More large aftershocks rattle Christchurch: will it ever end?

A post by Chris RowanPoor Christchurch just can’t seem to catch a seismic break this year: two more powerful aftershocks shook the city early Monday afternoon local time: the first a magnitude 5.3 (Geonet, USGS), the second a magnitude 6 (Geonet, USGS). The focal mechanisms of both shocks indicate mostly pure strike-slip motion, probably dextral strike-slip along a northeast-southwest oriented fault plane; the M6 quake also has a component of roughly east-west compression. These focal mechanisms are very similar to the magnitude 7 Darfield earthquake last September and the magnitude 6.3 rupture that badly damaged the city in February. These latest tremors occurred a few kilometres to the east of February’s quake, and from their location and the manner in which they ruptured, it looks like these earthquakes were on another, more easterly segment of the same fault.

Focal mechanisms and locations for the two large June 13 aftershocks, compared to the September 2010 and February 2011 ruptures. White lines are rough fault traces.

Fortunately, there appear have been few casualties this time around, but a new violent tremor is not good news for the many already damaged buildings in Christchurch. Peak ground accelerations from the M6 shock were quite high in the southern parts of the city (0.7-0.8g mostly, although one instrument recorded a 2.1g acceleration). The BBC reports that several buildings have collapsed, and there have also been further rockfalls, perhaps from cliffs already weakened by the previous earthquakes. Once again, the shaking has also caused extensive liquefaction in Christchurch and its suburbs, exacerbating the damage and destruction.

An SUV engulfed in mud caused by seismic liquefaction. Source: The Australian.

A clear thread running through many of the reports and interviews that I’ve been reading today is that the residents of Christchurch are fearful and fed-up with what is turning into a year of misery – and you can hardly blame them for wondering if they shouldn’t just give up and move. The risk of further large aftershocks is still present, of course, but one clear trend since the Darfield earthquake last September is that seismic activity is migrating east over time. The mainshock and aftershocks of February’s earthquake (the red circles in the figure below) are to the east of the Darfield mainshock and aftershocks (blue circles), and yesterday’s earthquakes (orange circles) are further east still. The whole fault system seems to be progressively unzipping from west to east, and the locus of seismic activity has now moved past Christchurch and is heading out to sea. The earthquakes may not stop right away, but at least they’ll probably be further away – and will therefore cause less damage.

Aftershocks of the September 2010 Darfield earthquake (blue), the February 2011 earthquake closer to Christchurch (red), and the biggest earthquakes on June 13 (orange). Click for a larger version. Data from Geonet.

Also, whilst it may be happening far slower than everyone would like, the seismic activity is slowly dying down. In the top figure below I’ve plotted the magnitudes of all of the earthquakes with a moment magnitude larger than 3 that have been detected around Christchurch since the Darfield earthquake last September; in the bottom figure I’ve plotted the number of earthquakes per day. There is a clear sawtooth pattern: the magnitude and frequency of the aftershocks decayed as expected following the Darfield quake, until February’s magnitude 6.3 tremor. This triggered a whole new flurry of activity, but note that the peak magnitudes are lower in this cycle, and the aftershock frequency decays more quickly. Yesterday’s events have temporarily boosted seismic activity again, but (as far as you can tell from limited data so far) to a still lesser degree.

Magnitude and Frequency of earthquakes near Christchurch since September 2010 Darfield Earthquake. Data from Geonet.

So, whilst the immediate seismic risk has by no means abated for the citizens of Christchurch, hopefully the worst is behind them.

Categories: earthquakes, focal mechanisms, geohazards, tectonics
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Comments (23)

  1. Brian Romans says:

    as always, nice explanation and graphics

  2. Lanthanide says:

    I’m curious as to why the USGS numbers are so far off Geonet’s?

    The quakes were first reported as 5.5 and 6.0 by Geonet, and were revised upwards today to 5.7 and 6.3. But your numbers are 5.3 and 6.0, which (particularly the first one) are quite a lot lower than Geonet’s.

    • Chris Rowan says:

      There are two different magnitude scales in play here: the USGS is using the moment magnitude scale, whilst Geonet is reporting the Richter scale magnitudes as well. They’re calculated slightly differently, but the moment magnitude is generally regarded as a better measure of the energy released by an earthquake.

  3. s says:

    It is interesting how most of the September aftershocks occurred near lyttleton area (red circles)

    • Lanthanide says:

      September aftershocks are blue, and out west. Red aftershocks are those since Feb 22nd, around Lyttleton/Port Hills.

