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Seismic T Waves

  1. Dec 1, 2014 #1

    davenn

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    a year ago ( 18 Dec 2013) I started a thread on the first observed/confirmed recording I did of a seismic T wave from a quake off the SW tip of the South Island of New Zealand.
    https://www.physicsforums.com/threads/seismic-t-waves.729045/

    Since then I have recorded them on 3 other quakes from the same region. The latest was overnight last nite

    this gram show the low frequency ( long period) recording of the quake an M 5.1
    P wave arrival at approx. 1313 hrs T wave not really discernible in the surface wave tail at 1330 hrs

    141201 1304UT zhi.gif

    This gram is from a 4.5 Hz geophone and shows a easily identifiable burst of T waves arriving at 1330 hrs

    141201 1304UT M 5.1 SW of Sth Is showing T wave arrival syde.gif


    Since that first observation a year ago, I have diligently looked at my events recorded around the New Zealand region and up towards Fiji. I have yet to see T waves from anywhere other than the area south of the South Island.
    I don really see this as a distance between sensor and event issue, as there is no significant difference in distance between sensor and this region than say to the Kermadec region immediately to the north of NZ.
    Yet on events from the Kermadecs, a much more active area, giving more opportunities to possibly see T wave events, they are not present.
    Both region paths are all water/oceanic less the last ~ 20km from the east Australian coast to the sensor.

    As yet I don't have an explanation as to why I only see then from the region immediately to the south of the South Island of NZ. I find it quite fascinating :)

    regards
    Dave
     
    Last edited: Dec 1, 2014
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  3. Dec 1, 2014 #2

    Bystander

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    Had to rummage in the mental closets a bit, and came up with S and P --- "T" is "new" to me, but it's been a while, and is what? Orthogonal to S?
     
  4. Dec 1, 2014 #3

    davenn

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    hi Bystander, greetings

    prior to a year ago, I hadn't heard of them either
    click on the link at the top of my first post, it will take you to the thread I started a year ago, in there are a couple of links to T wave information

    cheers
    Dave
     
  5. Dec 1, 2014 #4

    Bystander

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    Interesting. Just spent a couple minutes on G-earth looking at abyssal-shelf transitions. Tasman sea floor transit to Sydney isn't all that different (on the gross scale) from the Atlantic to Hatteras transit. Much humor during grad school forty years ago from the geology dept. (split my time between chem. and geo.) about the "Guns of Seneca" by another colloquial name along the Carolina Coast. Just looked that up and found a little hydrological tidbit that may or may not be of interest or use to you,
    http://legrandhydrogeology.blogspot.com/2009/06/anxieties-about-ground-water-above-cape.html .
     
  6. Dec 1, 2014 #5

    davenn

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    It would be interesting to know if USA east coast seismic stations ever recorded T wave arrivals from Mid Atlantic Ridge events
    Have some seismo friends in that part of the world ... I will have to ask them

    That was definitely an interesting read, something quite unusual ... thankyou :)

    Dave
     
  7. Dec 1, 2014 #6

    Bystander

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    Slightly higher frequency than what you've been getting, but there may actually be some relation between the two phenomena.
     
  8. Dec 1, 2014 #7
    The primary and secondary waves are expected during and before volcanic activity. The T-wave depending on what it accompanies could be a precursor to imminent activity of slip, slide. or a variety or combination of other movement.
    Keep tracking the activity and see if you can find some historical data that illustrates the same activity in or around the event.
    The is a young lady at Vesuvius in Italy who loves to follow-up on this sort of thing. Her name is Linda Davis. She is a former professor at CSUB.
     
  9. Dec 1, 2014 #8

    davenn

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    Hi Robert
    welcome to PF :)

    This doesn't have anything to do with volcanic activity

    EVERY earthquake produces P and S waves :)

    No, the T wave is a slower travelling wave that originates as a P wave at the quake focus under the ocean floor. When that P wave hits the seafloor sea interface it then propagates as a sound wave in the ocean till it in this case hits the continental shelf where it becomes a seismic wave again and is then recorded by my equipment some 20 - 25 km inland from the coast.

    did you have a read of the other thread that I did a year ago ?
    https://www.physicsforums.com/threads/seismic-t-waves.729045/
     
    Last edited: Dec 13, 2014
  10. Dec 9, 2014 #9
    I had to go look up T waves, Found a decent paper.
    http://www.earth.northwestern.edu/people/emile/PDF/EAO212.pdf
    They stated that T waves from quakes in the abyssal plane defied laws of
    geometrical optics, I disagree, it is just a smaller capture window.
    Think of a light bulb (point source of light) under a sheet of glass,
    The rays strike the glass from 0 to 90 degrees, most rays ether
    go through with an index offset, or are reflected off the front surface
    at greater than Brewster angle.
    Some small percentage is collected between the layers of different media,
    and gets trapped in the waveguide of total internal reflection.
    This can happen in the ocean, where the entire depth is the waveguide.
    The equations for sound in physical matter, and light in optics are very similar,
    C just changes to the speed of sound in your media.
     
  11. Dec 13, 2014 #10

    davenn

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    hi John

    Yes, T-phase waves are an interesting topic. Just recorded another burst within the last 24 hours from another quake off the SW corner of the Sth Is. of NZ (a M5.2)

    I emailed the Seismologists at the NZ Seismological Observatory and this was his response .....

    I really appreciated his bathymetry comments. For whatever reason this hadn't occurred to me and when one looks at the difference in the seafloor
    between the region to the south of NZ and to the north of NZ, then this becomes kinda an obvious reason as to why I record T-phase waves from
    one region but not from another.

    I have uploaded the PDF file he speaks of to my www site, as its a bit big to upload to here
    http://www.sydneystormcity.com/1275.full.pdf [Broken]

    cheers
    Dave
     
    Last edited by a moderator: May 7, 2017
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