Surface seismic waves and amplitude with distance

  1. OK so we all know that seismic waves decrease in amplitude at about 1/r, the decay is slower than P and S-waves but it's there.
    So how do you explain that when I see examples of seismograms of the same earthquake from stations at different distance you can sometimes notice that surface waves amplitudes increase as you get away from the source?

    Example: http://www.sciencebuddies.org/Files/3030/3/Geo_img046.gif
     
  2. jcsd
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  3. davenn

    davenn 3,816
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    Hi Sandro

    the amplitude of the surface waves at the Golden, Colorado station and the Colombian station don't look overly different in amplitude in that example.
    You may be fooled by the fact that there is almost no P and S wave amplitude at the Colombian station

    There's also no indication of the gain of any of the stations shown
    In all my experience, ( I run a seismograph station) and can compare my data with may other stations
    You will also probably find that the gain of the Palmer, Alaska station is reasonably low else the signal would have overloaded the recorder

    IF we all were set at the same gain, there would be 2 main reasons that would cause possible amplitude differences

    1) differences in rock densities between the quake epicentre and the stations concerned
    2) the type of ground that the station is situated on .... ie ... soft soils, solid bedrock etc
    soft soils will tend to amplify the lower frequency surface waves

    2 of my 5 seismometers can be seen online here ....
    My seismos

    There's some surface waves present there at the moment ( at the time of writing this post)
    from some Chilean quakes ... The P and S waves are non-existent

    cheers
    Dave
     
    Last edited: Jun 21, 2014
  4. Ok, so what about this video example? It clearly shows amplitudes are higher at longer distances for surface waves.
     
    Last edited by a moderator: Sep 25, 2014
  5. davenn

    davenn 3,816
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    That's still an unrealistic example as its just an animation

    The real info on the grams, as in your first example are the increasing times of the difference in the P and S waves
    It also shows well the spreading out of the surface waves in time this called dispersion. ( they spread out in duration with increasing time). This is because the velocity of the Surface Waves ( the Love and Rayleigh waves) are frequency dependant.
    The velocity of the Body waves, the P and S Waves, is not frequency dependant and therefore there is no dispersion of those waves so their duration shown on the seismogram is constant, even at distance

    In the real world, on real seismograms, and I have access to many records for any given large quake, I don't see the effect you are describing.

    Finally, just to be clear, Your amplitude .... you are referring to the height of the wave record Plus and minus movement from the centre line ? <--- that is amplitude
    Not the length of time the waves go on for ?? <-- That is dispersion

    Nothing else can be obtained from the grams. There is no seismometer info given, no info on ground type at their locations etc.
    The amplitude of the Surface Waves of a large shallow, crustal quake at the source, are huge. They are what cause the damage. The P and S Waves arrivals are difficult to even recognise if the quake is very close


    cheers
    Dave
     
    Last edited: Jun 21, 2014
  6. Hi, yes by amplitude I mean the actual physical amplitude of a wave as you described. :)
    Tomorrow I'm gonna send an email to my professor because I clearly remember he said that (while showing the video) in class.
     
  7. davenn

    davenn 3,816
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    The other thought I had whilst I was away...

    Seismometers designed for recording teleseismic events ( long distance > 1000 km) tend to have frequency responses that peak in the freq range of the surface waves 10 to 100 seconds
    The top seismogram on my www site I gave above is designed (filtered) for that. It peaks at ~ 30 seconds

    The bottom seismogram is of a short period sensor, it peaks at around 0.1 second ( 10 Hz). It wont record long distance surface waves overly well, if at all, on large quakes greater than ~ 2000 km or greater.

    both of your references are mainly showing you the differences in propagation speeds of the P, S and Surface waves and how the arrival times at different stations changes with elapsed time

    I have been working with seismograms for more than 20 years, and I'm still learning, I don't know it all, never will haha !! Gram interpretation is a complex subject. But as the years go by, patterns emerge, and one starts recognising the arrival of different waves, the direct travelling ones as well as those that bounce off the core and sometimes those that have travelled the opposite way around the earth before getting to the seismometer

    cheers
    Dave
     
  8. jim hardy

    jim hardy 5,026
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    I'll be curious to see if yours sees that Alaska 'quake a couple hours ago...
     
  9. davenn

    davenn 3,816
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    Yes I recorded it Jim, but because it was quite deep, ~ 108km, it wasn't huge on the seismographs
    Deep quakes don't produce much in the way of surface waves
    It was totally outclassed by 6.9, 6.3 and 6.2 quakes much closer to me in the 1.5 hours prior to the Aleutian Is M8.0 event

    here's a low gain long period channel on my system that shows the different events
    http://www.sydneystormcity.com/zlo1.gif

    Am at work at the moment .... give me a little time and I will save an image and label the events :)

    Dave
     
    Last edited: Jun 23, 2014
  10. davenn

    davenn 3,816
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    OK here we go

    [​IMG]

    you can see on that indicated line that the surface waves from the M 8.0 were minimal
    Had this been a shallow quake < 30 km the surface waves would have been almost full amplitude ( top to bottom of the screen) even on this low gain channel


    EDIT .... some updates ..... its been now downgraded to a M 7.9
    It produce a local Tsunami of ~ 0.2 - 0.7 ft
    This quake was quite a long way back (and deep down slip ) from the trench so hence why it didn't produce any significant tsunami



    cheers
    Dave
     

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