  4. Mary Evins-Poindexter says:

    My daughter is studying abroad in Dunedin and felt the library shake during that last quake. What fault is Dunedin located on? And what are the chances of one happening there? I was surprised she felt it there. I tried to do some research and decided it was better left to the experts. Luckily I found you….. Thanks Missy Poindexter

  5. parclair says:

    Nice post, thanks

  6. John from Oz says:

    Many, many years ago, while studying geology at University, I saw maps depicting sea floor spreading across the Pacific Ocean and noted that Paleomagnetism identified a number of transverse faults extending from the East-Pacific Rise to the subduction trenches to the west, which indicated the seafloor was moving at different speeds. These memories brought a few thoughts to mind:
    1) The Christchurch earthquakes indicate an East-West alignment of the newly discovered fault which differs from the Northeast trending faults associated with the Alpine Fault. I am wondering if the new fault is one of the transverse faults that extend across the Pacific. If so we might expect to see the movement extend out into the Pacific in the months and years ahead.
    2) If the the fault extends to the East, what is to stop it also extending to the West and eventually make a connection to the Alpine Fault? If this is to occur we could observe more strong earthquakes in the Mount Hutt area and beyond. The fact that there are some aftershocks to the west of Darfield is concerning.
    3) Because Banks Peninsula is made up of solid volcanic rock as opposed to the gravels that cover the Canterbury Plains, I wonder if its solid nature combined with its additional weight over the fault line has resulted in resistance to strike slip movement until sufficient pressure built up and forced the releases that occurred on 22 February and 13 June?
    4)Since the last earthquake I note many of the aftershocks are occurring to the East of Lyttelton Harbour which may indicate a breakout of the strike slippage from under the mass of Banks Peninsula. Alternatively, if the strike slip movement encounters more resistance, there could be another large earthquake under Banks Peninsula in the months ahead. Time will tell.

    • Daniel Harvey says:

      Hey there id be very greatful if you could suggest a search so i can find the geological maps showing the sea floor spreading faults across the pacific, i can only find the rises/ridges etc which are all north/south as opposed to east/west….. would be interesting to see.. i see it is such alongside the San Andreas/ Cascadia Faults, with lots of faults branching off westward so maybe it could be the same for us. Although hopefully not because if that is such then wouldnt we be witnessing the beginning of a new geological era? if so then we could probably expect to see more west/east transform faulting north & south of Christchurch & ie more “new faults”, ie earthquakes… but it could explain how they happen to fracture east/west under banks peninsula not conforming to the terrain. And yes ive been noticing the south/south east movement of the aftershocks & wondering if theres more faults under Banks Peninsula yet to anounce themselves…… But on the other hand it looks like the “zip” started in the west & is now moving westwards off-shore, ie away from chch, but what if theres more “zips” north & south of us? do we expect a future of frequent earthquakes as the “new faults” reveal themselves & accomodate the eastwards movement of the sea-floor spreading?

      BUT my initial amateur thought is that these quakes have been happening due to the south-wards migration of theMarlborough Fault Zone/ hikurangi fault/ main plate boundary, & Banks Peninsula is acting like a wedge to that movement with chch being in the middle? ie Christchurch & north canty is on a crumple zone, like a vice with pressure built up…. but then that wouldnt explain why theres other faults that seem to be evident under the peninsula south of where the “wedge” would inhibit motion, as there have been quakes all over the peninsula, not just in the area “between the vice”……..

      im not a geologist.. im just an “earthquake expert” ie Christchurch citizen… so some more info as to my questions/theory would be great. Thanks :)

      • John from Oz says:

        Hi Daniel

        Sorry I can’t help you with the maps.

        I’ve looked for them myself ever since the first earthquake but have been unsuccessful thus far. I think the original Pacific paleomagnetism research was done during the Glomar Challenger voyages in the 60′s.

        Pehaps Chris Rowan, author of the original report could assist since he seems to be working closer to the academic action.

        Or, alternatively, drop by the Geology Department at the University of Canterbury. I’m sure earthquakes are a hot topic at the moment. And they might have the desired information in those old fashioned sources of information known as books. Best of luck.

        John from Oz

    • Chris Rowan says:

      If you want to look at the Pacific Fracture Zones, then they show up quite nicely in the ocean bathymetry on Google Earth, which is mostly based on the seafloor gravity data available here.

      However, these structures reflect the plate motions between the Pacific plate and the Nazca,Cocos, and Antarctic plates, not the Australian/Pacific motion that is ultimately causing these earthquakes. Even a structural connection is unlikely due to there being a large submerged plateau of thinned continental crust – the Chatham Rise and Campbell plateau (see the maps here) between the Canterbury coast and the oceanic part of the Pacific plate. Any pre-existing faults in this region were probably formed during local rifting in the Eocene. If these are not reactivated faults, the more westerly trend compared to Marlborough faults may just be due to the Alpine fault accommodating most of the north-south component of motion.

