Seismic T Waves

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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
 
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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?
 
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davenn
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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?

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
 
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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 .
 
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davenn
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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.

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

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 .

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

Dave
 
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Slightly higher frequency than what you've been getting, but there may actually be some relation between the two phenomena.
 
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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.
 
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Hi Robert
welcome to PF :)

This doesn't have anything to do with volcanic activity

The primary and secondary waves are expected during and before volcanic activity

EVERY earthquake produces P and S waves :)

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.

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/
 
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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.
 
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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 .....

Hello Dave,

I've been asked to respond to your email. I'm not overly familiar with T-phases, but I have seen them on some ocean bottom seismometer data I've worked with. I've done a little bit of reading about them, and exactly how they are generated and propagate is still a bit of a mystery. A big factor appears to be the bathymetry in the source-receiver travel path. The T-phase is an acoustic phase that travels through the water and requires specific conditions. If anything interferes with it such as seamounts or island chains, it disrupts the channel it's travelling through and blocks it. I had a quick look at Google Earth and if you look at the travel path between Sydney and south of the South Island, and compare it to Sydney to the Kermadec region, the Sydney-Kermadec bathymetry is more complex. This may explain why the T-phase is weak or non-existent for Kermadec region events, or events further north in the Fiji/Solomon Islands/Samoa region. The depth of the earthquake may also be a factor as you need efficient coupling between the seafloor and the water. Earthquakes south of the South Island tend to be shallow, whereas events in the Kermadecs and further north are often deeper.

I've attached a paper from the Bulletin of the Seismological Society of America which seems to have a fairly good overview of T-phases and how they are generated and propagate.

cheers
John

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
 
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