NZ Earthquakes

  1. davenn

    davenn 3,975
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    hi Gang :)

    interesting set of earthquakes in the central NZ region in Cook Strait, between the 2 main islands

    [​IMG]

    the last M6+ event that I recall in this area ( and almost in the same spot) was a 6.2 in the 1980's. This a complex tectonic region where there is a change from oblique slip across the South Island changing to a subduction zone under the northeastern most corner of the South Island and under the North Island.
    These events have occurred in the centre of that zone and appear to delineate a fault striking approx. NW-SE. The USGS moment tensor supports this conclusion.

    I was in Christchurch, some 400km to the SW at the time of the M5.7 ( the first event) but didn't feel it. It would appear that only a few people in Christchurch felt the M6.5.
    The first event (M5.7) was preceded by several foreshocks of ~M1.5 - M2.5 in the 2 day before the event. The 5.7, 5.8 and 6.5 had their own good set of aftershocks so far totally in excess of 300 events.
    Unfortunately my digital seismograph computer decided to have a hard drive failure at the time of the M6.5 so am unable to show you a seismogram of the event.

    Regards
    Dave
     

    Attached Files:

    Last edited: Jul 22, 2013
  2. jcsd
  3. So does that mean that NZ is getting smaller, and the two isalnds will evenually meet up. Is the Cook Straight a result of the subduction. I am just wondering if the two islands have relative motion to one another or are experiencing uplift or whatnot.

    At home I assume. That is interesting. I always had the idea to set something up but never ever got around to it. I am now having moments of regret.
     
  4. StevieTNZ

    StevieTNZ 1,123
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  5. davenn

    davenn 3,975
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    Hi 256

    Both the North and South Islands are under compression but by different methods. As a result they are both shortening in they East-West dimension


    [​IMG]

    in the above image, Nth Is. cross-section the tectonics motion is primarily subduction with a bit of a strike slip component. Most of that subduction shortening is taken up by the uplift of the coastal ranges ( east coast) through reverse faulting. A lot of that east coast material (land) is Pacific seafloor that has been accreted to the Nth Is mainland.

    [​IMG]

    In this cross-section of the Sth Is., the tectonics is oblique slipping its roughly a 3:1 ratio. That is for every 3 metres of horizontal motion there's ~ 1 metre of vertical motion.
    The Southern Alps mountain chain is one of the youngest in the world at ~ 5 - 8 million years.
    Prior to then there was extensional tectonics across the Sth Is., resulting in a lot of volcanism


    Yes at home. have a look here for info my system

    cheers
    Dave
     
    Last edited: Jul 26, 2013
  6. davenn

    davenn 3,975
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    yeah I happened to see that the other day :)
    that discussion would have probably been better off in the earth forum ;)
    I always wondered when in NZ you were

    Dave
     
  7. davenn

    davenn 3,975
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    Dangit, only just got my online seismo running for several days and my www site host decides to do some changes to their servers and now the images are not uploading.
    Am still fighting with them trying to sort it out.

    Dave
     
  8. StevieTNZ

    StevieTNZ 1,123
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    Didn't feel this morning's 5.4.
     
  9. davenn

    davenn 3,975
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    deep sleeper huh ;)

    I see the USGS only gave it a M4.9, but they are often quite low in their magnitudes for NZ events

    and a M4.7 in upper Canterbury this afternoon

    cheers
    Dave
     
  10. StevieTNZ

    StevieTNZ 1,123
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  11. Hi Dave,

    I was struck by the difference in tectonic regime between the N and S islands in your diagrams. After looking into it seems as though New Zealand is the stage of an epic battle between the Pacific and the Australian plates! They are colliding and are fighting to remain afloat. To the North the Australian plate is winning, but to the south the Pacific remains afloat.

    [​IMG]
     
  12. davenn

    davenn 3,975
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    Hi Billiards

    yes, NZ is a geologists "field of dreams" or is that nirvana ? LOL
    I loved doing my geology studies there, there's just so much variety packed into such a small landmass

    and yes something I didn't point out was in the lowest corner South Island and further down south through the Puysegur - Macquarie trench, the subduction returns but in the opposite direction compared to the North Island, as can be see by the B-B' cross-section.
    The subduction zone there dips very steeply just a few degrees (~15) off the vertical. I personally don't know of another such steeply dipping zone.

    [​IMG]

    cheers
    Dave
     

    Attached Files:

  13. Wow that really is steep. I guess that means its a mature subduction zone. So is the theory that it will eventually detach and subduction of the Pacific plate will takeover?
     
  14. StevieTNZ

    StevieTNZ 1,123
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    Just had two severe ones.

    First one 6.2, 8km deep followed by a 5.7 8km deep. Data being revised. First it was 6.9 at 12km deep.
     
  15. davenn

    davenn 3,975
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    yup its coming in on my seismo here in Sydney as we speak

    http://www.sydneystormcity.com/sydln.gif


    and unlike the previous ones in this region over the last month ... these ones are on land only a few km's from the Seddon township :(
    Dave
     
  16. StevieTNZ

    StevieTNZ 1,123
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  17. StevieTNZ

    StevieTNZ 1,123
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    Am noticing a lot of small aftershocks. Lets hope there is no more big one.
     
  18. Evo

    Staff: Mentor

    Hold on Stevie!!!!
     
  19. davenn

    davenn 3,975
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    a quick little graphic I whipped up :smile:

    [​IMG]


    for non-New Zealanders, this is the NE tip of the South Island of NZ

    EDIT: updated graphic and another update

    Dave
     

    Attached Files:

    Last edited: Aug 15, 2013
  20. StevieTNZ

    StevieTNZ 1,123
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    A few severe aftershocks have occurred since the last post.
     
  21. davenn

    davenn 3,975
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    OK now that Im home from work and have had time to process the data
    here's the seismograms for the 2 largest events

    Mw 6.5 (MB 6.2) at 02:31UT (14:31 NZST)
    [​IMG]


    Mw 5.9 (MB 6.0) at 05:31UT (17:31 NZST)
    [​IMG]

    exactly 3 hours apart, give or take a few seconds ... freaky!!

    Dave
     

    Attached Files:

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