  7. terry says:

    I wonder if we could conceivably call this more of a sequence, much like the New Madrid sequence. The big quakes in that sequence aren’t considered aftershocks of the first one in December 1811, but were likely triggered by it.

  8. Paul says:

    Not sure is the recent events are aftershocks or new quakes. Either way its driving us all mad!

  9. Joanne says:

    I was wondering if there is a map of the North Canterbury faults anywhere and are these likely to activate as well? I live in Rangiora.

    • Mark says:

      There’s a NZ active faults database with interactive user settings here:
      Zoom in to the area, and play around with the slip rate and recurrence interval settings, and you’ll see that close to Rangiora there’s a long fault extending from Porters Pass to the North East and very close to Rangiora. The September Greendale fault quake probably put extra stress on this fault, but I don’t know that there’s an accurate estimate of recurrence interval for that fault – called the Porters Pass fault. So it may go sooner than it would have done, but nobody knows how soon that may be. It is capable of generating M7.5 quakes.
      Rangiora area appears in a seismic hazard maps as a greater risk than Christchurch (ie chance of having quakes generating ground shaking over a certain level, over a certain period of time), because is closer (than Chch) to the Alpine fault, and the mass of large faults extending to the North East from the Alpine fault through Marlborough.
      USGS seismic hazard map for NZ is here:
      The top scale on that map is up to peak ground acceleration of 4.8 m/s^2. Note that in areas of Canterbury in the three largest quakes since September, recorded peak ground acceleration was much higher than the maximum on this scale, several readings up to and over 20 m/s^2 were taken close to the epicentre(s).

      So what to do? Be prepared, but also comforted by the fact that in the three recent events in Christchurch, nobody was killed due to collapse of private dwellings.

  10. Tony says:

    Wow!!! Finally found a site where I can ask a question with some expectation of it being answered seriously…. I note in recent days that the number of new aftershocks in a north-south direction from Sumner through to Akaroa on the Banks Peninsula. This completely new trend is quite disturbing. Is this the start of a new fault line and how dangerous is it?. If it is, my understanding is that the hardness of the rocks on the Banks Peninsula, may in fact contribute to a larger quake than if the rock were softer, as in the immediate Christchurch area?

    • Chris Rowan says:

      Taking a quick look at the last week of aftershocks, they do seem to be mainly occurring southeast of last week’s big shocks on the Banks Peninsula. It’s unlikely to be a new fault line, and more likely to be some strain being taken up by old faults or fractures associated with the extinct volcanoes on the Peninsula, as explained towards the bottom of this helpful article.

      It is true that hard, strong rocks can accumulate more strain, which means larger earthquakes when they do rupture, but the earthquake generating faults hidden beneath the soft sediments of the Canterbury Plains are within hard bedrock, too. The stuff on top just inconveniently hides them, and amplifies the shaking.

  11. Ann says:

    Thanks so much for this. Since September, fear has not seemed a rational response to me. Even after being crushed by a building in February and in hospital for 2 months. But after the last week’s big shakes directly under our fair Sumner, many of us are downright scared. Everything has changed and fear seems quite a rational response. I still hope I’m wrong, and am over-reacting. But it seems a whole different ballgame now. I hope I’m wrong.

  12. jeff barnfield says:

    hi Chris,
    Haven’t spoken up for a while since my last email when some of my theories were dismissed by the so called experts.My main theory was that there are fault lines all under Christchurch faning out much like a braided river and that they are all interconnected.I have in front of me page A3 from the Christchurch press dated Saturday July 16 2011.It shows a map showing all the know faults under Christchurch which proves my theory! Unfortunately for my family we live vertually on the cross hairs of two fault lines being the Porthills fault being east/west and the New Brighton fault being north/south.At our location in South New Brighton/Southshore we have had many more quakes than have ever been recorded by GEO NET.In some of these unrecorded quakes the land/house feels like it is being lifed up then dropped hard all the time.On our particular property/section the I estimate that the eastern side has risen as much as 50 to 70 cm.This upward movement can easily be proved by the fact that before the Feb/June quakes if one slowly ran the garden hose the water would subsequently drain/flow quite quickly to the east, now since the major quakes it now flows quickly to the west, no Theodolite required to prove that one! So to me it would appear the extreme eastern coast line of Christchurch is rising and the centre of Christchurch is sinking.What are your thoughts?
    Regards from Jeff.

  13. jeff barnfield says:

    hi Chris. Re Statement on how much our land has risen should be 50 to 70 mm not cm!
    Regards from Jeff.

  14. Kate says:

    Hi Chris Rowan,

    Living in Christchurch at the moment and things have been quiet in a similar way prior to Feb and June… Currently is experiencing another big quake likely?

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