can enjoy the Christmas and New Year without any further activity & that 2012 is a much quieter year in terms of seismic activity for our quake weary province.

    • Passerby says:

      Your area in NZ gets quite a bit of rainfall; 2010 and 2011 have been marked by either unusually dry or unusually wet periods. The broad river valleys that feed silt and gravels into the estuarine environment underlying Christchurch and the known coastline degradation beg the question: is sediment deposition adding on stress to valley loaded fault systems – and – are accumulating valley and coastal erosional materials loading stress onto adjacent and presently quiescent fault arrays in the Bay??

      Modern environments of the Canterbury Plains and adjacent offshore areas, New Zealand — an analog for ancient conglomeratic depositional systems in nonmarine and coastal zone settings. (2003) D Leckie. Bulletin of Canadian Petroleum Geology 51(4):389-425.

      Christchurch coastal erosion technical discussion

  2. Tony Norriss says:

    Hi Chris,

    I was wondering if the 5.9 New Brighton quake was on the “Barbadoes Street” fault which I understand runs to New Brighton. What are your thoughts on that?

  3. Marc Valdez says:

    Inferring from this animation, I wondered whether, starting with the Darfield quake, the North Canterbury Plains had begun a process of rotating counterclockwise.

  4. Garry says:

    Hi Chris

    slightly confused units here ( about 25 km miles east of Christchurch ) – I’m taking that as kms, not miles

    Our workplace under 5 miles from the 6.0 epicentre, initial physical impressions were that the two were similar, with the second being largely similar apart from a “gutsy ” up blip towards the end of the sequence..

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