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M 6.6 Quake SSW of New Zealand and huge T-phase

  1. Jul 11, 2017 #1

    davenn

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    hi everyone

    this evening a sizeable and shallow M6.6 quake SSW of the South Is. of NZ has produced
    the largest T-phase event I have so far recorded on my seismic system

    M 6.6 - 198km NW of Auckland Island, New Zealand

    Time: 2017-07-11 07:00:01 (UTC)
    Location: 49.546°S 164.031°E
    Depth: 10.0 km

    Location map

    upload_2017-7-11_22-3-38.png

    Long period seismogram

    170711.070012.zhi.gif

    one of the short period sensors, a 4.5 Hz geophone
    this shows the labelled P and S wave arrivals followed by the very large amplitude T-phase wave arrival at around 07:22 UT

    170711.070012.sydz.gif

    I have shared a number of posts showing recordings of T-phase events over recent years
    but this one, to coin a phrase, blows all of them out of the water"

    As a reminder and also info to new comers to my posts
    The T-phase waves are ones that leave the seafloor at a point roughly above the quake focus and
    then travel through a deep sea path till, as in this case, they hit the continental shelf offshore of the east coast of Australia and then travel the last ~ 15 km through the ground to my recording equipment.
    The T waves travel much slower than the P and S waves and hence the considerable delay in their arrival, in this case, some 18 minutes after the P wave arrival.

    Regards
    Dave
     
    Last edited: Jul 12, 2017
  2. jcsd
  3. Jul 12, 2017 #2

    jedishrfu

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    THis is interesting. Do you have a reference for T-waves as in an educational video or article?

    T-waves are surface waves right?

    I tried to find one but kept turning up only P and S wave demos.

    http://www.colorado.edu/physics/phys2900/homepages/Marianne.Hogan/waves.html

    and this cool highschool teacher video which talks about surface waves:



    No hobbits were injured in this earthquake right?
     
  4. Jul 12, 2017 #3

    davenn

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    hi ya

    will look up again ... had supplied in posts in days gone bye ... will find them

    no, they are waves that travel in the SOFAR Channel at depth in the sea

    https://en.wikipedia.org/wiki/SOFAR_channel

    here's a thread I did back in 2013 on T-phase events ( there are a couple of others as well)

    https://www.physicsforums.com/threads/seismic-t-waves.729045/

    nope :smile: well out in the southern ocean


    Dave
     
    Last edited: Jul 12, 2017
  5. Jul 12, 2017 #4

    jedishrfu

    Staff: Mentor

    Wow, the I didn't know the SOFAR could be the channel of earthquake waves but of course it makes sense since they are acoustic.
     
  6. Jul 12, 2017 #5

    jedishrfu

    Staff: Mentor

    Does the SOFAR collect and amplify the T signal?

    Does this then cause greater surface damage?
     
  7. Jul 12, 2017 #6

    davenn

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    yes, an interesting condition. I personally only became aware of T-phase waves around 5 years ago when I first started seeing these burst of signal arriving on the seismo short period sensors well after the P, S and surface waves had arrived. It was with discussions with fellow amateur seismologists that we discovered the existence of T-phase waves.

    The T waves start as the P wave, at the focus of the quake, which is a longitudinal compressional wave ( same as sound) at the seafloor water boundary, they then propagate as a sound wave in water within the SOFAR channel that just acts as a waveguide, like a RF microwave waveguide. Actually a better analogy would be an atmospheric ducting waveguide that happens along coast lines and can produce low attenuation paths for ~ 100 MHz and up.
    Once the T waves hit the continental shelf, they again become P waves and their speed has been observed to increase.

    from WIKI....
    From what I am aware, the SOFAR channel is produced by similar processes within the ocean ... differing layers of water temperature and density.

    I am not sure if "collect" is the right word ( maybe it is) but if the sound waves in the water enter the channel at the correct angle, they will get trapped in the channel and propagated at reasonably low attenuation

    Amplified, no, I don't see a mechanism that would cause that. Just lower attenuation than compared through the ground path

    no, because of their overall small amplitude, I doubt that they would cause any damage, probably wouldn't even be felt.
    It's the surface waves of an earthquake that causes the damage. Their low frequency and rolling motions is what buildings cannot handle.

    cheers
    Dave
     
    Last edited: Jul 12, 2017
  8. Jul 12, 2017 #7

    davenn

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    A location / info diagram

    distance of quake from me 2000km
    T-waves have been recorded from quakes on the South Island of NZ and on the coast on the west side and to the south
    I have never ( as yet) seen T waves from earthquakes in the Kermadecs, Tongan and up into the Fiji region ( all north of the North Island of New Zealand).
    Discussions with others has at this time have surmised that the seafloor ridge extending NW 'ward from the west coast of NZ may be blocking the SOFAR channel

    T-phase info.JPG
     
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